Cricket Behavior and Neurobiology 9781501745904

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CRICKET BEHAVIOR AND NEUROBIOLOGY

CRICKET BEHAVIOR AND NEUROBIOLOGY EDITED BY

FRANZ HUBER Max-Planck-Institut fur Verhaltensphysiologie Seewiesen, Federal Republic of Germany

THOMAS E. MOORE Museum of Zoology University of Michigan, Ann Arbor

WERNERLOHER Department of Entomological Sciences University of California at Berkeley

Comstock Publishing Associates a division of Cornell University Press I Ith~ca and London

Copyright © 1989 by Cornell University All rights reseiVed. Except for brief quotations in a review, this book or parts thereof, must not be reproduced in any form without permission in writing from the publisher. For information, address Cornell University Press, 124 Roberts Place, Ithaca, New York 14850. First published 1989 by Cornell University Press.

The paper in this book is acidJree and meets the guidelines for permanence and durability of the Committee on Production Guidelines for Book Longevity o.fthe Council on Library Resources.

Library of Congress Cataloging-in-Publication Data Cricket behavior and neurobiology I edited by Franz Huber, Thomas E. Moore, and Y\1erner Loher.

p.cm. Bibliography: p. Includes index. ISBN 0-8014-2272-8 1. Crickets-Behavior. 2. Nmvous system-Insects. 3. Neurobiology. I. Huber, Franz. II. Moore, Thomas E. (Thomas Edwin), 1930- . III. Loher, Werner. QL508.G8C75 1989 595f26-dc20 88-43256

Cover illustration: Gryllus campestris, singing male.

Contents

1. 2.

3. 4.

5.

6. 7.

8. 9. 10. 11. 12.

13.

Contributors Preface Natural History Thomas J. Walker and Sinzo Masaki Reproductive Behavior Werner Loher and Martin Dambach Temporal Organization of Reproductive Behavior Werner Loher Structure and Function of the Endocrine System Werner Loher and Malcolm D. Zaretsky Vision and Visually Guided Behavior Hans-Willi Honegger and Raymond Campan Vibrational Responses Martin Dambach Mechanoreceptors in Behavior Werner Gnatzy and Reinhold Hustert Songs and the Physics of Sound Production Henry C. Bennet-Clark Neural Basis of Song Production Wolfram Kutsch and Franz Huber Phonotactic Behavior of Walking Crickets Thea Weber and John Thorson Evasive Acoustic Behavior and Its Neurobiological Basis GeraldS. Pollack and Ronald R. Hoy Biophysical Aspects of Sound Reception Ole Ncesbye Larsen, Hans-Ulrich Kleindienst, and Axel Michelsen Auditory Organ Structure, Development, and Function Eldon E. Ball, Brian P. Oldfield, and Karin Michel Rudolph

v

vii ix 1 43

83

114 147 178 198

227 262 310 340 364

391

vi

Contents

14. Central Auditory Pathway: Neuronal Correlates of Phono-

tactic Behavior Klaus Schildberger, Franz Huber, and David W. Wohlers 15. Perspectives for Future Research Franz Huber Glossa1y Glossary of Song Terms Thomas E. Moore Literature Cited Author Index Subject Index

423 459 477 485 488 533 542

Contributors

Eldon E. Ball; Department of Neurobiology, Research School of Biological Sciences, Australian National University, P.O. Box 475, Canberra City, A.C.T. 2601, Australia Henry C. Bennet-Clark, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, England Raymond Campan, Neuroethologie, Universite Paul Sabatier, 118, route de Narbonne, F-31062 Toulouse Cedex 3, France Martin Dambach, Lehrstuhl Tierphysiologie, Zoologisches Institut der Universitat zu Koln, Weyertal119, D-5000 Koln 41 (Lindenthal), FRG Werner Gnatzy, Zoologisches Institut, Fachbereich Biologie, J. W. GoetheUniversitat, Postfach 11 19 32, D-6000 Frankfurt am Main 11, FRG Hans-Willi Honegger, Institut fur Zoologie, Technische Universitat Miinchen, Lichtenbergstrasse 4, D-8046 Garching, FRG Ronald R. Hoy, Section of Neurobiology and Behavior, Seeley G. Mudd Hall, Cornell University, Ithaca, New York 14853-2702, USA Franz Huber, Max-Planck-Institut fur Verhaltensphysiologie, D-8130 Seewiesen, FRG Reinhold Hustert, Zoologisches Institut, Universitat Gottingen, Berliner Str. 28, D-3400 Gottingen, FRG Hans-Ulrich Kleindienst, Max-Planck-Institut fur Verhaltensphysiologie, D-8130 Seewiesen, FRG vii

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Contributors

Wolfram Kutsch, Fakultiit fur Biologie, Universitat Konstanz, Postfach 5560, D-7750 Konstanz, FRG Ole Nresbye Larsen, Biologisk lnstitut, Odense Universitet, Campusvej 55, DK-5230 Odense M, Denmark Werner Loher, Department of Entomological Sciences, University of California, Berkeley, California 94720, USA Sinzo Masaki, Laboratory of Entomology, Faculty of Agriculture, Hirosaki University, Hirosaki 036, Japan Axel Michelsen, Biologisk lnstitut, Odcnse Universitet, Campusvej 55, DK-5230 Odense M, Denmark Thomas E. Moore, Museum of Zoology, University of Michigan, Ann Arbor, Michigan 48109-1079, USA Brian P. Oldfield, Department of Physiology and Pharmacology, University of Queensland, St. Lucia, Queensland 4067, Australia Gerald S. Pollack, Department of Biology, McGill University, 1205 Docteur Penfield Avenue, Montreal, Quebec H3A 1B1, Canada Karin Michel Rudolph, Nikolaus-Lenau-Platz 12, D-7032 Sindelfingen, FRG Klaus Schildberger, Max-Planck-Institut fur Verhaltensphysiologie, D-8130 Seevviesen, FRG John Thorson, The Old Marlborough Arms, The Passage, Combe, Oxford OX7 2NQ, England Thomas J. Walker, Department of Entomology and Nematology, University of Florida, Gainesville, Florida 32611, USA Theo Weber, Max-Planck-Institut fur Verhaltensphysiologie, D-8130 Seewiesen, FRG David W. Wohlers, 1832 West Genesee Road, Baldwinsville, New York 13027, USA Malcolm D. Zaretsky, Department of Electrical Engineering and Computer Science, University of California, Berkeley, California 94720, USA

Preface

The world of crickets is a world of scientific adventure and human fascination. Most cricket species are musical, sound-producing insects that have long been part of human life and lore. For instance, according to Polynesian creed, crickets are embodiments of the souls of loved ones, and in many countries a cricket singing at the hearth is thought to bring luck and protect the home against evil spirits. The fierce rivalry behavior of male crickets was well known in ancient China; games were organized and bets were waged on the outcome of their battles. But it is the elaborate behavior of crickets, and especially their acoustic communication, which has always drawn the most attention from biologists and the general public. This multiauthored book offers an introduction to cricket behavior, especially acoustic behavior, and neurobiology. It tells about the manifold strategies crickets employ in matching development with seasons and habitats, finding mates, and avoiding parasites and predators, and it describes the physiological mechanisms, especially the neuronal mechanisms, underlying cricket behaviors. Even though 64 genera and 116 species of crickets are mentioned, representing the 13 subfamilies ofGiyllidae throughout the world, this book is not a monograph; rather it deals with selected aspects of cricket biology assembled under the umbrella of behavior and neurobiology. In fact, most information available from such combined studies comes from fewer than a half dozen species in one subfamily. \!Vhy have crickets remained so interesting to so many generations of students of animal behavior and physiology and become model systems for behavioral physiology and neuroethology? Probably because crickets display elaborate behaviors that can easily be studied in the field and in the laboratory. Some of their behavioral patterns can be measured with high resolution, even in partially restrained animals. These behaviors provide important insights into many sensmy, neuromuscular, neuroen-

ix

X

Preface

docrine, and central nervous system structures and mechanisms at the whole-animal level down to the level of single cells. Furthermore, there are abundant species for comparative studies, several species are readily reared in captivity under seminatural conditions, crickets do not sting, and most do not have painful bites. We hope this book stimulates cross-fertilization between different biological fields and encourages scientific progress. An evolutionary biologist with a particular interest in natural history, life cycles, geographic distribution, mechanisms of speciation, or species relationships may gain as much from it as might a behavioral ecologist who is trying to understand how different environments influence and shape specific behaviors and to what extent genetic backgrounds or even individual experiences are responsible for optimizing cricket life and reproduction in a given habitat. Students of comparative behavior and physiology can learn about the various tactics employed by closely and distantly related species. Acoustic cues-important as they maybe-are only part of a cricket's communication network. Other sensory stimuli, such as substrate vibration, light, or the use of chemicals, often neglected in the past, may complement the acoustic channel or may play primary roles in such cricket behaviors as the selection of habitats and burrow sites, aggregation and migration, and antiparasitic strategies, to mention only a few. Those interested in the circadian organization of behavior will discover that various activities affecting reproductive success are optimized over time by means of internal clocks located in the nervous system. Although neuroendocrine and hormonal effects and mechanisms in crickets are less well understood than in other insect groups, some progress has been made in demonstrating the structure and distribution of neuroendocrine cells in the brain and in showing how certain hormones affect behavior. Distinct hormonal titers can now be measured in conjunction with particular behavioral states and motor patterns, offering new approaches to and understanding of sexual receptivity, the functioning of the reproductive system, hormonal feedback, and the interaction between hormonal and nervous systems throughout a cricket's life. The book summarizes what is currently known about a cricket's visually guided behavior and vision, taking into account structural and functional features of the compound eyes and the visual pathway. Visual cues are used to localize burrows and shelters and, with the help of landmarks, to orient the animal. Photoreceptor cells specialized for polarized light perception have been found in the dorsal rim area, and new information has been gained about color vision in crickets. Chemoeeception in crickets is a rather neglected field of research, although numerous chemoreceptors are present on the surfaces ofpalps, antennae, and other body parts. Knowing more about their structure and function will help us understand how crickets use them to select certain habitats, food plants, and even sexual partners.

Preface

xi

Vibration-sensitive structures outside the auditory system function to a surprising extent. Crickets can detect disturbances produced by predators approaching on the ground or in flight over a considerable range. A digger wasp-cricket interaction is analyzed in some detail in chapter 7. Crickets also use self-generated vibrations and air particle movements as well as movement -dependent mechanosensory input as feedback devices to stabilize motor patterns and make them more efficient. Many mechanoreceptors occur over and within the cricket's body. The book discusses their role in eliciting grooming, in controlling body position, and in escape behavior, and thereby illustrates how they interplay with other sensory modalities during complex behavioral sequences, even in situations previously thought to be the exclusive domain of acoustics. Not surprisingly, more than half the book is devoted to acoustic behavior, for which crickets are famous, and it is here that the most significant advances are presented. The biomechanical, neuromuscular, and neuronal events associated with sound production by crickets are quite well understood, as is the integration of sounds into overall cricket behavior. The ontogeny of sound pattern production and its genetic basis in males is beginning to be elucidated, and even though they do not sing, we need to examine critically how similar the nervous machinery is in females. The concept of a central pattern generator is examined in cricket stridulation, as is increasing knowledge about how different sensory systems interact by feedback to adjust sound production and songs to the environment, optimizing their efficiency both in loudness and in pattern structure. Considerable new insights concerning the st1ucture and the biomechanics of cricket ears are discussed, including the complex processes of sound conduction and cross-body interference and their role in acoustic orientation. How the hearing system is adapted to small body size and to the frequency range in which crickets broadcast is now better known. Frequency tuning and discrimination in the ear are described, but the underlying mechanisms are still not well understood. The auditory pathway from the ear to the prothoracic ganglion and further on to the brain has been studied at the single cell level, and we have discerned how carrier frequencies and song patterns are encoded at the various levels and how behaviorally relevant information is extracted. Some of these findings may have relevance for all hearing animals. Correlations and even causal relationships between a particular auditory neuron and aspects of phonotactic and avoidance behavior are elucidated. In the brain, cells that act in conspecific partner-recognition have been localized and identified, thereby providing insight into the neuronal hardware of innate releasing mechanisms. This progress was achieved because questions resulting from behavioral studies guided the neurophysiological approach and because sophisticated behavioral and neuronal approaches were combined. Certainly the next and most urgent task will be to study the con-

xii

Preface

nections between neurons forming the recognition and localization network and to explore how they drive the corresponding motor output. There is evidence for parallel and multineuronal processing within the nmvous system of crickets. Thus the concepts of hierarchical organization and command neurons are put into question; they may have to be replaced by ideas of complex interactions between parts of the neiVous system, including multimodal and multisensory processing. Recent studies have demonstrated a high degree of plasticity in cricket neiVous systems, and this information needs to be related to neuronal development and behavior, beginning at the level of juveniles. The time may even be near when crickets will become models for experiments on simple forms of learning. Quite a few unexplored avenues remain-for example, causes and mechanisms of aggression and territoriality, of spacing of individuals within a population, and of differences in sympatric life styles. We are just beginning to appreciate the complexity of sound communication in the natural habitat, where the frequency, pattern, and loudness of sound signals are changed by the environment. There is still little understanding of age-dependent changes in cricket behavior, the genetic basis of senderreceiver interactions, and temperature effects in communication. We know little about how different species of crickets choose their specific habitats or display satellite strategies. All these gaps in our knowledge call for more comprehensive combinations of field and laboratory analyses. Many topics must await a subsequent book. Here we provide a comprehensive treatment of comparative cricket behavior, one composed of contributions from people who have sought to explain some of life's incredible complexity by seeking physiological mechanisms that control or enhance the behaviors they have studied. It is at once a book about communication, comparative physiology and anatomy, and environmental interaction, for some of the studies and all of the behaviors relate to activities in which crickets engage in nature. Untold numbers have enjoyed crickets; relatively few have studied them seriously; and even fewer have stimulated others to search for answers to basic biological and philosophical questions through their publications and their personal magnetism and enthusiasm. It is for all who may come to love the world of crickets that this book is written. Assembling a book with many authors requires careful selection of contributors who are cooperative and compatible, who have interesting perspectives to offer, who are active in the field, and who are able to detail the present state of cricket research both at conceptual and at experimentallevels. We believe we have succeeded beyond our original hopes, and we most sincerely thank our contributors for their patience and their good efforts. We also thank Robb Reavill, Science Editor, Marilyn M. Sale, Managing Editor, Helene C. Maddux, Associate Manuscript Editor, and

xiii

Preface

Cynthia Gration Marketing Manager, Cornell University Press, Lois Smit and Margo Quinto, copy editors~ and Mark F. O'Brien~ Theresa Duda, John B. Klausmeyer, Celia 0. Riecker and Margaret Van Bolt University of Michigan~ for their assistance, enthusiasm~ and care in bringing this book to completion. 1

1

1

FRANZ HUBER

THOMAS

E.

MooRE

WERNER LOHER

Seewiesen, Federal Republic of Germany Ann Arbor, Michigan Berkeley, California

CRICKET BEHAVIOR AND NEUROBIOLOGY

A cricket is a lovely thing that likes to sing. Rose Ann Walker, age 9 (Kevan 1974) Hark to those tinkling tones,-the chant of the suzumushi! -If a jewel of dew could sing, it would tinkle with such a voice! Hoshu Tokunaga, 1871 (Hearn 1898)

CHAPTER ONE

Natural History Thomas J. Walker and Sinzo Masaki

Crickets are arrwng the insects that impose upon human consciousness without biting, stinging, or destroying. Because their songs are generally perceived as pleasant, they rank with butterflies and fireflies as insects to enjoy. The acoustic conspicuousness of crickets has also been important in their enjoyment by researchers in field and laboratory biology. Taxonomic, ecological, and behavioral studies of crickets have been prompted and made easier by the persistent, locatable, identifiable calls of the males. It is not surprising that most chapters in this volume present studies made possible or facilitated by the acoustic behavior of crickets. Although crickets have long been known to biologists, the literature dealing with their natural history is varied and diffuse. Fragments are included in faunal works such as those of Chopard (1969), Matsuura (1976-83), and Otte and Alexander (1983). Life cycles have been reviewed by Alexander (1968a) and by Masaki and Walker (1987). B. B. Fulton (19151 wrote and beautifully illustrated a bulletin on the tree crickets (Oecanthinac) of New York. More recently, Walker (1984) produced a 54-page bulletin on Florida mole crickets (Gryllotalpinae). Furthermore, one-third of Gwynne and Morris's (1983) Orthopteran Mating Systems concerns crickets. These noteworthy concentrations of cricket lore do not alter the fact that most accumulated knowledge of cricket natural history is scattered thinly through entomological and biological journals. This chapter presents aspects of the natural history of crickets that are not dealt with in later chapters and yet are important as background for all studies of crickets. Aspects of taxonomic and ecological diversity, zoogeography, and especially life cycles are included, together with aspects of dispersal and migration. Crickets are confronted with the tasks of finding food, partners, and oviposition sites. They have natural enemies, pathogens, and parasites to avoid or overcome. The ways that crickets cope with these problems are described. Finally, the cultural and eco1

T. J. Walker and S. Masaki

2

nomic aspects of cricket natural history are considered. We begin by considering the diversity of crickets.

Diversity Phylogeny The more than 2,600 modern species of crickets are apparently descended from a common ancestral species that lived ca. 300 million years ago (Fig. 1.1). Sharov (1971) proposed that crickets originated when their immediate ancestors adapted to hiding under shelters; the posterior portions of the forewings became flattened over the somewhat flattened body, with the anterior portions retaining their primitive lateral positions. Because the lateral (anterior) portions of the forewings of the flattened protocricket were necessarily modest, the dorsal (posterior) portions (including the stridulatmy areas) were dominant. The stridulatory areas of male forewings expanded, and the forewings themselves became shorter, exposing the tips of the folded hindwings in species that fly. Male forewings eventually became principally acoustic devices-as attested by the contrast in male and female forewings in all calling species and by the retention of forewings by males in many flightless species that have wingless females. The specializations of the male cricket's forewings permit them to vibrate in one principal mode at a time and at relatively low ENSIFERA

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Latitudinal clines in size of adult crickets. A, Two univoltine species. B, Two species that are bivoltine in the south and univoltine in the north; direction of clines at 35-40° is temporarily reversed in zone of voltinism transition. These are Japanese egg-overwintering species. Measurements were of head width of female fall adults; measurements in A corrected for regression on altitude; those in B based on specimens collected at< 150m altitude. (From Masaki and Walker 1987, courtesy of Evolutionary Biology.)

Fig. 1.9

When a species with a wide latitudinal range shows a transition in voltinism, the direction of the cline in adult size should be temporarily reversed, resulting in a "sawtooth" pattern. Masaki (1978) documented such a pattern for Dianemobius mikado and D. nigrofasciatus (Fig. 1.9 B). The latitudinal clines in adult size ofunivoltine crickets have a seasonal parallel. As the adult season progresses, the costs to an individual that

20

T. J. Walker and S. Masaki

delays its final molt to become a larger adult become greater than the benefits. Consequently, adults that mature late in the reproductive season should be smaller than those that mature early. In Scapteriscus acletus egg laying occurs in April through June, but adults mature as early as September of the previous year. Fall-maturing males average 9.4 mm in pronotallength (= 740 mg). In the spring this dimension gradually declines to 8.5 mm (= 580 mg). Females show similar trends (Forrest 1987). Dispersal and Migration Moving about to find food and mates is essential in all cricket life cycles; and, for species that live in transient habitats, individuals must move to new habitats, at least in some generations. Adopting a distinction made by Southwood (1962), we distinguish interhabitat movements as migratory and locomotion within a habitat as local. For local movements, adult and juvenile crickets generally walk (see chapter 5). Adults of tree-dwelling species are more likely to use flight for local movements than are those of ground-dwelling species (Waloff 1983). Most migration is by flight, but the fact that flightless species also colonize newly available habitats shows that some interhabitat movement is by other means. For example, Simberloff and Wilson (1969) reported that flightless crickets (Cycloptilum spp.) colonized experimentally defaunated red mangrove islands. Rafting, swimming, "ballooning," or phoresy seem the major possibilities; and none can be excluded. Wings and Winglessness Most crickets, like other Orthoptera, have two pairs of wings, with the meso thoracic wings (forewings or tegmina) somewhat thickened and providing some protection for the membranes metathoracic wings, which are the principal organs of flight. The metathoracic wings are folded fanlike at rest (Fig. 1.10 B). All crickets seem to have evolved from a Permian species that had hindwings specialized for flying and male forewings specialized for call-

Fig. 1.10 Wings in crickets. A-E, Gryllus rubens: A, long-winged male; B, longwinged male with tegmina (forewings) removed; C, long-winged male artificially dealated by tugging on hindwings; D, short-winged male; E, short-winged male with tegmina removed. F-H, Trigonidium cicindeloides: F, long-winged female (note tibial tympana); G, dealated long-winged female (note tibial tympana); H, short-winged female (no tibial tympana). I, Cycloptilum bidens female (no wings; see Fig. 1.3 K for male). J, K, Prognathogryllus oahuensis: J, male with acoustic tegmina; K, female with vestigial tegmina. L, Tafalisca lurida male (tegmina not acoustic). (F-H from Ingrisch 1977, courtesy ofS. Ingrisch; J courtesy of D. Otte; K modified from Insects of Hawaii, vol. 2, by E. C. Zimmerman,© 1948, University of Hawaii Press.)

Nat ural His tory

21

22

T. J. Walker and S. Masaki

ing. Many recent species have both specializations, but flying and calling have been lost many times, both independently and in the same phyletic line. Three subfamilies were evidently founded by cricket species that had lost one or both specializations. Phalangopsines generally lack hindwings and are not known to fly; all mogoplistines lack hindwings (and cannot fly). Most males in both subfamilies have acoustically specialized forewings (and call). The myrmecophilines seem to have evolved from a mogoplistine ancestor that had lost its forewings (i.e., had no wings at all) and was mute as well as flightless, as in the modem mogoplistine genera Arachnocephalus and Oligacanthopus. Within each remaining subfamily, there are generally species that cannot fly, species that cannot call, and species that cannot fly or call. Calling, and its loss, will be addressed further below. Flight Crickets that fly have well-developed hindwings as well as forewings that partially protect the hindwings at rest (Fig. 1.10 A, B). Crickets that do not fly may have no hindwings (with or without forewings; Fig. 1.10 I, K), vestigial hindwings (always with forewings; Fig. 1.10 D, E), or fully developed hindwings (Fig. 1.10 A, B). The latter crickets can be classified as flightless through behavioral tests (e.g., Walker 1987a) or through dissection-since flight requires well-developed metathoracic muscles, which are lacking in some long-winged insects (Harrison 1980, Roff 1987). The occurrence of both local and migratory flights is related to habitat. Most ground-dwelling crickets fly only when moving between habitats, and many species are flightless. Local movements are generally by walking. On the other hand, many vegetation-inhabiting crickets (e.g., most oecanthines, eneopterines, and trigonidiines) make some of their local

movements by flying from one part of a plant to another or from one plant to another in the same habitat. Migratory flights are most characteristic of crickets that live in transient habitats. To the extent that flight is restricted to migration in grounddwelling crickets, the possession of flightworthy wings is correlated with habitat impermanence. For example, species that occupy early successional habitats ar·e usually macropterous or dimorphic (see below and Fig. 1.11 A-C); species that live in woods (which are relatively permanent) are often flightless (Fig. 1.11 D, E). Vegetation-inhabiting crickets that make local flights often migrate as well, as illustrated by the sudden appearance of numerous adults in habitats where juveniles do not develop (e.g., Walker 1963). Not all ground-dwelling crickets eschew local flights. Two species of mole crickets (Scapteriscus acletus and S. vicinus) are known to fly both within and between habitats. Within-habitat flights were proved by capture of marked mole crickets terminating their flights where they were released one or more nights before (e.g., 67% of 647 recaptures of S. acletus) (Ngo and Beck 1982). Local flights help these two species find

Natural History

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Frequency distributions ofhindwing length (HW) expressed as proportion of tegmina! (forewing) length (FW) for five species of Gryllus from Florida. Species A invaded south Florida from the West Indies and is 100% long-winged. Species B and C live in early successional habitats and are dimorphic in hindwing length. Species D and E live in woods and are 100% short-winged (except that D produces a few flightless long-winged individuals when reared in the laboratory). Long-winged morphs of B and C frequently fly to lights, and A sometimes flies in pest proportions. The sequence from A to E probably represents the evolution of flightless crickets from flying forebears. (Modified from Walker and Sivinski 1986.)

Fig. 1.11

mates and oviposition sites (Forrest 1983a). Evidence for migratory flights includes the rapid spread of these two species from ports of introduction in southeastem United States and the landing oflarge numbers in habitats where they could not have developed (Walker and Fritz 1983). Frequency of migratory relative to local flights can be estimated by comparing the numbers of flying mole crickets caught in traps in fields (where these two species develop) with those in woods (where they do not and therefore must be migrants) (Fig. 1.12) (Walker and Fritz 1983).

24

T. J. Walker and S. Masaki

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Monthly migration indices for two species of Scapteriscus mole crickets, calculated from numbers that terminated their flights at sound-baited traps in woods and in fields. Because mole crickets that landed in woods (W) (where they do not breed) had to be migrants, and those that landed in fields (F) could include both migrants and locally flying crickets, W!F (migrants/[migrants + local fliers]) estimates the proportion of trapped mole crickets that were migrants. (Data from Walker and Fritz 1983.) Fig. 1.12

Wing and flight dimorphism. Wing dimorphism-the occurrence of long- and short-winged morphs within the same population-is widespread among crickets (Masaki and Walker 1987). Wing dimorphic species (Figs. 1.10 A-H; 1.11 B, C) occur in at least six of the seven major subfamilies that fly. We know of no documented examples from oecanthines, but wing length has more than one mode in some herb-inhabiting oecanthines (T. J. Walker, unpublished). Wing morphs are usually discrete and easy to distinguish. Individuals that have the folded hindwings projecting beneath the forewings are long-winged (macropterous), and individuals that have hindwings shorter than the forewings are shortwinged (micropterous or brachypterous) (Figs. 1.10 A, D, F, H, 1.11). Wing dimorphism generally reflects flight dimorphism, as evidenced by short-winged morphs never flying and by long-winged morphs flying to lights or sound in large numbers. However, as noted above, long-winged crickets do not necessarily fly. For example, we know of no reports oflongwinged Gryllus campestris flying; and even though thousands of G. rubens may fly to one trap in one month, many long-winged rubens probably never fly (Walker 1986b, 1987a). Long-winged crickets should be viewed only as potential fliers.

from

Natural History

25

Two aspects of wing dimorphism are of special importance in understanding cricket natural history: its adaptive significance and how the morph of an individual is determined. In wing-dimorphic species the short-winged morph reproduces earlier and is more fecund than the long-winged morph (Tanaka 1976, 1986, Roff 198Gb, Zera and Rankin 1989). On the other hand, flight-capable, longwinged individuals have the advantage of being able to escape a deteriorating or crowded site and to colonize new or sparsely occupied ones. Thus some individuals are specialized to exploit the home habitat, and others are specialized to leave it and colonize new ones. As expected, wing-dimorphic species occupy habitats that sometimes last long enough to favor the short-winged morph and yet are unstable and transient enough to favor (or require) the long-winged morph. The genetic basis of wing dimorphism is twofold. In the first place, genetic differences among individuals are sometimes responsible for their developing into different morphs (Roff 1986a, Walker 1987a, Zera and Rankin 1989). The maintenance of these differences may depend on the environment favoring different morphs at different times and places, with the result that neither set of morph-determining alleles is eliminated. Of equal or greater importance is the fact that a genotype may be specifically adapted to switch development to one morph or the other in response to environmental cues. In some cases these cues are indicators of season (e.g., photoperiod, temperature) and condition of the habitat (e.g., food, crowding! that can cause an individual to develop into the morph that is appropriate to the particular season or habitat in which the adult must function (Masaki and Walker 1987). For example, Veazey et al. (1976) reported seasonal ,changes in morph frequency for two dimorphic Gryllus species, with long-winged morphs being most frequent during parts of the year most likely to have nocturnal temperatures favorable for flight and to provide habitats favorable for colonization. An intriguing aspect of environmentally controlled wing development in wing-dimorphic crickets is that crickets reared under constant conditions (e.g., temperature, day length, food, and crowding) may remain dimorphic-even after eight generations of 100% selection for long and short wings (Walker 1987a). Computer modeling and theoretical considerations indicate that, in circumstances that are truly uncertain (i.e., no accessible environmental cues predict which morph is appropriate), natural selection favors genotypes that use inconsequential environmental differences to produce a mixture ofmorphs. This type ofmorph determination has been termed adaptive "coin flipping" or stochastic polyphenism (Cooper and Kaplan 1982, Walker 1986a).

Migratory flights and oogenesis. For many flight-capable insects, migration is the first major activity of the postteneral adult. After the migratory flight the wing muscles are histolyzed, and females use the materials in oogenesis. Males may or may not have an analogous ontogeny; they

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T. J. Walker and S. Masaki

cannot colonize by themselves, and females should mate before emigrating. The importance of females as colonizers is supported by the fact that, in wing-dimorphic crickets, females are more often long-winged than males and usually form the majority in samples of migrants. The partitioning of adult activity into early migration and later flightlessness and reproduction is especially clear-cut in species that shed their hindwings.

Loss of hindwings. Hindwing shedding, dealation, occurs in some species of at least three subfamilies of crickets (Tanaka 1986, Masaki and Walker 1987, Roff, 1987). Individuals that have no hindwings projecting from beneath the tegmina are not necessarily short winged-what is concealed may be wing stumps rather than short wings (Fig. 1.10 C, G). If they have stumps, the wings they lost were long, because only longwinged individuals dealate. Dealation occurs both in monomorphic longwinged species, such as Anurogryllus muticus and Loxoblemmus amoriensis, and in wing dimorphic species, such asAllonemobius fasciatus and Velarifictorus parvus. Most long-winged crickets do not dealate; and those that do may not have flown or been able to fly (e.g., all Anurogryllus arboreus have long wings but shed them while still teneral) (Walker 1972). Repeated migratory flights. Most crickets that migrate probably make a single, early migratory flight (followed by wing-muscle histolysis in species that make no trivial flights). However, in one of the few cases in which cricket flights have been studied intensively, migration can occur repeatedly. The pest mole crickets Scapteriscus vicinus and S. acletus migrate (as evidenced by their landing in habitats unsuitable for development) throughout their principal period of maturation (fall through midspring) and well beyond (early summer) (Walker and Fritz 1983). That individuals fly more than once is known from recaptures of marked individuals (up to 58 days between flights for S. acletus) (Ngo and Beck 1982) and from holding mole crickets in buckets of soil and recording when they fly out (Forrest 1986). Perhaps the most convincing evidence that repeated migration occurs (as opposed to repeated trivial flights) is that flights become more frequent when caged crickets are crowded or deprived of food (Walker, unpublished). This translates into their flying from a newly found habitat when it becomes unsuitable. Even when conditions are kept optimal, females will sometimes fly from oviposition buckets (Forrest 1986). Some long-winged Gryllus rubens fly several times during periods of days or weeks, especially when deprived of food and oviposition sites (Walker 1987a). Habitat Selection Whether crickets are dispersing within their home habitat or terminating a long migratory flight, they must decide where to stop. Walking

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27

crickets can conveniently monitor critical features of the habitat (e.g., soil moisture, vegetation, presence of conspecifics) as they travel; and for them, habitat selection should be straightforward (see chapter 5). However, in experiments with Allonemobius fasciatus and A. allardi, two closely related ground crickets that occupy wet and dry grassy areas, respectively, Howard and Harrison (1984a, 1984b) elicited no choice of habitat in A. allardi (the dry-ground species), except that females preferred to lay their eggs in wet soil. A. fasciatus (the wet-ground species) settled in wet areas when exposed to a gradient, but in oviposition tests females laid no more eggs in wet soil than in moist. Neither species was influenced in its choice of habitats by the presence of the other species. Crickets that fly away from their home habitat and over land or water that they cannot colonize must make quick decisions as to whether to descend or keep flying and must do so with very limited information about potential stopping places. The problems that migrating crickets may encounter are well illustrated by swarms of Gryllus bimaculatus landing on ships as far as 900 km off the coast of West Africa (Ragge 1972). Perhaps their flying that far supports the speculation that migrating crickets avoid landing on bodies of water. Two features of habitat selection by migrating crickets are well substantiated: (1) migrating crickets frequently terminate their flights at conspecific sounds (Fig. 1.12) (Campbell and Shipp 1979, Walker 1986b); and (2) at the beginning of their migratory flights they are refractory to cues that later cause them to land (e.g., they will fly directly away from a soundbaited trap that will soon attract and capture hundreds of conspecifics) (T. J. Walker, unpublished). Results of phonotactic tests with tettigoniids (Morris and Fullard 1983) suggest that migrant crickets can find suitable habitat by homing on sounds other than the conspecific calling song. Specifically, flying crickets may detect conspecifics by the song's carrier frequency even though the species-specific amplitude modulation is obscured, and they may land at the songs (or carrier frequencies) of other species that occur in suitable habitat. Landing at any insect sound would, for example, preclude landing on open water. Experimental results, incidental to other studies, support the hypothesis of acoustical habitat selection. Ulagaraj (1974) reported 27 Scapteriscus acletus landing at a continuous tone of 2.7 kHz, while none landed at a silent control (and 270 landed at a call-simulating, modulated 2.7-kHz tone). Sound traps that broadcast the synthetic call of one cricket catch significant numbers of crickets of other species and subfamilies (Mangold 1978, Walker 1987a). For example, a trap broadcasting Gryllus rubens call (4.8 kHz, 50 pulses/s) in a forest captured 113 migrating S. vicinus (3.3 kHz, 130 pulses/s) in 51 days (T. J. Walker, unpublished). In north Florida, species of the following subfamilies land at the calls of species of other subfamilies: Gryllotalpinae, Gryllinae, Nemobiinae, Oecanthinae, Trigonidiinae. Whether woodland species favor woodland

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sounds (other than their own calls) and pasture species favor pasture sounds merits investigation. Acoustic detection of habitat is one of two special functions known for tympanal organs in flying crickets. The other is bat detection (see chapter 11). Since the frequencies of the echo-locating calls of bats are much higher than those of sounds likely to be used in locating suitable habitats, determining the tuning curves (hearing spectra) for the tympanal organs of migrating crickets could refute one function or the other. In the case of Trigonidium cicindeloides it could refute both, since that species has no calling song and develops tympana only when long winged (Fig. 1.10 F-H) (Ingrisch 1977).

Food Selection and Feeding Most, if not all, crickets are omnivorous. This by no means implies that their diets are similar. Indeed, within a single genus, natural diets may vary from nearly 100% herbivorous to largely predaceous (Fig. 1.13). The chief basis for declaring that most crickets are omnivores is that, when captured and deprived of natural foods, they will eat a wide variety of organic materials. For example, species from the eastern United States will generally survive and develop on a diet of ground dry pet food. Walker (1957a) reared tree crickets (Oecanthus nigricornis) from egg to adult by provisioning juveniles solely with ground dog food, lettuce, or aphids, as well as with various combinations of foods. The reared adults were of normal size, except that the ones fed only ground dog food were stunted. The sole exception we know to the rule that caged crickets will eat a variety of foods is a Japanese nemobiine (Parapteronemobius sazanami), which lives on the beaches of Honshu and Kyushu. Furukawa (1970, pp. 65-66) reported that it "denies every food (including dried shrimp) except fresh crab meat." The natural foods of crickets can be learned by watching crickets in nature, by dissecting the alimentary canal of field-collected crickets (Fig. 1.13), by examining feculae, and by examining food caches of burrowing crickets. Laboratory food choice experiments and studies of mandibular morphology are of limited value in predicting natural diets (Gangwere 1961). Many crickets are predominantly herbivores, and their feeding on plants and fruits valued by man is of major economic consequence. Vegetation-inhabiting crickets often eat the blossoms, fruit, and leaves of the plants they occupy. Ground-inhabiting crickets cut and eat small or young plants. Mole crickets feed on grasses and other plants both above and below ground. Relatively few crickets are known to take living prey. When this is recorded, the prey are usually inactive or nearly defenseless (e.g., eggs, pupae, molting insects, scale insects, aphids). The most formidable preda-

29

Natural History vicinus

abbreviatus

n=188

87

actetus

223

Feeding habits of three species of Scapteriscus mole crickets, based on examination of gut contents of field-collected juveniles and adults. (Data from Matheny 1981 and E. L. Matheny, Jr., and J. A. Reinert, unpublished.)

Fig. 1.13

tors among the crickets are certain mole crickets, which, because of their size and subterranean agility, have little trouble in dispatching most of the insects and annelids they encounter as they tunnel through the soil. Oecanthines feed avidly on aphids that are offered them, but examination of crop contents reveals that most individuals of most species feed chiefly on plant tissues and fungal spores (Fulton 1915, Gangwere 1961). Even small crickets can be active predators; for example, Ana;dpha gracilis, a trigonidiine, was noted feeding on blood-engorged sand flies (Christensen and Herrer 1975). Cannibalism is occasionally a problem in laboratory culture of crickets. Those eaten are generally much smaller than the attacker, or they are molting. In nature, cannibalism is probably less frequent because of fewer contacts and more escape routes. The natural diets of common ground-dwelling crickets are poorly known. In analyzing crop contents and fecal materials of Gryllus pennsylvanicus andAllonemobius allardi, Gangwere (1961) found mostly "organic debris" and dicotyledonous leaf materials, lesser amounts of insect remains, and least amounts of spores, pollen, and fragments of grass leaves. Gangwere (1961) summarized other records of natural foods of Gryllus spp., including cow manure, dead vertebrates, mantid eggs, and live termites. Monteith (1971) concluded that G. pennsylvanicus and Allonemobius sp. were major predators of apple maggot pupae and observed the older juveniles and adults detecting and quickly excavating pupae he had buried 1-3 em in the soil. Some plant-dwelling crickets are known from one or a few plant species, but host-specific crickets do not necessarily eat the plant they dwell on. For example, Oecanthus pini in southeastern United States occurs only on pines, and its color and resting behavior is specially adapted to concealment in pine foliage . Captive 0. pini feed little, if at all, on host pine needles, but they feed readily on lettuce, aphids, and ground dog food. The fact that we know of no case in which a host-specific cricket feeds

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primarily on the tissues of its host is not surprising in view of how poorly known are the natural diets of plant-dwelling crickets. Because myrmecophilines live as unwelcomed guests in ant nests, their feeding behavior is of special interest (Wheeler 1900, Henderson and Akre 1986). Although they can smvi.ve on other foods, myrmecophilines use two techniques to feed directly from their hosts. One is by using the mouthparts to strigilate oily secretions from the surface of the legs and body of an ant (Fig. 1.14A). They are allowed to do this apparently because the feeding resembles the grooming behavior of their hosts. Indeed, the cricket usually gains access when the ant is self-grooming or being groomed by another ant. (Myrmecophilines also mouth the surfaces of dead ants and greasy nest walls.) The second technique mimics the mutual feeding behavior of their hosts. The cricket elicits trophallaxis by antennating the ant and, if accepted, by manipulating the ant's mouthparts with its maxillary and labial palps. The ant may then allow the cricket to feed on regurgitated gut contents for a few seconds before aggressively terminating the process (Fig. 1.14 B).

Mating in Call-less Species All crickets reproduce sexually. Facultative or obligatory parthenogenesis, as known in other orthopteroids (Lamb and Willey 1975), is not known for crickets. Most crickets use calling songs in sexual pair formation, as outlined in chapter 2. Here we discuss call-less sexual pair formation. Occurrence of Call-less Species In most cricket species sexual pairs are usually formed by the female homing on a male-produced, species-characteristic calling song. However, in many genera or subfamilies that generally form pairs acoustically, males of one or more species produce no calling song. Call-less species in eastern United States include Gryllus ovisopis (grylline; other Gryllus spp. call), Scapteriscus abbreviatus (gryllotalpine; other Scapteriscus spp. call), Hapithus brevipennis (eneopterine; its sister species, H. melodius, calls), and Tafalisca lurida, Oligacanthopus prograptus, and Falcicula hebardi (eneopterine, mogoplistine, and trigonidiine; these genera have no calling species, but calling species occur in many other genera of these subfamilies). Species that do not call range from having well-developed courtship and aggressive songs (e.g., G. ovisopis) to being totally mute (Otte 1977). Those that are mute may have male forewings with stridulatory files (e.g., H. brevipennis), male forewings that are female-like (T.lurida, Fig. 1.10 L), or no forewings (0. prograptus). Pair Formation without Calling Pairing has been studied in only a few call-less crickets. InAmphiacusta maya (a phalangopsine) the final nymphal instars and, later, the adults

Natural History

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A

B

Two species of Myrmecophilus ingesting materials from ants. A, M. nebrascensis feeding on secretions of a Pogonomyrmel( ant. (From Wheeler 1900.) B, M. manni receiving regurgitated gut contents from a Formica ant. (Drawn from photograph in Henderson and Akre 1986.J

Fig. 1.14

congregate in natural cavities during the day; mating occurs in these diurnal aggregations (Boake 1984a). In Phaeophilacris spectrum (another phalangopsine) adult females aggregate, and a male may join the group and defend it from other males (Dambach and Lichtenstein 1978). In Gryllus ovisopis populations in north Florida most individuals become adult within a few days during mid-September, and females are quickly mated without the aid of airborne pheromones or long-lasting trail pheromones (T. J. Walker, unpublished). In Trigonidium cicindeloides (a trigonidiine) pairs are said to be formed through random contact (Ingrisch 1977). Some vegetation-inhabiting mute crickets, including T. cicindeloides, make substrate vibrations during courtship (Matsuura 1984). Body jerking and tapping the substrate with the abdomen or foretibia are among the techniques used. The same or similar signals could function in pair

T. J. Walker and S. Masaki

32

formation. (For more on vibrational signals, see Bell 1980, Belwood 1988, and chapters 2 and 6, this volume.) What Causes Loss of Calling? Circumstances that correlate with loss of calling song are high densities, sedentary colonies, and synchronous maturation. These facilitate alternative means of pair formation but are not sufficient to explain loss of calling. Calling should be lost when its costs are increased or when its reproductive benefits are reduced relative to alternative means of pair formation. Circumstances that increase the energy costs of calling include small size (small insects are not as efficient at producing sounds that will carry long range as are large insects: Michelsen and Nocke 1974; chapter 8, this volume) and living under or on the ground (sound travels poorly through or along the ground: Markl 1968, Wiley and Richards 1978, Paul and Walker 1979). Risks of calling are increased by acoustically orienting predators and parasites (see below and Belwood 1988). Satellite males (i.e., conspecific acoustic freeloaders) can decrease the benefits of calling relative to other pair forming strategies (Cade 1979). The fact that calling has been lost independently in many phyletic lines suggests that intermediate steps in song loss should be encountered fairly frequently, and they are. Southern populations of Hapithus agitator call, and nmihern ones do not (Alexander and Otte 1967a). Some males of Gryllus integer call for hours each night, and some do not call at all; selection for much calling or no calling quickly produces lines that are significantly different in calling frequency (Cade 1981b). Males of Anurogryllus arboreus call for less than an hour each evening and spend most of the night searching for females, which are in burrows. Most females are found and mated by searching males; mated females generally do not go to calling males (Walker 1983a; and T. J. Walker, unpublished).

Natural Enemies Crickets have many natural enemies. Their life expectancy under field conditions is low compared to their longevity under protected laboratory conditions. For example, male Anurogryllus arboreus held in the laboratory smvived 69 days on average (T. J. Walker, unpublished); under field conditions estimated mean survival was less than 1 week (Walker 1980a). In most circumstances neither food shortages nor cold nor drought are common causes of death in the field. A variety of natural agents are common causes. Pathogens Cricket pathogens have been studied chiefly as causes of mass mortality in laboratory colonies or commercial cricket "farms" and as potential biological control agents of pest species. Only in the latter case does the

Natural History

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mortality occur under natural conditions, ensuring its relevance to natural history. Teleogryllus commodus is a major pest of pastures in New Zealand and Australia. In samples from more than 200 sites in Victoria, Australia, Reinganum et al. (1981) found significant frequencies of two fatal pathogens: 43% of the samples were infected with cricket paralysis virus, and 5% had the fungus Metarhyzium anisopliae. Similarly three introduced species of Scapteriscus mole crickets are important pests of turf and pastures in southeastern United States. In assays of individuals collected in their South American homeland, infection rates for the pathogens Metarhyzium anisopliae, Aspergillus sp., Beauveria bassiana, and Serratia sp. ranged from 1 to 20% (H. G. Fowler, personal communication, 1985). Other mole cricket pathogens, identified from Brazil or Florida, are an iridovirus, a microsporidian (Pleistophora sp.), and two more fungi (Entomophthora sp. and Sorosporella sp.) (Pendland and Boucias 1987, Boucias et al. 1987; D. G. Boucias, personal communication, 1988). Gregarine sporozoans occur in the midguts of some field-collected gryllines and nemobiines (Corbel1964). In Gryllus veletis and G. pennsylvanicus frequency of infection is 30-70%, and pathological effects include slower development and reduced spermatophore production (Zuk 1986). In laboratory colonies or in mass-reared crickets, fatal diseases have been attributed to a rickettsia and three additional viruses (Martoja 1963, Huger 1985). In such circumstances cannibalistic feeding on the dead or dying may unnaturally aid the transmission of pathogens. Parasites Metazoan parasites of crickets include nematodes, mites, and parasitoid wasps and flies. Little is known of the extent of parasitism under natural circumstances, but because crickets .sometimes reach pest proportions when introduced to new geographical areas without their parasites, several parasites of pest species have been introduced in hopes of permanently suppressing pest populations.Other parasites are currently under study.

Nematodes. Nematodes are commonly found within crickets, but in most cases their effects, if any, on the longevity and reproduction of the host are unknown (Webster and Thong 1984). Two groups that have received special attention because they are lethal and have potential as biological control agents are Mermithidae and Steinemematidae (Poinar 1983, Walker 1984). The mermithids are pale, threadlike worms much longer than the host when fully developed. They are most often seen when one emerges from a recently collected cricket. Mermithids invade their definitive hosts in several ways (Poinar 1983), and the life cycles of those occurring in crickets have apparently not been studied. Unlike mermithids, steinemematids do not develop singly in a host, and they kill

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the host quickly though indirectly. They invade the host as infective larvae, release a symbiotic bacterium that kills the host, and undergo one or more generations while feeding on the host cadaver. The end result is thousands, or hundreds of thousands, of bacterium-carrying, infective larvae. Neoaplectana sp., a steinemematid that is a major mortality agent of Scapteriscus mole crickets in Brazil and Uruguay, has recently been established in Florida as a biological control agent (Hudson and Nguyen 1989, Hudson et al. 1988).

Mites. Large red mites are occasionally found on crickets. McGregor (1929) reported that in California's Imperial Valley many crickets (Gryllus sp.) were so weakened from attacks of Euthrombium sp. that they could barely move about. The mites were mostly attached on the dorsum beneath the wings. Egg-parasitic wasps. Two genera of scelionid wasps, Leptoteleia and Oethecoctonus, contain, so far as is known, only species that develop as solitary endoparasites of cricket eggs laid in plant tissues-specifically the eggs of oecanthines and eneopterines (Masner 1978). One or more species in four other families of wasps have been reported to parasitize oecanthine eggs: Mymaridae, Eulophidae, Eupelmidae, Eurytomidae (Thompson 1951, Herting 1971). Ectoparasitic wasps. Larvae of Rhopalosoma spp. (Rhopalosomatidae) and Larra spp. (Larridae) are ectoparasites on adult or large juvenile crickets and kill the host as they complete their development. Little is known of Rhopalosoma spp., except that the larvae develop on eneopterine and nemobiine crickets (Gurney 1953, T. J. Walker, unpublished). Larra spp. develop only on mole crickets and, largely because of their potential as biocontrol agents, are better known (Smith 1935; Castner 1984, 1988). Whereas in most sphecids the female provisions her brood with permanently paralyzed prey, the Larra female only briefly paralyzes the mole cricket that she has chased from its tunnel-just long enough to remove any other Larra egg and lay one of her own. The mole cricket then resumes an active underground existence until2-3 weeks later, when the Larra larva (Fig. 1.15) reaches its final instar and kills and devours its host. Species of Larra have been introduced to Hawaii, Puerto Rico, and Florida in efforts to control introduced mole crickets. Phonotactic flies. Female tachinids of the genus Euphasiopteryx are attracted to certain cricket calls and larviposit in the vicinity. The larvae attach to crickets they contact, burrow in, and develop endoparasitically (Cade 1984, Fowler and Kochalka 1985, Walker 198Gb, Fowler 1987). The host cricket is killed in 6-8 days when the mature larva or larvae emerge to pupate. E. ochracea is an important mortality agent for Gryllus integer and G. rubens in southern United States. E. depleta parasitizes Scapteriscus

Natural History

35

Fig. 1.15 Wasp larva ectoparasitic on a mole cricket. Larra bicolor developing on Scapteriscus vicinus. When the larva (indicated by arrow) reaches about twice this size, it will kill its host and devour the remains before pupating.

mole crickets in South America and has recently been released in Florida for biological control of introduced pest mole crickets (T. J. Walker, unpublished). The ease with which larvipositing females can be attracted to broadcasts of crickets' songs should facilitate further study of these poorly known flies.

Other endoparasitoids. In studies of field-collected grylline crickets, fully developed fly laiVae occasionally emerge from dying adults. These pupate and produce tachinid, sarcophagid, or conopid flies (e.g., E?Coristoides sp., Blaeso((ipha sp., Stylogaster sp.) (Thompson 1951; T. J. Walker, unpublished). Predators Crickets are generally tasty food for vertebrate and invertebrate predators. Their palatability and the ease with which some species can be reared have resulted in crickets being used as food for many zoo and laboratory animals. The extent to which various predators feed on crickets in the wild is poorly known, but the fact that crickets are frequently found in the guts of birds, mammals, reptiles, and amphibia confirms the idea that crickets are at risk from a multitude of vertebrates. They are also palatable to generalist invertebrate predators such as lycosid spiders and mantids. Two subjects of particular interest and relevance to predation on crickets are phonotactic predators and specialist predators.

Phonotactic predators. The calls that make cricket males conspicuous to their females and to naturalists can also reveal them to hungry, insect-eating predators. Use of cricket calling songs to locate prey has

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T. J. Walker and S. Masaki

been experimentally demonstrated in cats, little blue herons, and geckos (Walker 1964a, Bell 1979a, Sakaluk and Belwood 1984) and should be expected whenever the predator hears well at the cricket's carrier frequency. No invertebrate predators are known to hunt crickets by their sounds, but predaceous katydids (e.g., Listroscelinae, Decticinae) seem likely candidates. Walker (1979) used pitfall traps baited with calling crickets to test for previously unrecognized acoustically orienting predators; results were negative. Similarly, Cade and Rice (1980) failed in attempts to attract the cricket-eating toad Bzifo marinus to its prey's call.

Specialist predators. Generalist predators often feed on crickets and probably account for most deaths of crickets in the wild. In addition, a few predators are known or suspected to specialize on crickets. Several groups of sphecid wasps provision their nests mostly or entirely with crickets, including species of Liris (Larrinae), which prey on gryllines (see chapter 7); Isodontia (Sphecinae), which prey on oecanthines; and Chlorion (Specinae), which prey on gryllines (some species are Larra-like parasitoids of Brachytrupes spp.) (Bohart and Menke 1976, Steiner 1976). The larvae of certain bombardier beetles are specialized predators of the eggs of mole crickets. In Japan Stenaptinus jessoensis larvae attack the eggs of Gryllotalpa africana and undergo hypermetamorphosis within the egg chamber (Habu and Sadanaga 1965, 1969). In the New World Pheropsophus aequinoctialis larvae feed on eggs of Scapteriscus spp. (Hudson et al. 1988).

Defensive Strategies Crickets have evolved a variety of ways to thwart their enemies. We categorize their defenses as hiding, fleeing, attacking, and threatening. Hiding When crickets are not seeking food, mates, or oviposition sites, they generally conceal themselves. Many species hide in naturally occurring crevices, but others construct their own hiding places. Vegetation-inhabiting species often hide in tree holes or leaf curls or squeeze beneath loose bark. A few can tunnel in soft wood. Caged individuals of Tafalisca lurida, a New World eneopterine usually found on red mangrove, tunneled into the cork stoppers of their water vials and completed their concealment by folding their long antennae within their retreat (T. J. Walker, unpublished). Mjobergella warra, an Australian grylline, evidently makes blind tunnels in the wood of downed trees (Otte and Alexander 1983). Ground-dwelling species often hide beneath debris, stones, and fallen logs. In areas where the soil shrinks during dry spells, the resulting cracks provide a haven and access to moist soil.

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37

Burrowing. Many ground-dwelling species improve their places of concealment by excavation. For example, most gryllines (especially the more robust ones) v.rill, under appropriate circumstances, either enhance natural shelters-by enlarging soil crevices or spaces beneath rocks or logs-or dig a simple burrow. Unbranched, blind borrows without noteworthy chambers are often occupied for short periods and generally seJVe only for shelter. For example, wandering males of Anurogryllus arboreus dig simple burrows, hide in them during the day, and abandon them to call and search for females the next evening !Walker 1983a). On the other hand, simple burrows are sometimes a site for mating and oviposition (e.g., Gryllus campestris: Turcek 1967; Teleogryllus commodus: Evans 1983). Mole crickets and some gryllines, including species of Brachytrupes, Anurogryllus, Apterogryllus, and Cephalogryllus, construct elaborate burrows and occupy them for long periods (Chopard 1938, Bilttiker and Bilnzli 1958, Bell 1979b, Otte and Alexander 1983, Walker 1983a). All mole crickets spend most of their lives underground in tunnel systems they construct with powerful forelegs specially modified for digging. Temporary tunnels, probably used in foraging, are made just beneath the surface and are apparent as trails of upthrust soil. Deeper, more lasting tunnels and chambers are made by compacting the soil with the pronotum and pushing soil to the surface or into old tunnels. Mole cricket burrows are extensive and frequently modified. Few have been described in detail, but the acoustic burrows of males (see Chapter 8) and the egg chambers of females (Chopard 1938) are evidence of their subterranean construction skills. The powerful fossorial forelegs of mole crickets enable them to tunnel out of sight in a few seconds when placed on loose soil. Gryllines use a different, slower burrowing technique. The mandibles loosen the soil, which is then carried in the mouth parts, kicked rea1ward by the legs, or bulldozed with the head IBilttiker and Bilnzli 1958, Alexander 1961). Juveniles of Brachytrupes membranaceus occupy pcnnanent burrows that they enlarge as they develop. The burrow of the adult is 50-80 em deep, has a chamber where food is stored, and boasts an emergency exit as well as a main entrance (Buttiker and Bunzli 1958). Mating occurs within the male's burrow, and females may remain in the male's burrow in sand-plugged side tunnels (Costa and Petralia 1984). Anurogryllus arboreus also has permanent burrows, which include a deep tunnel for defecation and one or two chambers for living and food storage. Mating usually occurs in the male's or female's burrow; in either case the male leaves, and the female uses the burrow for oviposition and brood rearing (Walker 1983a; T. J. Walker, unpublished). If the male does not attract a female to his home burrow, he abandons it anyway after a few days and begins a wandering existence.) Camouflage. Crickets that fail to find (or build) dark retreats benefit from colors and patterns that make them difficult for visually hunting

38

T. J. Walker and S. Masaki

predators to detect. Host-specific oecanthines match their host's colors and may have pattems that aid concealment. For example, Oecanthus pini has a brown head and pronotum and pine-green tegmina. At rest the cricket keeps its head and pronotum next to a branchlet amid the brown needle-bundle sheaves and holds the rest of its body parallel to and among the green needles (Walker 1963; T. J. Walker, unpublished). Except for oecanthines, most vegetation-dwelling crickets are brown or gray rather than green, supporting the notion that most hide in crevices or against dark backgrounds during the day. The occurrence of green trigonidiines and eneopterines (e.g., Cyrto;dpha spp. and Orocharis vagina/is) suggests that the rarity of green is a matter of selection rather than phylogenetic restraints. Ground-inhabiting crickets sometimes match the color of their substrate. A noteworthy case is Dianemobius csikii, an inhabitant .of sandy beaches. Other Dianemobius species are mostly brown to black, but D. csikii has dark specks on a light yellow, light brown, or even whitish background. To some extent it matches local variation in the color of the beach sand. For instance, crickets on Shimokita beach, where the sand is usually dark, are dark, whereas those from Tokunoshima Island beach, where the sand is much lighter, are pale. Members of each population are difficult to spot on their own sand and more easily seen on the sand of the other population (S. Masaki, unpublished). The hatchlings of two woodland Gryllus spp. have a bright yellow thorax that sharply contrasts with the black head and abdomen (T. J. Walker, unpublished). The resulting spots of black and yellow may make it difficult for a predator to recognize them as edible. (Cott [1957] termed this phenomenon "disruptive coloration.")

Freezing. When some mole crickets (e.g., Scapteriscus vicinus) are excavated, they remain motionless for several seconds. Until they start to run or dig, they are difficult to find. Males of A. arboreus often climb perches at sunset and begin to call. When a caller is approached before darkness, he leaps from his station and freezes wherever he landseliminating motion and sound as cues to his whereabouts (T. J. Walker, unpublished). Ventriloquism and silence. Calling males are acoustically conspicuous to most vertebrate predators and to some parasitoids. Anyone who tries to catch calling crickets by homing on their calls soon leams that some calls are notably difficult to localize and that crickets usually cease calling when they are approached. Neither ventriloquism nor silence has been carefully studied in crickets, but larger crickets calling from more exposed places are the ones that quit calling soonest as they are approached; and some large, exposed callers produce brief, irregularly repeated chirps that are more difficult to localize acoustically than regularly repeated chirps or continuous trills. A study of what artificial cricket songs are most attractive to phonotactic

Natural Histocy

39

parasitoids such as Euphasiopteryl(. ochracea should give clues as to the auditocy capabilities of the flies and to acoustic avoidance strategies of host crickets. Fleeing Most crickets are quick and agile. When a predator tries to capture a cricket, the cricket may dash into a retreat, such as a burrow or soil crack, or concealment, such as beneath leaf litter or to the other side of a stem or leaf. It may leap once or twice and freeze or conceal itself, or it may leap and fly like a grasshopper, or it may continue to distance itself from danger by running along a branch or stem or through grass clumps. Males of the riparian nemobiine Thetella oonoomba, when disturbed at night, leap into the water and swim straight away from the bank (Otte and Alexander 1983). A lakeside nemobiine, Pteronemobius lineolatus, leaps onto the water surface in daytime when disturbed; as it swims back to shore it can orient by celestial cues (Beugnon 1986). Myrmecophilines survive as unwelcome guests within ant colonies by dodging and darting among their hosts. Rarely is one captured and killed (Wheeler 1900, Henderson andAkre 1986). Even if a cricket is seized by a predator, it may succeed in fleeing by breaking loose from one or both hindlegs or its hindwings. Attacking Most crickets have no means of fighting back when threatened. However, the larger more robust species can kick viciously with spine-lined, spur-tipped tibiae and deliver a painful bite. When attacked by Liris niger, Acheta domesticus assumes characteristic postures that may make it harder for the wasp to sting the cricket and/or make it easier for the cricket to kick effectively (see chapter 7, Fig. 7.6 B, C). A few species have chemical weapons. Most notable are some mole crickets that have anal glands from which a sticky material is ejected. Neocurtilla hexadactyla, attacked by Larra spp., often entangle their attackers with the secretion and escape; Gryllotalpa oya can eject a mucilaginous liquid from the glands to a distance of at least 23 em (Baumgartner 1910, Tindale 1928, Castner 1984). The anal glands of Scapteriscus mole crickets secrete a smelly, nonsticky fluid that may deter attack (W. G. Hudson, personal communication, 1982). When an individual of A. arboreus is chased into a blind tunnel, it will often exude a drop of anal fluid and hold it between itself and the potential attacker. Threatening Males of Brachytrupes membranaceus, a giant among crickets, will stridulate when cornered and, if pressed further, will attack and bite vi-· ciously (D. Otte, personal communication, 1986). Similar warning sounds are made by some large tettigoniids (Sandow and Bailey 1978), but smaller crickets and katydids do not stridulate defensively or counterattack. Threats can be false. The trigonidiine Phyllopalpus pulchellus may be an

40

T. J. Walker and S. Masaki

example of a false chemical threat. Its metallic blue body and reddish head and thorax make it resemble a bombardier beetle, which delivers a powerful blast of benzoquinones when attacked. Cultural and Economic Roles

In Western folklore, a cricket in the house is an omen of good or bad luck, but in Eastern cultures crickets have had a much more prominent role. In China and Japan crickets are actively sought and brought into the home as music-making pets and, in China, as pugilists. Crickets became an important part of Chinese culture more than 1,000 years ago. During the T'ang Dynasty (A.D. 618-906) members ofthe imperial court kept crickets in cages so that their songs might be more conveniently enjoyed, and the custom gradually spread throughout the country. Cages ranged from delicately carved art objects to simple bamboo containers. Later, during the Sung Dynasty IA.D. 960-1279) the sport of cricket fighting was established, and it and the keeping of caged crickets for their songs remained a noteworthy part of Chinese culture into the twentieth centmy. The importance of cricket fighting is attested by the many books that describe the methods of collecting, rearing, and fighting crickets. Even though more than 60 varieties of fighters are recognized in Chinese cricket manuals, all belong to four species (Velarifictorus aspersus, Teleogryllus testaceus, T. mitratus, and Gryllus bimaculatus). Fighting crickets were reared, collected, or bought, kept in special pottery cricket houses, and fed special diets (e.g., rice mixed with fresh cucumbers, boiled chestnuts, lotus seeds, and mosquitoes). For a fight, two male crickets of approximately equal weight were placed in a large clay bowl and excited by touching their antennae and cerci with mouse whisker hairs affixed to one end of a small stick. The outcome of a match usually became clear after a few minutes of fierce fighting, when one of the combatants was disabled or killed. Gambling was an important part of cricket fighting, and bets on a single match sometimes were equivalent to thousands of dollars. It was thought that cricket champions were the incarnation of great warriors of the past, and they were treated with great respect -sometimes including solemn burial in a small silver coffin (Anonymous 1928, Hsu 1929, Yasumatsu 1965). Cricket fighting is apparently in eclipse in the Peoples' Republic of China, but it is still popular in Hong Kong, where the best fighters are imported from the mainland (Kevan and Hsiung 1976). What makes a winner in a cricket fight has been investigated by Alexander (1961), Burk (1983), Simmons (1986a), and Dixon and Cade (1986). In Japan, as in China, the appreciation of cricket music can be traced back more than 1,000 years. For example, the two species most esteemed for their songs, the matsumushi (Xenogryllus marmoratus) and the suzumushi (Homoeogryllus japonicus), are in poems of the Engi period (A.D. 901-922). Crickets were caged for their songs as early as A.D. 1095 (Hearn

Natural History

41

1898). When the 17-syllable haiku form of poem became popular (late Muromachi period, A.D. 1336-1573), the concept of seasonal words emerged. By one of these words, seasonality is given to each haiku. Words relating Lo crickets and katydids are regarded as autumn words and constitute nearly half of about 70 autumn words concemed with insects. Mushi is the Japanese word for insect in general, but in poems it means insects singing in autumn-in most if not all cases, crickets. The keeping of caged crickets for their songs led to the trade of insect seller (mushiya) by the seventeenth century and to thriving businesses in nineteenth-centmy Japan (Hearn 1898). The market has waned, but there is still at least one professional grower of singing crickets; and the suzumushi and several other species of singing insects are still sold in department stores in Tokyo, along with hom and stag beetles for children (M. Konishi, personal communication, 1986). In the Suzumushi Temple in Kyoto thousands of suzumushi are reared in front of the altar. However, this seems to have no religious meaning and is just a tourist attraction. Commercial trade in crickets is currently thriving in the United States. These crickets, alas, are not raised as songsters but as fish bait and as food for laboratory and zoo animals. The species reared is invariably Acheta domesticus, an Old World species that was Milton's "cricket on the hearth." Although cricket growers are secretive about the magnitude of their business, sales are probably about 53 million annually. Purina Company, the producer of Cricket Chow, reported selling 445 metric tons to growers in one state (Louisiana) in 1985. Dried, chopped crickets are sold as medicine in China.Xi-shuai, used as a diuretic, incorporates four species of gryllines (Inagaki et al. 1984). In many countries crickets are used as food for humans. Those most often eaten are Brachytrupes sp. and mole crickets. In Taiwan, Thailand, and Burma, Brachytrupes sp., either raw or fried, are sold in food markets (Sonan 1931; Taylor 1975; I. Matsuura, personal communication, 1986). A few cricket species are major agricultural pests. In southeastern United States, introduced Scapteriscus mole crickets, principally S. vicinus, cause an estimated $35 million damage annually, mainly to turf and pasture (Southem 1982). In New Zealand, Teleogryllus commodus reaches numbers that consume as much pasture as 21 ewes per hectare (Blank and Olson 1981). In 1983, to control T. commodus in Nm1hland, New Zealand, farmers applied 950 metric tons of bait (Blank 1984). Other species sometimes are damaging, but the effects are usually more sporadic. For example, the eneopterine Hapithus agitator scars oranges in Florida (Bullock 1973), Gryllus spp. eats alfalfa pods in South Dakota (Walstrom 1971), Teleogryllus emma harms forage crops in Honshu (Kobayashi and Oku 1973), tree crickets damage raspberries by eating the flowers and young peach trees by ovipositing (Smith 1930, Elliott and Dhanvantari 1973), and Anurogryllus arboreus destroys pine seeds and pine seedlings in Louisiana (Campbell 1971). Brachytrupes spp. are pests of crops in Africa and Asia (Bi.i.ttiker and Bi.i.nzli 1958, Szent-Ivany 1958). Crickets are sometimes pests merely because of their numbers. Flights

T. J. Walker and S. Masaki of a Gryllus sp. in southern California have been so large as to clog the cooling system of large compressors in a natural gas pumping station (Caruba 1980). So many Scapteriscus sp. mole crickets flew one evening at Disney World in Florida that a portion of the tourist attraction had to be closed (1. Hagedorn, personal communication, 1980). Finally, it should be mentioned that crickets are valued for their songs in at least one respect in this age of television. Producers sometimes signal that the action in a show is taking place outside on a summer's evening by dubbing in the songs of crickets. They often use the song of Oecanthus JUltoni, a sound Nathaniel Hawthorne described as "audible stillness" (1846) and declared, "If moonlight could be heard, it would sound just like that" (1851). Conclusions

Because of their musical songs, crickets are among the most appealing and accessible of insects. Nonetheless, only a small number of species in very few parts of the world have been carefully studied, and most of these species belong to one subfamily (Gryllinae) and live at temperate latitudes, where most scientists studying them also live. Species that are small, tropical, or vegetation-dwelling have generally been neglected. The most serious limitation to really understanding crickets stems from the fact that nearly nothing is known about most aspects of the natural history, behavior, or neurobiology of the majority of cricket taxa. Acknowledgments

T . .J. Walker thanks R. D. Alexander for introducing him to cricketsi J.D. Spooner, R. E. Love., J. J. Whitesell, D. L. Mays, J. B. Walker, and J. E. Uoyd for sharing the joys and hardships of fieldworki J. J. Owen for finding the Hawthorne passagesi and B. A. Hollien, William Cade, Werner Loher, Dan Otte, and Scott Sakaluk for their help with this manuscript. Some of the research reported here was supported by NSF grant BNS 81-03554. S. Masaki thanks Ichiro Matsuura for continually giving him invaluable biological information on crickets, and Masayasu Konishi for information on past and present insect sellers in Tokyo. Drawings not credited in their legends were made by S. A. Wineriter, except 1.3 K, by Kathy Bates, 1.3 Land 1.10 G, by Harry McVay, and 1.8.

CHAPTER TWO

Reproductive Behavior Werner Loher and Martin Dambach

The acoustic behavior of most species of crickets is intimately connected with reproduction. Since cricket songs are direct expressions of a sophisticated sexual behavior and models of animal communication, physiological research in the past has primarily concentrated on the mechanism of acoustic behavior, such as sound production, sound reception, and signal processing for phonotaxis and avoidance (see chapters 811, 13, 14). By comparison, other modes of communication involving tactile, vibratory, chemical, and visual stimuli have received only limited attention in crickets (see chapters 5-7). Moreover, the neurobiological approach to acoustic behavior has been used mostly on single, often restrained insects in a standardized laboratory environment where the complexity and variability of the natural habitat and its influence on intact insects and populations was not considered. On the other hand, behavioral ecologists follow a different pathway, one that is directed toward the evolution of reproductive behavior. They are interested in how sexual selection has shaped behavioral strategies, in terms of competition and mate choice, which finally result in reproductive success (Blum and Blum 1979, Gwynne and Morris 1983, Thornhill and Alcock 1983). Behavioral ecologists recognize the variability of environmental conditions and the effect of this on reproduction and survival. However, they could profit from knowing the current physiological state of the insects and devise verifiable experiments to test their predictions. In short, scientists from both disciplines should consult with one another because the two approaches are complementary (Huber 1985). Here we look at reproductive behavior from both perspectives. We show acoustic behavior to be the prime mode of information transfer, but point out other, sometimes multisensory and multimodal forms of communication with equally adaptive value to the individual. Together they lead to alternative mating strategies accompanied by environmental changes and 43

W. Lober and M. Dambach

44

subsequent genetic fixation, documented by a great variety of behavior patterns displayed at various stages of reproductive life. Reproductive behavior consists of a series of repetitive sequences, which we treat in chronological order as follows: pair formation; sexual recognition, courtship, aggression; mate choice; mating; postmating behavior; and oviposition (Fig. 2.1).

Pair Formation Tactics of Female Attraction by Calling Males Competition among males for females to achieve mating is one of the primary evolutionary forces in sexual selection (Darwin 1871). In crickets, the most common strategy employed by males to attract females for mating is to emit a calling song, a function discovered by Regen (1913) in the field cricket Gryllus campestris. For stridulation, rubbing the tegmina together produces a pulsed sound with a characteristic carrier frequency and species-specific temporal patterning (see chapter 8). Conspecific male crickets have evolved a variety of strategies to create broadcasting conditions favorable for attracting females. Grylline crickets owning a burrow call while standing in front of it facing the entrance. Crickets occupying a crevice sing close to the surface to prevent sound attenuation. The loudness of the call depends on the resonance properties of parts of the elytra (Nacke 1971; chapter 8, this volume), on the angle ofthe raised forewings during stridulation, and on sensory structures Isee chapter 9). For the calling song of Teleogryllus commodus the tegmina are raised at a 65° angle above the abdomen, whereas the soft courtship song is produced with the forewings hardly lifted, and the aggressive song requires an intermediate tegmina! position (Loher and Renee 1978) (Fig. 2.1). Under laboratory conditions, T. commodus males may remain stationary for hours during emission of the calling song, a fact that should allow females to track the sound source with some precision. Males ofthe short-tailed cricketAnurogryllus mulicus call from or near the burrow entrance in a thumbprintlike depression that amplifies and directs the sounds upward toward flying females (Walker and Whitesell 1982). Other A. muticus males fly into the area, choose a spot bare of vegetation, and periodically tum in full circles about the vertical body axis while emitting their continuous trills, thereby radiating the song in all directions (W. Loher, unpublished). Still other males of the species, singing in open fields, change their location repeatedly and thus increase their advertising range. Although different males emit the calling song cumulatively throughout the night, which coincides with the females' nocturnal flight activity, their individual stridulatory periods last no longer than 2-3 h and they are repeated each night during the same time (Walker and Whitesell 1982; chapter 3, this volume). Time-dependent singing is also common in Gryllus campestris, where males are either day-

Reproductive Behavior

45

f---------j

0.5 sec.

Te/eogryllus com modus

MATING

Fig. 2.1

Singing, reproductive, and aggressive behaviors in Teleogryllus commodus. Calling of male attracts both sexes. a, Sex identification by antenna! touch

from a female elicits courtship and courtship songs, followed by copulation and transfer of a spermatophore; upon separation, the male guards the female; after having produced a new spermatophore and later regaining sexual readiness, the male courts again and another mating follows; alternatively, the female may lay eggs. b, Sex identification by antennal touch from a male leads to aggressive chirping, termination of the encounter, or fighting. Note that during fighting both males hold their tegmina slightly higher than usual for other aggression, which amplifies sounds produced, thereby introducing bluff between fighting bouts.

or night-active or both. The temporal division of stridulatory activity across the 24-h day seems to enhance mating chances of males that sing during both day and night (Honegger 1981a). However, in field studies on the same species low nightly temperatures prevented female locomotion and attraction to calling males, and copulations were reported only during daytime (Rost and Honegger 1987; chapter 3, this volume). Another short -tailed cricket,Anurogryllus arboreus, greatly improves its broadcasting range by climbing about 1m up a tree trunk. Calls from such a height radiate sound energy 14 times better than those from the ground (Paul and Walker 1979). Males sing for only about an hour after sunset,

W. Loher and M. Dambach then climb down from their perches and search for female burrows or dig their own (Walker 1980b, 1983a). Yet a further way to enlarge the broadcasting radius is to use the substrate itself for sound amplification, as do mole crickets. Scapteriscus acletus and S. vicinus males dig burrows with hom-shaped entrance tunnels (Nickerson et al. 1979). The horn serves as an acoustic guide, amplll}ring the sound. Gryllotalpa vineae and G. gryllotalpa males construct two entrance side-by-side tunnels with acoustic guide characteristics. The resulting call of G. gryllotalpa is so loud that it can be heard for about 600 m (Bennet-Clark 1970b, 1975; chapter 8, this volume). Several tree cricket species enhance sound radiation by constructing a sound guide or baffle from the plant on which they stand. The Oecanthus burmeisteri male chews a pear-shaped hole in the center of a leaf and during stridulation presses the forewings against its edges. A 2.5- to 3.5fold amplification of the calling song results (Prozesky-Schulze et al. 1975). An interesting male strategy has been described for the field cricket Gryllus integer (Cade 1975, 1979, 1980). Nocturnally singing males are surrounded by loose aggregations of females and silent males, no doubt attracted to the caller. These satellite noncalling males intercept approaching females, court them, and try to mate. As many as 12 noncallers have been found within a 0.5-m radius of a singing male. At dawn the number of calling males increases dramatically, and many previously silent crickets join in. But even then, some satellite males do not call. During this rise in acoustic activity calling songs become attenuated, and their function may change from long-distance attraction to stimulation of female receptivity, although this has not been documented. Besides, due to the foregoing nightly singing activity of a caller, males and females already are close by. There is circumstantial evidence that calling males mate more often than noncalling ones. However, that presumed benefit of calling is offset by parasitoid flies, Euphasioptel)'l( ochracea, which are attracted by the song. The female flies deposit their larvae on and around the host; the larvae penetrate the host cuticle and can consume a cricket within 7 d. Since satellite males become parasitized less often than singing crickets, parasitoid flies constitute a selective force against daylight calling behavior. Indeed, calling duration in G. integer males is reduced in comparison with that of other species not parasitized (Cade and Wyatt 1984). Laboratory experiments in which calling and noncalling crickets were separated and bred for several generations have demonstrated that calling and satellite behaviors are genetically distinct, though their expression depends on a variety of factors, such as population density, the time of day the behaviors are performed, aggression, and the presence of natural enemies (Cade 1981a). The satellite male phenomenon is not a special adaptation of Gryllus integer but also seems to be present in other species such as Teleogryllus commodus (Evans 1983), T. oceanicus (L. Orsak, unpublished), and Gryllodes supplicans (Sakaluk 1987).

Reproductive Behavior

47

Aggregation of both sex.es. Calling male crickets attract not only females but also males, thus increasing competition for mates. When T. commodus is in a migratory mood, both sexes fly to habitats where conspecific crickets call, a phenomenon that translates into good feeding and burrowing grounds and availability of the opposite sex. This finding has been confirmed by using loudspeakers as decoys. On arrival, females are attracted to the sound source by phonotaxis, whereas males silently segregate and usually move from the nearest singing cricket or loudspeaker toward more distant ones. The nearest-neighbor distances in low- and high-density populations were, on average, 8 m and 1.5 m, respectively (Campbell and Shipp 1979). Spacing may increase when the males make contact, causing aggressive chirping and fights (Fig. 2.1). Attraction to singing males has also been documented in mole crickets, and during a 1-h period after sunset both sexes fly into an area where males call from burrows (Ulagaraj and Walker 1973). Cricket species not normally known to migrate, such as G. integer, G. veletis, and T. oceanicus, aggregate phonotactically by walking or flying toward sound sources; they sing at intermale distances similar to those ofT. commodus (Cade 1981b). Such aggregations of chorusing males then resemble leks, offering increased mating opportunity due to the influx of attracted females (Alexander 1975). However, it has not been shown that a cricket singing in a congregation attracts, on average, more females and mates more often than a solitary caller. Limits of acoustic communication efficiency. Low-frequency sound emission in small crickets is inefficient because of the disproportionate ratio between the size of the sound-producing wings and the sound wavelength (Michelsen and Nocke 1974; chapter 8, this volume). A second problem is the cricket's immense energy consumption during sound production, which in A. muticus is 25 times higher than during silence (Prestwich and Walker 1981). Not surprisingly, such impediments have led to alternative mating strategies, particularly in small insects such as the nemobiine species Bobilla victoriae, which is 1 em long and lives in grassy areas. Males gather in groups and are very mobile. They do not possess territories and, instead, search the terrain for females; by calling intermittently from various sites (A. R. Evans, unpublished), they enlarge their advertising range. Parasitoid flies are likely to inhibit daytime acoustic signaling (Cade 1975). Mammals (Walker 1964a) and birds (Browning 1954, Bell 1979a) reportedly find and capture singing crickets. Geckos waylay cricket females attracted by a male's song (Sakaluk and Belwood 1984). Silence not only protects from predatory attack but also saves noncallers much energy. Notwithstanding such advantages, silent males do not attract females and presumably suffer a reduced number of mating opportunities. Thus, evolutionary pressure appears to favor new modes of communication and living habits, depending on environmental conditions, which

48

W. Loher and M. Dambach

make sound production and reception partly or entirely obsolete. Gryllus ovisopis, for instance, has lost the ability to call but is still capable of courtship and aggressive chirps (Walker 1974). More likely reasons for no longer emitting the calling song are the sedentary way of life of that micropterous species, its nonfluctuating habitat, and the high population density. Chance encounters and short-range communication by antenna! contact and olfaction evidently suffice to bring about mating. Another reason why the calling song might have been secondarily lost is the chance that the original calling song of G. ovisopis might have been confusingly similar to that of a related species Gryllus fi.Iltoni, which is present when G. ovisopis adults first appear seasonally. Yet another species from Florida, Hapithus brevipennis, is completely mute, and some of the explanations given for song loss in G. ovisopis might also explain the total loss of acoustic behavior in H. brevipennis (Walker 1977). A more extensive discussion of call-less males is presented in chapter 1. Chemical Cues and Substrate Vibration That olfaction can indeed seiVe as a supplementary communication mode in pair formation has been shown in four species of Nemobiinae. These crickets live in soggy, decayed wood, in tussocks of marshes (Thomas and Alexander 1957), and in leaf litter (Paul 1976b). The males retain their full acoustic repertory, but females produce a scent. When males are exposed to paper impregnated with this scent, rapid antenna! touching and palpation as well as calling and searching are elicited. Maleimpregnated paper has no effect on males (Paul 1976a). Some Australian crickets living in deep caves (Apterogryllus, Apteronemobius) have lost their acoustic signaling ability altogether, and no longer possess organs for sound production and reception. In particular, Apteronemobius darwini, which live together in colonies, have evidently developed other communication modes using olfaction or contact chemoreception, but no proof is yet available (Otte and Alexander 1983). Other Australian species of the subfamily Trigonidiinae have changed from a nocturnal to a diurnal way oflife. They have large compound eyes, and their stridulatory apparatus has partly or completely disappeared (Otte and Alexander 1983). For instance, males of the trigonidiine Balamara gydia, a species that lives in high density on reeds and grasses in swampy areas, are very mobile and spend much of their time searching for females (A. R. Evans, unpublished).Although the male still has a complete stridulatory apparatus, no calling song is emitted. Both sexes have lost the tympanal organs; substrate vibration appears to have replaced acoustic signaling. These crickets produce percussion signals by tapping the reed they sit on, either with their abdomen (sternal taps) or with the maxillaty palps (palpal taps). Shaking the whole body causes other vibratory signals. A male attracted visually to a female jumps on the same reed and performs sternal taps. When the female is receptive, she replies in kind. It is uncertain whether olfaction is involved in bringing the sexes together (A.

Reproductive Behavior

49

R. Evans, unpublished). Such adaptations to particular habitats and the

consequences of small size have led to more efficient mating systems, which seems to confirm the hypothesis that the evolution of new signals may cause the deterioration of previous ones (Otte 1977).

Sexual Recognition, Courtship, and Aggression In acoustic insects, competition enters a new stage after a female is close to a male. Competition may be intensified, because calling also attracts other males to the same location (Campbell and Shipp 1979). Subsequent behaviors are then determined by the sex of the cricket a male encounters. However, behavior during the initial moments is remarkably similar in either case: they touch one another, primarily with their antennae. Depending on the sexes, the ensuing behavior then ranges from courtship to indifference to aggression (Fig. 2.1). Chemotactile Signals Sex recognition by contact chemoreception was investigated in Teleogryllus commodus (Renee and Loher 1977) and is briefly reviewed here. Surgically blinded and deafened males were able to distinguish between female and male antennae when touched with such antennae mounted on a stick. The behavior that followed such contact was categorized arbitrarily into a hierarchical continuum from high sexual receptivity, as demonstrated by the courtship song (+9), to fierce aggressiveness, indicated by aggressive sounds (-8), through no response or indifference (0) (Fig. 2.2 a, b). The. intermediate steps between these extremes were milder forms of these behaviors, such as antenna! vibration following the stimulus, or turning away 180° from the stimulus in preparation for mating. Crickets touched with a male antenna ran away from the stimulus, rocked, or bit at the antenna. Whether the extreme behaviors were exhibited depended on the age of the test male, the age of the antenna donor, and on the prior treatment of the antenna (see below). Eighty percent of males up to 5 d old responded to male and female antennae from mature donors with aggressive soundsi the remaining 20% ran away from the stimulus. Following onset of sexual maturation on day 6, as demonstrated by the possession of a spermatophore, 80-90% responded to female antennae with courtship (Fig. 2.2 a), and 95-100% responded to male antennae with aggression (Fig. 2.2 b). Mature recipients reacted to antenna} stroking with extreme behaviors only when the antennae were derived from sexually mature crickets. After test antennae were soaked in chloroform and air dried, they could no longer elicit the reactions described above. Test males would either avoid them or remain indifferent, as they would with a human hair, a bristle, or a fine thread. Since the solvent apparently rendered the antennae inactive by removing the wax layer, separate male and female sex recognition pheromones are sug-

W. Loher and M. Dambach

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gested. Other parts of the body, such as the flight wings, can elicit the behavioral reactions, too. It is assumed that the pheromones are present in the wax layer all over the body, but neither the production site nor the structure of these compounds is known. In electroantennogram studies exposure to male and female odor or to clean air did not change the spike frequency rate of the antenna! neurons. That result is an argument against the volatile nature of the sex recognition pheromones and in favor of chemotactile substances (Renee and Lober 1977). In Gryllus campestris, sex is revealed partly by the reaction of the partner. Males sing during the day and, upon seeing a cricket approach the burrow, emit a warning chirp. An incoming male either flees or lunges forward and attacks, whereas a female behaves passively, enduring the male's antenna! inspection until he proceeds to courtship (Huber 1955). A pheromone is probably perceived by a G. bimaculatus male during palpation of the female's body, which then induces courtship, whereas the absence of that substance when palpating a male leads to antenna] fencing and fighting (Hormann-Heck 1957). Sex recognition by odors at close range has been demonstrated in Acheta domesticus, Gryllus integer, and Gryllus sp. (Otte and Cade 1976). In choice experiments, males preferred a chamber containing female odor to one without it, and females selected a chamber with male odor. Conspecific odors were clearly preferred to heterospecific ones. Earlier experiments on A. domesticus, in which wooden blocks were placed among large groups of crickets in a cage, indicated that these blocks were impregnated with two substances: an attractant, possibly a contact pheromone, and a volatile repellant. Both chemicals might regulate the local distribution of these crickets when in close quarters (Sexton and Hess 1968). Close-range chemoreception may help G. integer satellite males locate and intercept approaching females (Cade 1979). Multisensory and Multimodal Signals Crickets employ a multimodal signaling system for courtship. In the tree crickets Oecanthus nigricornis and 0. foltoni, females are first attracted from a distance by the calling song. The male presumably becomes aware of her by antenna! contact, which stimulates courtship.

Fig. 2.2 Three-dimensional plots of reactions of14-d-old Teleogryllus commodus males to being touched with stick-mounted antennae from females or males from last larval ins tar to adults 1-14 d old. a, Males responding to touch with antennae from females. b, Males responding to touch with antennae from males. X axis, age of animals providing antennae. Y axis, number of males responding by behavioral state of response. Z axis, behavioral response states: 0, indifference; -8, fierce aggression with aggressive sounds produced, +9, sexual receptivity with courtship sounds produced, others intermediate. Fifty males tested for antennae of each age. (After Renee and Loher 1977, courtesy of Physiological Entomology.)

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Courtship singing not only invites a female to mount him but also may be attractive to nearby females. In T-maze studies females of both species usually made the correct turn at the junction when exposed to taped conspccific courtship songs or to scent emanating from two males. Another component of courtship is substrate vibration, consisting of tremulation and percussions of the male's body against the plant on which both sexes are sitting (Bell1980). Male scent could arise from the metanatal glands at the tegminal base, which empty their secretions into a cavity from which a mounted female feeds. When the cavity was closed with wax, the male's ability to attract females in 0. nigricornis was reduced but not abolished, and Bell (1980) believes that the locus of scent production is elsewhere on the body. Less ambiguous is the case of the European species 0. pellucens, in which tufts of glandular hairs arising from the metathoracic cavity have been described (Hohorst 1937); a recent scanning electron microscopy study confirmed their structure (Fig. 2.3l. The exits of the three pairs of metanotal glands are located on the bottom of the cavity, and the hairs are thought to be the source of the male scent. Males with the forewings cut and an intact glandular cavity were still able to attract females from distances of 5-15 em, but with their cavity washed out or taped over they failed to do so (Hohorst 1937). The trigonidiine species Balamara gidya exhibits multisensory courtship behavior, including sternal and palpal taps as well as body shaking, all of which generate longitudinal and transversal waves in the substrate. The male even stridulates with his forewings, but the tibial tympanal organs in both sexes are not developed, and the resulting energy is most likely absorbed by the substrate and perceived by the subgenual organs. These behaviors are accompanied by antenna! touching, during which the males court the mostly unreceptive females. After lengthy courtship sessions, previously unresponsive females may become receptive, and their willingness to mate is then expressed in a series ofpalpal taps. Malemale confrontations are rare, but males become aggressive toward other males in the presence of a female and drum a series of sternal taps, should one male challenge another during the latter's courtship. Tapping also occurs when a female moves away during courtship. In that case, male taps render her immobile (A. R. Evans, unpublished). Species of the subfamily Phalangopsinae live in groups, and pair formation signals have been minimized. Antenna} touching plays a major role in maintaining group structure and coherence.Amphiacusta maya is rhythmically active in the course of the 24-h day. During daylight the crickets live socially in hollow trees or under overhanging embankments. Within 20 min of dusk they disperse and apparently forage throughout the night; also, nighttime oviposition has been observed. At dawn all members of the group return to their hideout. The main social interactions, such as fighting, courting and mating, occur during the early daylight hours, accompanied by acoustic signals. After that period of activity the group settles down, and by noon the crickets are quiescent for the rest of the day

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b

Fig. 2.3 Scanning electron micrographs of metanotal glands from an Oecanthus pellucens male. a, Dorsal view of thorax and base of abdomen. b, Enlarged view of anterior portion of left metanotal gland cavity, showing two exits from glands and anterior and posterior tufts of glandular hairs. al-a3, three pairs of exits from metanotal glands; AH, anterior tufts of glandular hairs; CN, central nodule of metanotal glands; FW, cut bases of forewings (tegmina); HW, hindwings; MT, metathoracic notum; PH, posterior tuft of glandular hairs; PN, pronotum; Tl, first abdominal tergum.

(Boake 1984al. The signal for dispersion at dusk could be waning light intensity, triggering circadian-controlled locomotion, whereas waxing light may make them look for shelter. Finding the same hole repeatedly could be facilitated by trail pheromone markers or visual orientation cues. Long-range acoustic signals during the return movements and phonotaxis are conspicuously absent. Another peculiarity of this species is the close resemblance of courtship and aggression songs, which can only be distinguished in a behavioral context (Boake 1984a). Furthermore, singing does not seem to be primarily meant for females, since noncalling or artificially silenced males have as much mating success as stridulating ones when confronted singly with females . Only when a silent male competes with other males does his inferiority become evident and result in a low copulation rate; a silent male in courtship is more often aggressively interrupted by other males, particularly those at or near the top of a dominance hierarchy, than is a singing male. Interference from competitors reduces the time and attention a male can direct to a female (Boake and Capranica 1982) . Thus the song during courtship has a warning function directed mainly toward nearby males. Such sounds could even have the opposite effect and make the female leave before the courting male has taken care of his adversaries. A male stridulating in front of a

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female strokes her with his antennae between bouts of chirps, which apparently has a calming effect, apart from identitying species and sex (Boake 1984b). In the African species Phaeophilacris spectrum, also phalangopsine, both male and female lack tympanal organs, the female is apterous, and the stridulatory apparatus is absent, although the male has a pair of forewings. Unlike stridulating crickets, where during sound production the tegmina are moved laterally in closing and opening strokes, special wing articulations allow P. spectrum only forward and backward flicks. These are performed singly or in units of 4-7 wing flicks at a rate of 8-12 Hz, which elicit periodic air disturbances (Kamper and Dambach 1979). Recently, techniques to visualize aerodynamic flow fields (Heinzel and Dambach 1987) revealed that wing flicks generate traveling air vortex rings. Using air particle movements as a means of soundless communication is distinctly different from the use of infrasound. A traveling vortex ring may also be suitable for pheromone transport (see chapter 6). Sex recognition derives from the reaction of the partner: upon antennal touch, two males react with single wing flicks and fighting may follow, whereas females behave passively and allow thorough body inspection. Mutual antennal stroking can then lead to courtship, which consists of a series of wing flicks and vertical presentation of the wings. Male aggression in P. spectrum develops with the onset of sexual maturity and leads to the formation of a linear dominance hierarchy, where the ranks are determined by fighting. The aggressive bouts range from mild antenna! touch to lashing, kicking, wing flicking, and wrestling. Bodily harm, however, is rare (Dambach and Lichtenstein 1978) (Fig. 2.4). Mate Choice In crickets, intrasexual selection relates to fighting ability and competition for access to females, but intersexual selection for mating depends on the power to attract the opposite sex and to display favorable behavior and appearance. In both forms of selection a cricket male frequently takes the active role and spends much time and energy in calling and fighting until selected by a female. While the final choice is hers, and while it also has been rationalized that a female's larger parental investment in the form of eggs as compared with the male's low-cost sperm would give her cause to choose the right male (Trivers 1972), the female's selectivity must also have been formed under the influence of male behavior. In return, males may have reshaped their own strategies to counteract the females' prerogative. In crickets communicating acoustically, females select their mates at long range on the quality of the calling song, especially on its temporal structure, with the syllable repetition rate being the most prominent factor (Popov et al. 1974, Thorson et al. 1982; chapter 10, this volume).

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f Fig. 2.4 Aggression between Phaeophilacris spectrom males. a, Antenna! flagellation by male on right. b, Preparative posture for fighting. c, d, Alternating production of single wing flicks. e, f, Wrestling with spread mandibles. Note laid-back antennae and maxillary palps. (After Dambach and Lichtenstein 1978.)

However, some trade-off between different song parameters is indicated by the following examples. To find out which components of a calling song add to its attractiveness, females of Acheta domesticus were tested by playing synthesized songs in which each single parameter was systematically changed by leaving all other parameters as constant as possible. The efficacy of the various signals was judged by measuring the degree of directionality toward the sound source of a female's approach in an arena. Stout et al.

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(1983) concluded that a calling song with a carrier frequency in the natural range (4-5 kHz), a chirp repetition rate of 1.5-2/s with three-syllable chirps of 25-ms syllable duration, and a syllable period of 50 ms (equal to a syllable repetition rate of 20 Hz) was more attractive than any other combination. However, the syllable repetition rate was still the most prominent factor. Doherty (1985b) repeated similar experiments with Gryllus bimaculatus on a walking compensator. He found that some females even tracked tone bursts of chirp length and rate and that at the margins of the syllable repetition rate the chirp rate became a factor. But the whole question of trade-off in cricket phonotaxis still awaits a full answer. Since differences of song composition between conspecific males do exist, females may follow the song with the best possible combination of acoustic parameters. In earlier attempts to find the attractive components in the calling song of the same species, chirp rates from calling songs of males high or low on the ladder of dominance hierarchy were simultaneously played back to females in a Y maze (Crankshaw 1979). Although the calls from dominant males had a subjectively determined low chirp rate and other characteristics that made them preferred over those from subordinates with higher chirp sequences, no acoustic analysis was made and the calls were transmitted at the same sound pressure level. Further studies ofA. domesticus have shown that, apart from the acoustic signal preference, certain dark targets of various shapes can be attractive as well. Pheromone traces from previously tested females in the same arena are claimed to divert attention from formerly attractive optical targest (Atkins et al. 1987). Females respond to acoustic and optical targets by adopting different walking modes (Weber et al. 1987), whereas in the presence of both signal modalities the attractive range of acoustic signals is somewhat narrowed, with a broad range of attractive syllable repetition rates. (Stout et al. 1987). More field studies are needed to determine the role and efficiency of the multimodal stimuli emitted by a male and its immediate environment in the course of female attraction. In the mole crickets Scapteriscus acletus and S. vicinus, male calling and female flight occur in synchrony during the first hour after sunset (Ulagaraj 1975; Ulagaraj and Walker 1973). During that short period, when predator pressure is reduced and temperatures are still high enough to allow flight (Forrest 1983a), females have optimal opportunity to choose among singing males. The importance of permissive nocturnal temperatures is underlined by the case of the Puerto Rican mole crickets S. didactylus and S. imitatus; elevated temperatures allow male calling and female flight throughout the night (Forrest 1983b). In S. acletus and S. vicinus, flying females selectively responded to louder singing males and either landed near the burrow of the stridulating male or accumulated in pitfall traps underneath a loudspeaker emitting calling songs. There is a correlation between sound pressure level and body size of a caller because large males generate more muscle

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power for wing stridulation. Scapteriscus males are, unlike many other crickets, on average significantly larger than females, which supports the idea that sexual selection favors large body size for the production of louder calls and therefore results in better mating opportunity (Forrest 1983a). Whether large males are, apart from having more frequent access to females, also superior in mating frequency, or sperm activity, or survivorship of offspring has not been studied. As to the question of how an approaching flying female discriminates among calls, it is unlikely that loudness or audiofrequencies affect that function. Differences in the temporal structure of the calls are more plausible cues, as has been shown for Acheta domesticus (Stout et al. 1983) and Gryllus bimaculatus (Doherty 1985b, Simmons 1988b). Females of both these species orient on the ground. A preference for large males has also been observed in G. bimaculatus females (Simmons 1987c). In experimental matings larger parents produced large offspring; but genetic contribution, in order to persist, must be strongly influenced by changing environmental conditions. During pair formation in nature, often initiated over some distances, females cannot at first choose body size per se, but rather its consequences, such as a particular calling song configuration presented loudly and, often, the possession of a burrow (see chapter 8). At close range, females might choose a male by what he represents, for instance being the winner of a fight or the producer of a more intense sex pheromone (Hormann-Heck 1957), if it exists. Since G. bimaculatus females, when allowed to choose their own males for mating, produce superior offspring, such important consequences may not rest with the selection of only one, but several, male characteristics (Burk 1983, Simmons 1987c). Females' preference for longer callings songs has been demonstrated in Gryllus integer (Hedrick 1986). Are longer calls more attractive because they deliver more energy, or do they facilitate female orientation? In laborato:ry trials, females were attracted by trills of 1-4 min duration, while calls of 5 s were ignored. Breeding experiments have shown that both calling song types are stable over time, that they are genetically different, and that the preferred longer calling bout is a heritable male trait (Hendrick 1988). Recently, studies on the g:ryllines Gryllus pennsylvanicus and G. veletis under seminatural conditions and in the field showed that females were more attracted to older males than to young ones. Apart from age, other factors such as body weight or size, song duration, or the degree of protozoan infestation of the callers did not matter (Zuk 1987a). Whether the deciding factor resides in the chirp structure or in another sensory modality is unknown. The possession of a burrow has long been recognized as an asset for a cricket male in attracting a female; females prefer burrow owners to males without a shelter (Alexander 1961). However, a population increase due to new arrivals of both sexes, flying in because of acoustic attraction, can

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decrease the number of available burrows. In Teleogryllus commodus, clusters of both sexes form under rocks and in cracks, aggression gradually decreases, and the crickets then enter a so-called inactive state, in which males rarely fight and females take little notice of them (Evans 1983). Such changes can be ascribed to high population density, and the behavior is by no means stable. Some males can quickly become aggressive again, whereas inactive males nearby do not react (Evans 1983). The duration of inactive and active periods is unknown. Immigrant males ofT. commodus, before joining a community or digging their own burrows, often resort first to altemative strategies: they either roam around, relying on chance encounters with females, or they enter a burrow and try to evict the owner. The intruders apparently recognize burrows by smell. Habitat simulation studies have shown that empty artificial burrows containing soil scrapings from inhabited burrows are adopted immediately. In fights for burrows, however, the resident male wins more often than the attacker, particularly when the burrow houses a female (Evans 1983). Aggregating behavior has also been seen in Gryllodes supplicans, and silent males surrounding a caller may act as satellites and intercept phonotactically attracted females (Sakaluk 1987, Cade 1975). Simmons (1986a) conducted an interesting study of the role of population density on intermale competition in G. bimaculatus. The intensity of fighting behavior during male-male encounters was inversely proportional to population density: at low density, males bit and flipped their opponents over; at high density, threatening sufficed to terminate the encounters. Introduction of burrows, or the presence of a female, at once caused fierce fighting between burrow owners and their challengers, regardless of population density. Three factors were found to influence competitive ability: body size, the number of fights a male had won before, and the possession of a burrow, preferably with a female in it. Successful males also called more frequently than losers, thereby attracting more females. In this context it is thought that, after mating, a female can decide whether she wants to receive a full sperm load or not and that the decision depends on the size of the mate (Simmons 198Gb). Accordingly females leaving the burrow after mating with a small male would remove the spermatophore earlier than after mating with a large male. Females also prefer to stay with a large male, remove the spermatophore only when it is empty, and mate several times in sequence. It would be fascinating to discover how a female distinguishes between different-sized males. But before we do that, two points should be considered. First, small G. bimaculatus males not only must court longer until a female mounts them but also often fail to attach a spermatophore (Simmons 1988a). Could it be that during the mating procedure they also have difficulties in threading the spermatophore tube properly into the spermathecal duct? Teleogryllus commodus females remove inadequately placed spermato-

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phores after mating when sperm does not begin to flow into the spermatheca. In such circumstances, females continue to behave like virgins and leave their partners in search of other males, and the mates cannot prevent them (Loher 1981). Similarly, the attachment of a spermatophore to a G. birnaculatus female does not guarantee insemination. Second, does a mated female stay with a large male longer because he can prevent her from leaving by guarding her? The functions of guarding behavior will be discussed later. The function of the courtship song after the female has already made a choice and approached the male deserves some comment. Since, at least in field crickets, the courtship song is less species-specific and stereotyped than the highly discriminating calling song, courtship and antenna! touch could identity the partner's sex and quality. More important, however, the courtship song could persuade the female to mount the male, which is suggested by the following example. Mating success of A. dornesticus males made soundless by cutting their forewings and confronted singly with a female dropped from 74% to 0%. Mating efficiency was restored to 62% when the male's silent courtship behavior was accompanied by a taped courtship song, thereby confirming the important function of that song type for the onset of mating (Crankshaw 1979). Do courtship songs need specific qualifications to induce a female to mount a male, or does any courtship song do? Trials on Teleogryllus oceanicus groups under laboratory conditions that permitted the formation of a dominance hierarchy, provided the answer (Burk 1983). As expected, ranking among males was established by fights, and the outcome depended on individual agressiveness, the possession of a burrow, fighting history, and, possibly, body weight as an index of body size. Females mated more often with successful fighters than with losers. In each case the releaser for mounting was the courtship song from the superior male. However, females did not discriminate among courting males, because a defeated male was accepted like a winner when singly confronted with the female, provided he emitted a courtship song. The fact that socially dominant males have more access to females and mate more often than subordinate males relates to the extreme aggressiveness of the superior male. Within his territory, the dominant male is attracted to courtship parades of lesser males and will physically disturb them. Other subordinate males do not even try to court females when a dominant male is around. Hence, it is not the quality of the courtship song but simply its presence that typically identifies the top competitor, and a female chooses the only courting male (Burk 1983). These behavioral data allow yet another interpretation, in which the initiative is put on a dominant male, who merely triggers the female into mating. The argument goes as follows. Through repeated fighting a male arrives at the top ranking. He actively suppresses courtship of subordinates and becomes the only courting male. Receptive females have no

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choice but to accept him, since the courtship song in T. oceanicus is the mating stimulus. In the gregarious cricket Amphiacusta maya, courtship is definitely not the tool for mate choice (Boake 1984b). Its function here is primarily one of warning other males and preventing interruption of courtship. Mating Most cricket species adopt and maintain the evolutionarily primitive female-above-male posture for copulation (Alexander 1964). There are cases where that initial posture changes into a final end-to-end position, with the male and female facing in opposite directions. The male remains standing, which causes the genitalia to twist (Anurogryllus) (Fig. 2.5), or he lies on his back (Gryllotalpa) (Alexander and Otte 1967b). Spermatophores All cricket males use spermatophores for insemination. For depletion a spermatophore is either attached to the female's genitalia, or it is held by the male and retained. In most species of Gcyllinae a spermatophore consists of an ampulla containing sperm, an anchor plate, and a tube of species-specific length. During copulation the spermatophore is transferred to the female, with the ampulla protruding outside and held in place by the attachment plate slipped into the female above the subgenital plate, while the tube is threaded through the genital chamber into the spermathecal duct aperture. The mechanism of spermatophore depletion has been studied in Acheta domesticus by Khalifa (1949). The spermatophore wall is composed of several layers, including a prominent, thick semipermeable membrane separating an evacuation fluid from the liquid within the two pressure bodies located in the posterior part of the ampullar cavity. The two fluids differ in osmotic pressure. When the tip of the spermatophore tube breaks off or is dissolved, the evacuation fluid permeates the inner layer and causes the pressure bodies to swell, thereby pushing the spermatophore content from the ampulla into the tube. Whether, on arrival at the spermathecal duct, the spermatozoa swim actively into the spermatheca or are physically propelled by spermatophore pressure is still an open question. This type of spermatophore occurs also in Teleogryllus commodus (Fig. 2.6), where it lies ready, fully formed and hardened in the male's dorsal cavity, hours before use during the calling period (Loher 1974). In Gryllodes supplicans and in several African species of the genus Teleogryllus, the spermatophore ampulla is capped with a large fibrous mass that is consumed by the female after mating (Boldyrev 1915, Alexander and Otte 1967b). Nemobiine species produce a spermatophore shortly before mating which is transferred in a soft stage (Mays 1971). A second type of spermatophore is found in Gcyllotalpinae (Neocur-

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a

b

Fig. 2.5 Copulatory postures of crickets. a, b, Female-above-male position: a, Teleogryllus commodus; b, Phaeophilacris spectrum . Note vertically raised tegmina of the male; the female is apterous. c, End-to-end position in Anurogryllus muticus.

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Fig. 2.6 Cricket sperrnatophores. a, Teleogryllus commodus. b, Anurogryllus muticus. c, Gryllodes supplicans with sperrnatophylax. (Adapted from Alexander and Otte 1967b.) d, Oecanthus pellucens. !Adapted from Boldyrev 1915.)

tilla), Phalangopsinae (Phaeophilacris l, and also in some Gryllinae (Anurogryllus, Discoptila), but only from A. muticus is relevant information on sperm transfer available (H. J. Lee, unpublished). The spermatophore consists of a bulbous ampulla and a short, thick spermatophore tube (Fig. 2.6). It is not ready and stored in the male but produced during copulation and held by the male. His abdomen contracts periodically and may exert pressure to empty the spermatophore into the female's spermatheca. Since mating lasts only about 3 min and the spermatophore is not released, sperm transfer has to be completed during copula. When the couple separates, the male retains the empty spermatophore, which is rubbed off and rarely eaten (H. J. Lee, unpublished). Thus, the structure of the spermatophore, its production before or during mating, sperm transfer after or during copulation, and the time sperm transfer takes are the major differences between the two described mating mechanisms, as exemplified by T. commodus and A. muticus. It is to be expected that these differences will be reflected also in postmaling behavior. Mating posture, on the other hand, does not indicate one spermatophore type or the other because, although males of A. rnuticus and Phaeophilacris spectrum produce the same type of spermatophore and hold it during spem1 transfer, the first species maintains the end-to-end position, whereas the second prefers the female-above-male posture (Fig. 2.5b).

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Mating Duration and Frequency The duration of mating is highly variable among species and unrelated to their taxonomic affinity or to the type of spermatophore produced; only a few examples are given here to make the point (Alexander and Otte 1967b). Mating times of only a few seconds, during which a spermatophore is thrust into the female, have been reported for some Nemobiinae (Gabbutt 1954, Mays 1971). Pairing lasts 30 s inAcheta domesticus (Khalifa 1950), 60 s in the gryllotalpine Scapteriscus sp. (Walker 1984), 110 s in Gryllodes supplicans (Sakaluk 1987), and up to 12 min in Gryllomorpha dalmatina (Boldyrev 1927); in each case the spermatophore is attached to the female. Mating in species with sperm transfer during copulation takes 3 min in A. muticus but up to 75 min in Phaeophilacris spectrum (Dambach and Lichtenstein 1978). Copulation times of 90 min have been recorded for Discoptilafragosoi (Boldyrev 1928). The frequency of mating, whether with the same or with another partner, depends on several factors. Females ofT. commodus stay receptive after mating, whereas those ofA. muticus become defensive, refuse further copulations, and withdraw into the burrow. The male's ability for renewed mating depends on the time required to produce the next spermatophore and to regain sexual readiness. Both interoals are the same for Orocharis sp. (Eneopterinae), and a male can copulate and pass a new spermatophore eight times in about 30 min (Alexander and Otte 1967b). In another eneopterine species, Hapithus agitator, the interoal between two matings is 12 min, whereas the grylline T. commodus takes an average of 65 min to produce another hardened spermatophore and a further 60-70 min to mate again (Lober and Renee 1978l. The short-tailed grylline Anurogryllus arboreus is unusual because, while copulating for 10-16 min, it continues to call, and more females are attracted. Upon separation the male mates immediately with one of the waiting females (Walker 1980b, 1983a). In nemobiine crickets, the mating time schedule is complicated by fake copulations, which are apparently necessary for stimulating spermatophore production. Fake coupling without transfer in Pteronemobius is followed by extended courtship lasting 20-40 min, during which a spermatophore is formed. The female then mounts a second time and stays coupled for 20-30 min while the spermatophore is transferred (Mays 1971). The female chews on spurs at the base of the hind tibiae, presumably only drawing hemolymph, as no glandular tissue has been found in that region. In Nemobius sylvestris, a small, sperm-free spmmatophore is transferred during the first copulation. Courtship is predominant in the next 60-70 min, during which the female repeatedly mounts the male and licks some secretion at the base of the forewings (Gabbutt 1954, Mays 1971; Campan and Demai 1983). Then, after the first spermatophore has been removed, a three-times-larger and sperm-containing second spermatophore is transferred in a 1-s thrust! A quite different mating strategy occurs in another nemobiine species,

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Bobilla victoriae (A. R. Evans, unpublished). The male protrudes a spermatophore 5-12 min into his courtship and copulates 10 min later in the rapid fashion typical for some nemobiine species. However, when he moves backward to slip under the female, a female sometimes picks his protruding spermatophore off and eats it without having copulated. Courting then follows again after 3 min, and soon another spermatophore appears. Usually, however, several matings take place within 30-40 min, either with the same female or with another nearby, and each spermatophore is eaten shortly after dismounting. Females may profit from eating the spermatophores, which may have nutritional value. Multiple Matings The foregoing examples raise a basic issue: why do female crickets mate more than once in a lifetime? Multiple matings serve a number of purposes: (1) guarding against unsuccessful inseminations; (2) balancing for deterioration of sperm; (3) increasing genetic diversity; (4) selecting a strong mate, after having already mated with a weaker one; (5) acquiring resources controlled by the male; (6) increasing egg production.

Guarding against unsuccessful inseminations. In Teleogryllus commodus, 20-30% of matings are unsuccessful, most likely because of the excessively long spermatophore tube (Fig. 2.6a), which has to be threaded into the aperture of the spermathecal duct. If that procedure fails, the attached spermatophore cannot be emptied. Unsuccessful matings have also been reported for A. domesticus and G. integer (Sakaluk and Cade 1983). Balancing for deterioration of sperm. Old, once-mated T. commodus females lay a considerable number of sterile eggs in spite of a large supply of highly motile sperm in the spermatheca. These crickets had derived from eggs of field-collected females. Since young, mated females also deposit 10-20% of their eggs unfertilized (W. Lober, unpublished), a male may have either transferred some nonviable sperm, or the physiological conditions for sperm maintenance in the spermatheca were deficient. Studies on the viability of sperm are highly desirable. Increasing genetic diversity. Variation in traits is the basis of selection. Adaptive characteristics in a cricket population may change over time and from location to location due to differential selective pressure of periodically appearing parasites (Cade 1981bJ. The mechanism of spenn competition is likely to depend on the shape of the spermatheca (W. F. Walker 1980). Whereas the elongated organ in the desert locust allows total displacement of recent sperm away from the exit of the spennatheca, and the last male's sperm fertilizes all eggs (Hunter-Jones 1960), spheroid spermathecae, like those in tsetse flies, increase sperm mixing and, therefore, genetic variability of offspring (Jordan 1972). Crickets seem to be intermediate between these two extremes.

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Spermathecae from virgin and mated females ofTeleogryllus commodus . The spermatheca from the virgin female is spherical (upper right, translucent); several sperm loads have tra nsformed the mated female's spermatheca into an ovoid shape (lower left, opaque).

Fig. 2.7

In sperm competition experiments G. integer females were mated first with a fertile male and then with an irradiated, sterile male, or vice versa (Backus and Cade 1986). When fertile males mated last, females produced significantly more offspring than in the reverse combination, but the data also suggested some sperm mixing. The shape of the G. integer spermatheca, which is spheroid but slightly elongated, would account for these results. In a similar study of G. bimaculatus, double matings by a normal male followed by an irradiated male, or vice versa, resulted in hatching rates that suggested sperm mixing. Therefore, when two normal males mate with the same female and the second inseminates her three times, his ejaculates dilute the first male's sperm, and sperm from the second male is used more often to fertilize eggs (Simmons 1987b). According to W. F. Walker's correlation between spermathecal anatomy and sperm precedence, G. bimaculatus is expected to have a spherical spermatheca, enhancing sperm mixing. In T. commodus, a virgin female's spherical spermatheca changes after mating to an ovoid shape, particularly when it contains 3-4 spermatophore loads (Fig. 2.7). Therefore, dynamic changes of spermathecal shape should be considered.

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Selecting a strong mate. A mate is considered strong when he occupies a burrow, which, as in mole crickets, considerably increases the amplitude of the calling song. A burrow also protects against parasites and predators and provides a safe oviposition site. Mating repeatedly with such a superior male dilutes already present sperm and favors the burrow owner's ejaculates (Simmons 1987b). Since in T. commodus mating stimulates oviposition (Lober and Edson 1973), which in nature normally happens in the male's burrow (Evans 1983), it is likely that most of the eggs laid are fertilized by the burrow owner. Acquiring resources controlled by the male. Males of Gryllodes supplicans produce spermatophores with a sperm-free spermatophylax. After mating, females consume the spermatophylax. Sometimes mates or unmated females snatch it from the female's mouth parts and eat it themselves, which suggests that it might be nutritious. Females of G. supplicans are also highly mobile during the night, thereby gaining access to several males and further matings. For such females, the consumption of the spermatophylax and later of the empty spermatophore ampulla might constitute an important source of nutrition (Sakaluk 1987). The repeated matings and avid consumption of spermatophores sometimes before, but usually after, mating of Bobilla victoriae make it likely that in some Nemobiinae these spermatophores are also of nutritional value. The disadvantage of eating the spermatophore too soon, and thereby consuming sperm, is probably balanced by the high frequency of mating, which can amount to the production of 15 spermatophores in 8 h (A. R. Evans, unpublished). Monopolizing a female will eventually secure sufficient sperm transfer, whereas mating with other females in alternation during the same mating session assures a male's sperm dispersal. If it can be proved that the spermatophores are composed of highly nutritional material, their consumption could be a paternal investment in the offspring (Trivers 1972). In an extensive review Gwynne (1983) discussed male nutritional investment in orthoptera. Increasing egg production. The number of eggs laid and hatched was compared after one versus multiple matings in A. domesticus and G. integer (Sakaluk and Cade 1983, Cade 1984). A. domesticus females to which males had constant access produced significantly more eggs and progeny than once-mated females. Similar results were obtained with G. integer when the numbers of offspring from once and twice-mated females were compared (Sakaluk and Cade 1983). Also, offspring production increased significantly with the amount of time a spermatophore was attached to a female's genitalia. In T. commodus there is no positive correlation between multiple matings and the production of eggs and progeny. A comparison between females that were successfully once versus twice mated (with the latter

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inseminated a second time either within 24 h or after 10 days) failed to demonstrate significantly higher egg production and hatching rate in twice-mated females (A. Persson and W. Loher, unpublished). Similarly, egg production did not differ in once-mated versus twice-mated G. bimaculatus females, but inseminated females produced significantly more eggs than virgins (Bentus et al. 1977). We need data from many more species before generalizing for crickets.

Postmating Behavior Postmating behavior is concerned with mechanisms that assure complete transfer of spermatophore contents and with paternity of offspring. Evidently males from species with retained spermatophores that are emptied during copulation have only to be concerned with defending the mate against copulatory attempts from competitors. A number of mechanisms have evolved, both in males and females, none of them peifect but all contributing to preserve a successful mating. Long Copulation Duration The long-legged cricket Phaeophilacris spectrum spends between 30 and 75 min in copula. The male holds the spermatophore during the entire period, but it is unknmvn whether transfer of spenn takes that long or whether copula serves to prevent the female from mating with other males (Dambach and Lichtenstein 1978). After separation, oviposition follows shortly. Copulation times in Discoptila fragosoi vary between 15 and 90 min, during which time the female is retained on the male's back by a secretion from the male's tegmina! base, on which the female feeds (Boldyrev 1928). Guarding Another strategy to protect the male's mating efforts is to guard the female (Fig. 2.1), a behavior widespread among the Gryllinae (Alexander and Otte 1967b). Guarding a female has three functions: (1) to prevent competitors from inseminating the same female and thereby possibly displacing the guarding male's sperm before oviposition, (2) to hinder the female from removing the attached spermatophore before its content has migrated into her spermatheca, and (3) to monopolize the female for further matings. Guarding a female to prevent her from removing the attached spermatophore before it is empty, an idea first expressed by Gerhardt (1913), assumes that guarding duration and the time it takes to fully deplete a spermatophore are correlated. Indeed, guarding and spermatophore emptying both take 60 min inA. domesticus (Khalifa 1950) and 40 min in G. campestris (Huber 1955). When, in the latter species, the male is removed shortly after mating, the female picks off the spermatophore after 20 min;

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at that point the sperm transfer is far from completed (W. Loher, unpublished). Spermatophores ofT. commodus take about 70 min to empty, and guarding lasts ca. 80 min, during which time the male produces a new spermatophore; but another 35-40 min are needed until the male is ready for further mating. A female isolated after mating picks off her spermatophore after 30 min, when it is still partially full. How does aT. commodus male prevent his mate from eating the attached spermatophore before sperm transfer has occurred, and how does he stop the female from leaving? According to observations in the laboratory, the male stays in close body contact with the dismounted female. If she makes conspicuous movements, such as bending back to pluck off the spermatophore with the mandibles, the male butts her with his head and she ceases at once. Attempts to leave and to break contact cause aggressive chirping, rocking, and jerky pursuit. Upon renewed touch, the couple quiets down again. After about 20 min into the guarding phase, the female's behavior changes: she visibly calms down, grooms herself, and feeds. In addition to this substantial reduction in locomotor activity, she no longer reacts to calls from neighboring males (see below). These changes aid the male in his attempt to keep the female and monopolize her for further matings. Such behavioral sequences as those described have also been observed under field and seminatural conditions in burrows and crevices, where most matings take place (Evans 1983). In such locations, shor1age of space combined with thigmotactic effects further facilitate the male's task to keep the female in the burrow. Therefore studies performed under laboratory conditions, where couples were mated in 10 X 10 x 10 em cells forcing frequent body contacts, were closer to reality than originally expected (Loher and Renee 1978). Recent field studies in Australia confirmed the above laboratory observations (W. Loher, unpublished). Yet there were still cases where females observed in the field or in a habitat simulation left males shortly after mating (Evans 1983). Two reasons could account for this. First, mating was unsuccessful because, although a spermatophore had been attached to the female, its tube did not find the spermathecal duct aperture, and sperm transfer was blocked. Consequently, no behavioral changes occurred and the female left, attracted to the call of another male in the vicinity. Laboratmy work on T. commodus showed that 20-30% of the matings were unsuccessful, even when the couples were together overnight and had opportunities to recopulate. The second reason for leaving a burrow might be that the substrate in the burrow was unsuitable for oviposition. Females can lay eggs as early as 10 min after insemination and, in habitat simulation studies, were observed to leave a burrow after mating and oviposit in a nearby tussock (Evans 1983). Gryllus bimaculatus females preferentially mate and stay with large males, and several inseminations can occur. Small males are abandoned after separation, and the spermatophore is removed before its contents

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can migrate into the female's spermatheca. In view of the female's alleged ability to decide paternity for her offspring after mating, the role of guarding behavior as a male-control mechanism over females, at least for G. bimaculatus, has been questioned. Accordingly, the function ofpostcopulatmy mate guarding was reduced to preventing "access of conspecific males to females while a new spermatophore is produced" (Simmons 1986b). While that function presupposes that a defending male must have the means to keep the female with him for at least 60 min-the time it takes for the attached spermatophore to empty and for the male to produce a new one-it should also be considered that large, strong males might better prevent a female from leaving than could smaller males. Furthermore, it was not established in that study whether or not the matings had been successful. A spermatophore attached to a female does not guarantee sperm transfer to the spermatheca (Loher 1981). In Simmons's study (1986b) females that had mated with vel}' small males (0.5-0.6 g) and had then left the burrow, did not remove the spermatophore until 20 min later. However, sperm begins to migrate to the spermatheca within minutes after a couple's separation. Thus a substantial amount of sperm could already have reached the spermatheca before the spermatophore was removed, thereby counteracting the presumed intent on the part of the female to refuse sperm from an inferior male (Simmons 1986b, fig. 3).A further weak point ofthat study is the unnatural environment in which male-female interactions of G. bimaculatus were studied: a large, open arena without landmarks and obstacles and no opportunity to hide. This species lives in grass fields and tussocks, with grass stems and blades limiting its visual reach and its locomotion. It is then not surprising that females, after having mated in an open, exposed arena, abandoned males of any size and searched for shelter (Simmons 1986b). In a confined environment, which G. bimaculatus prefers for mating, normal guarding behavior might then be manifested. Cricket reproductive behavior is not always dominated by female choice, and the male's role not always secondayY; both sexes and their behaviors complement one another in sexual selection. Diversion Tactics Boldyrev (1915) first described a fibrous white mass attached to the spermatophore of Gryllodes supplicans which the female ate immediately after mating. He named that structure spermatophylax (protector of sperm) because he reasoned that during its consumption the sperm could be migrating into the female's spermatheca before the ampulla was eaten. This hypothesis has been verified. Although the female begins to feed on the spermatophylax immediately after spermatophore attachment, sperm is transferred in 50 min (which is as long as it takes to consume the spermatophylax) and before she continues to feed on the ampulla (Sakaluk 1984, 1985) (Fig. 2.6cl. The metanotal glands of male tree crickets (Fig. 2.3) divert the female's

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attention from the attached spermatophore. In Oecanthus pellucens the female is attracted by the male's courtship song, and, upon mounting, spermatophore transfer takes place immediately and lasts only for 1 min. After transfer, she normally stays and licks the male's metanotal glands. In 92% of 500 observed copulations, the female did not lick the secretionfilled thoracic cavity before copulation, and mating occurred even when the secretion was removed or the cavity closed with tape. But after copulation, when females could not find the secretion, they left at once and consumed the full spermatophore instead. The mating act triggers feeding, and females normally stay on the male's back for 12-18 min, until the cavity is empty. This time span coincides with the spermatophore's average depletion time, 15 min (Hohorst 1937). After leaving, the female eats the spermatophore within 1 min. In Oecanthus fi.Iltoni, female precopulato:ry licking of the thoracic cavity for over half an hour has been observed, followed by about the same period of licking after mating. Precopulato:ry feeding in that species possibly appeases the female and holds her on the male's back (Fulton 1915). In contrast, postcopulato:ry feeding in Oecanthus pini may last as long as 65 min and may keep the female available for a further mating because a second spermatophore can be transferred within 70 min after the first (Walker and Gumey 1967). Rendering Females Unreceptive Making a female unreceptive to further matings has been observed by West and Alexander (1963) in a short-tailed cricket now called Anurogryllus arboreus (Walker 1973). Switching off receptivity by stretching of the spermathecal wall owing to the influx of material from the spermatophore, or the transfer of chemicals with it, might provide inhibito:ry stimuli affecting the central nmvous system. Females of Balamara gidya become unreceptive after the transfer of a spermatophore and of a gelatinous substance (A. R. Evans, unpublished). Indirect prevention of further mating by temporarily tuming off phonotaxis has been reported for inseminated G. integer females (Cade 1979). Whether mating is prevented for reasons mentioned above or because of a deposited spermatophore acting as a mating plug, as in certain acridine grasshoppers (Loher and Huber 1966), remains to be seen. Consequences of Mating for Females Mating in T. commodus has three overt effects on a female's behavior (Loher 1981). After 20 min into the guarding behavior, but also when the female is alone, her locomotor activity gradually subsides; she becomes more or less sedenta:ry and remains so even when guarding behavior ends 80 min after mating. Running-wheel experiments have shown that the female's reduction in walking activity, which occurs mainly during the dark phase and is under circadian control, can last from 2 to 10 d (88% of 58 females) to as long as 38 d (Loher 1981; chapter 3, this volume).

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As a second effect of mating, the male's calling song, a powerful attractive stimulant during premating behavior, which elicits walking even when played during daytime, no longer causes locomotion. This phenomenon may account for a couple staying together for another mating, in spite of competitor's calling in the neighborhood. Although the female is unresponsive to calling, she still responds to the courtship of her nearby mate and mounts him for another copulation. Mated females again react positively to the calling song within 2-38 days and then approach the sound source. However, the time course of phonotaxis differs from that of the return of circadian-controlled locomotion, which suggests that mating affects the two behaviors independently. Neither of the physiological mechanisms that reduce locomotor activity is understood. The third consequence of insemination is oviposition. In the virgin state, females store their eggs in the ovaries. Mating stimulates oviposition and several hundred eggs are deposited (Loher and Edson 1973, Pohlhammer et al. 1975). For oviposition females are more or less stationary and they lay their eggs within a limited area. But reduced locomotion after mating is not related simply to egg laying: when mated females are prevented from oviposition by unsuitable substrates, locomotion still does not return (Loher 1981). Oviposition

Chemical Mediation of Egg Release For fertilization to occur, an egg arriving from the ovary at the genital chamber has to pass the aperture of the spermathecal duct. Only a single spermatozoon succeeds in entering the egg through one of the three micropyles (Pohlhammer 1978). In Teleogryllus commodus only 70-80% of the eggs laid are fertilized and do hatch; the rest fail to develop (W. Loher, unpublished). So far, several mechanisms have been demonstrated which ensure fertilization of eggs by sperm from the last mating. However, fertilization would be even more efficient if egg release were triggered by insemination itself. This is, indeed, the case forT. commodus, where, within 12-24 h after insemination, the female deposits hundreds of eggs (Loher and Edson 1973, Loher 1981) as a consequence of prostaglandin (PG) activity (Loher et al. 1981). That role of prostaglandins was first discovered in Acheta domesticus, where injections of PGE 2 induced oviposition (Destephano and Brady 1977, Destephano et al. 1982). InT. commodus, the function of PGE 2 in oviposition is as follows. During mating, the male attaches to the female's genitalia a spermatophore containing sperm, a PG synthetase complex, and an inactivated PG precursor: the polyunsaturated arachidonic acid that is bound to phospholipids. This load migrates into the female's spermatheca, where several fatty acids are already present, among them arachidonic acid. ln the spermatheca the phospholipase A2 cleaves the esterified arachidonic acid

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from the [3-carbon position of phosphatidylchloine and makes the fatty acid available to the enzyme complex for further PG synthesis. These lipids then induce an intense bout of egg laying (Loher et al. 1981, StanleySamuelson et al. 1983). When radioactive arachidonic acid is injected into immature males, it immediately becomes incorporated in the testes and accessory glands; in sexually mature males, it can also be found in the spermatophores produced daily. The radioactive compound is then transferred by mating to a female's spermatheca, but it can also be traced to the hemolymph. In both cases, the radiolabeled compound has been incorporated in several PGs (Stanley-Samuelson et al. 1987). Nanogram quantities of PGE 2 applied to the genital chamber of virgins suffice to release oviposition behavior, and so do transplantations of spermathecal contents, which include PGs, from mated to virgin females (Loher et al1981, Ai et al. 1986). A variety ofPGs and related compounds injected into virgins lead to differential egg release according to their chemical structures. The highest number of eggs laid was associated withE-series PGs, which have in common a prostanoid backbone with a five-membered ring (Stanley-Samuelson et al. 1986). While there can be no doubt that PGs induce oviposition, and that paternal investment in offspring includes sperm and a PG precursor that appears in the form ofPGs in the hemolymph, there is still some debate as to the pathway the spermathecal PGs take to get into the hemolyn1ph (Sugawara 1987). These compounds cannot penetrate the walls of the spermatheca or its duct, as shown by transplantation of intact spermathecae from mated females into the hemocoel of virgins, apparently because that organ is lined with a cuticular layer (Ai et al. 1986). As to the locus ofPG action, PGE 2 applied directly to oviductal musculature failed to elicit contraction; the central nervous system might be the target. Further, in Acheta domesticus a long-term egg-laying stimulus is thought to be associated with sperm (Mmtaugh and Denlinger 1985). Although females mate shortly after the imaginal molt and their eggs are fully mature by the age of 7 d, oviposition in this species occurs only at the age of 12-14 d. Apparently, a factor not further described is transferred to the female's spermatheca in a latent form and activated there. Denervation of the spermatheca followed by mating did inhibit oviposition, and, since implantation of a sperm-filled spermatheca into virgins failed to induce egg laying, it was implied that the "factor" was not transmitted humorally but neurally. These data are inconclusive for two reasons: First, denervation of a spermatheca may have blocked the transport of its contents. Second, when a spermatheca is excised, the severed end of the spermathecal duct clogs up at once; hence a transplanted organ cannot release its material. Only when a transplanted spermatheca is cut open inside a recipient to allow the content to enter the hemocoel can egg laying be stimulated, as has been shown for T. commodus (Ai et al. 1986), a fact previously overlooked in the same species by Sugawara (1986).

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Egg-Laying Behavior A substrate suitable for egg laying must satis:ty several conditions. It has to offer maximum protection for the eggs, optimal conditions for their development, and sufficient nutritive material for the emerging larvae. Within the various suborders of orthopteran insects, different oviposition strategies have developed. Saltatoria lay their eggs in soil or plant matter and implant them deep in the substrate. Caelifera lay eggs by stretching the abdomen, often to twice or three times its original length. On the other hand, Ensifera employ an ovipositor consisting of three pairs of parallel valvulae in the Tettigonioidea or two pairs in the Gryllidae. Within the superfamily Gryllidae three types of ovipositors are found, each adapted to the particular species-specific, egg-laying strategy and substrate selected for oviposition: 1. The soil-laying species (most Gryllidae) possess an ovipositor with a long, slender shaft and a sharp, lance-shaped tip. 2. Species that oviposit in soft plant matter (e.g., Trigonidiinae) have a saw-edged, saber-shaped ovipositor. 3. Species that lay eggs in coarse plant material (all Oecanthinae) have an ovipositor with a tooth-edged tip. In the subterranean Gryllotalpinae the ovipositor is missing; in the Myrmecophilinae, which inhabit ant nests, the ovipositor has been modified and is apparently no longer used for inserting eggs into the soil (Cappe de Baillon 1922).

Structure of cricket ovipositor. Several studies on the morphology of the ovipositor and associated structures in Gryllidae have been made (Cappe de Baillon 1922, Snodgrass 1933, Matsuda 1976). The ovipositor consists of two pairs of long valvulae. The dorsal valvulae (valvulae 3 according to general nomenclature) are connected with the ventral pair (valvulae 1) by a tongue-and-groove joint (Fig. 2.8). The ridges on the dorsal valvulae slide freely in the grooves ofthe ventral valvulae. The base of the ventral valvulae consists of a skeletal plate, the valvifer, originating from the eighth sternum. The dorsal valvulae and the membranous rudiments ofthe second pair arise from the ninth tergum (Snodgrass 1933, Matsuda 1976). The movements of the valvulae are generated by a set of strong muscles which includes the ventral muscles of the seventh abdominal segment and the main lateral and ventral muscles of the eighth and ninth segment. In the lance-shaped apical region the right and left lower valvulae are held together by a mechanism that snaps apart when tension develops during egg transport down the ovipositor. After the release of an egg, the valvulae are reattached. The upper and lower valvulae always remain together because of the ridge-and-groove mechanism. However, the terminal tips of the upper and lower valvulae can be forced apart in a dorsoventral plane: through retraction of the dorsal valvulae a pair of sclerotized knobs is pulled against the sloping edges of the ventral val-

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cercus

terga

~

1.valv.(ss)

sterna

({~3.valv.

~c!) '1.valv. Fig. 2.8 Posterior abdomen of a Gryllus assimilis female,lateral view, demonstrating the relationship between the ovipositor and abdominal segments (ovipositor not shown full length). Cross-section of the ovipositor (below) shows tongue-andgroove joints between dorsal (third) and ventral (first) valvulae. (After Snodgrass 1933.)

vulae, thereby causing the free tip to gape open. When the ovipositor of G. bimaculatus is opened, ejected eggs can be maneuvered into spaces between other eggs that have already been laid. The cuticle of the valvulae bears scales and hair sensilla on its inner and outer surface (Sellier 1971, Sugawara and Loher 1986). The scales on the inner side grip the surface of the egg and move it along the ovipositor (Austin and Browning 1981). In G. bimaculatus five types of sensilla trichoidea, among them two with an apical pore, as well as campaniform sensilla, have been found on the ovipositor. The outside of all four valvulae is studded with hair receptors, which are concentrated on the lance head and in the basal region. On the inner side only the dorsal valvulae bear hair sensilla (Markus 1985). Their sensory function lies in testing the substrate, triggering and controlling the motor patterns of the valvular muscles.

Egg laying in soil. Laboratory observations on the sequential steps in oviposition behavior have been carried out inA. domesticus (Destephano

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et al. 1982), G. bimaculatus (Dambach and Igelmund 1983), and T. commodus (Sugawara and Loher 1986). In G. bimaculatus the female tests the substrate surface with the mouth parts and by tapping the ovipositor. When the surface is moist, the ovipositor is inserted (Destephano et al. 1982). Egg laying is, however, initiated only when the ovipositor detects more moisture in the deeper layers of the substrate. To find out whether sensory information from the ovipositor alone is sufficient for triggering egg laying, an oviposition chamber with a horizontal parafilm partition inserted 3 mm below the sand surface was used. The partition divided the substrate into two portions, which allowed their consistency to be varied independently (M. Dambach, unpublished). The number of punctures in the parafilm partition, as well as the eggs laid in the lower portion, could be counted. The results confirmed the general observation that sensory information received from a moist substrate is necessary to initiate both insertion of the ovipositor and egg laying. With moist sand in the upper layer and dry sand below the parafilm, the ovipositor was frequently inserted but always withdrawn. The parafilm showed numerous punctures, but in the lower portion only a few or no eggs were found. When the depth of the parafilm partition was increased, sensory information coming from the lance head (about one-sixth of the total ovipositor length) was sufficient for triggering egg laying. Experiments with various substrates have indicated that the ovipositor can also detect differences in substrate texture. The stimulus for egg laying is most likely a combination of moisture and substrate consistency. These observations are supported by electrophysiological data on several species of Teleogryllus (Ai and Sasa 1977, Ohsaki and Ai 1979). Mechanical stimulation of the ovipositor generated impulses from the respective afferent nerves, which triggered a neural network within the terminal ganglion, providing the basis for rhythmic movements of the genitalia and the ovipositor valves. Differences in ovipositor length are presumed to be the result of adaptations to soil conditions (Masaki 1986). It is then curious that Eurepa marginipennis, the cricket with the longest ovipositor (up to 39 mm), lives on tree trunks (Otte and Alexander 1983). Where it lays its eggs is unknown.

Insertion of ovipositor and egg transport. Sequences of normal oviposition have been described for G. bimaculatus (Dambach and Igelmund 1983) and T. commodus (Sugawara and Loher 1986). The G. bimaculatus female raises herself by means of the hindlegs and inserts the tip of the ovipositor into the substrate, often to its whole length when soil conditions are right. After a short lift of the abdomen, followed by a brief rest period, an egg appears at the base of the ovipositor and is pushed distally through the ovipositor canal by rhythmically alternating movements of the valvulae. When the upper substrate layer is removed, the egg transport through the proximal part of the ovipositor can be watched (Fig. 2.9). Eggs are deposited singly at intervals of 10-20 s; they form a cluster of 3-7 eggs, arranged around the tip of the ovipositor. After laying, the ovipositor is

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Fig. 2.9 Phases of egg transport between the valvulae of the ovipositor in Gryllus bimaculatus. a, b, Soil substrate removed to expose the position of the egg near the base. c, Egg is midway down the ovipositor. d, Egg has reached the lancelike head of the ovipositor, whose lateral attachment is uncoupled.

pulled out and reinserted into a site usually close to the previous one. A female lays 20-50 eggs in 15 min and up to 1,000 eggs overnight. If a female is disturbed during oviposition, she moves abruptly forward and pulls out the ovipositor. The action of the valvulae is halted, and the eggs remain in whatever position they have reached. In this way, stopaction pictures can be obtained of the various phases of egg transport (Fig. 2.9) . During egg movement the lateral halves of the ovipositor extend sideways, making room for the egg, until the lateral joint between ends of the two lower valvulae snaps open. After egg release, the valvulae become reattached. In T. commodus a similar sequence, consisting of testing, elevating the body, penetrating with the ovipositor, and a short lifting of the ovipositor by slightly retracting the last segments of the abdomen, has been observed (Sugarawa and Loher 1986). During the rest phase of a few seconds up to 3 min, the egg is located in the genital chamber (Pohlhammer 1978), and fertilization probably occurs then. By mounting a transparent window into the seventh sternum, the events occurring in the lateral oviducts and the genital chambers during oviposition can be observed (Sugawara and Loher 1986). Just after the ovipositor-penetration phase an egg proceeds from one of the lateral oviducts to the genital chamber, in synchrony with the retraction of the chamber. The subsequent short lift of the ovipositor indicates the presence of an egg in the genital chamber.

Egg laying in soft plant material. Egg-laying behavior common among tettigoniids (Marrable 1980) is displayed by crickets of the subfamily Trigo-

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Fig. 2.10 Ovipositing female and details of ovipositor structure of Trigonidium cicindeloides. a, Scanning electron micrograph of ovipositor and cerci, right lateral

view; terminal saw-tooth region is open due to fixation process. b, Female ovipositing into a rush stalk (Juncus sp.) .

niinae. The eggs are deposited deep into plant tissue by a sickle-shaped ovipositor bearing sawlike teeth at the tip, presumably to facilitate penetration of rushes (Juncus sp.) and grass nodules (Ingrisch 1977). For oviposition, the female of Trigonidium cicindeloides bends its abdomen to insert the ovipositor into a substrate area, that has been tested previously with the mouth parts (Fig. 2.10). After oviposition, the penetration site is closed with a plug, which is licked. The grass-inhabiting eneopterine cricket Euscrytus japonicus has a uniquely dorsoventrally flattened ovipositor, probably used to insert eggs between stem and leaf sheath (S. Masaki, unpublished).

Drilling and egg laying in coarse plant material. The oviposition habits of tree crickets (Oecanthinae) represent another type of egg laying in comparison to those of ground crickets. Eggs are laid exclusively into the bark of trees or bushes or into the pith of herbs. The ovipositor is inserted by a drilling process. From the classic investigations of North American species by Fulton (1915) and a recent study on the European Oecanthus pellucens (Dambach and Igelmund 1983) it appears that individual species are highly selective in their choice of vegetation, as expressed in their specific patterns of egg deposition. However, the basic behavioral sequence- substrate inspection, drilling, egg deposition, and plugging the bore hole-seems to be common to all species. The 0. pellucens female climbs up and down a plant stem and tests the surface with the maxillary palps. When a suitable oviposition site has been found, she bites a hole in the bark (Fig. 2.11). Advancing with the forelegs and middle legs, the female positions the tip of the ovipositor above the prepared hole, usually on the first trial. The ovipositor bores through the bark and into the pith within 5 - 9 min. After she withdraws the ovipositor

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Fig. 2.11 Sequence of Oecanthus pellucens female ovipositing into a cocklebur plant stem (Agrimonia eupatoria L.l. a, Biting through the selected drilling site. b, Ovipositor placed

on the chosen spot. c, d, Progressive drilling with ovipositor. e, Maximum insertion of ovipositor. f, Withdrawal of ovipositor.

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from the bore hole, she fills it with a foamy mass that later hardens to a plug. In the last phase of withdrawal, valvular motions help to plug the opening. Finally the female bites oif hairs from the plant stem and deposits them on the plug. Then she pushes her head forward 4-5 mm, bites a new hole, and repeats the egg-laying procedure. Characteristic coordinated movements of the fourvalvtilae accompany the egg-laying process. Generally, the two lower valvulae appear to be synchronized in their caudal and rostral movements during drilling and egg transport. For drilling, the coupled lower valvulae move forward and backward. The front endings act as wedges, pushing against the respective bevels on the upper valvulae and forcing them apart, thereby scraping and tearing with the teeth through the plant tissue. During withdrawal, the lower pair of valvulae is uncoupled. 0. pellucens lays up to 16 eggs, usually one at a time in a neat row, 4-6 mm apmt. In rare cases two eggs per bore hole have been found. The North American tree crickets have been subdivided according to egg deposition patterns (Fig. 2.12), which fall into three groups (Fulton 1915). In group 1, which includes 0. niveus [= 0. angustipennis] and 0. exclamationis, the eggs are deposited singly in the bark of trees and bushes. In group 2, among which are 0. quadripunctatus and 0. nigricornis, the eggs are deposited in rows in the pith of small stalks and twigs. 0. latipennis, representing group 3, uses a single hole in the outer layer of a stem to deposit 4-12 eggs, which are placed in the pith in two groups, one above and one below the initial bore hole.

Modifications of egg-Laying behavior. Most egg-laying behavior in crickets has been observed in the laboratory, often under restricted conditions, where a choice in substrate was limited to moist sand or peat. In nature the conditions for egg laying are quite different, and the behavior of a cricket is much more complicated. Gryllus campestris, for instance, can breed successfully in the laboratory when dishes of moist peat are offered as a substrate for egg laying. Under natural conditions, however, the eggs are laid in a burrow. Observations of females in artificial burrows have revealed that after copulation the female pulls in plant material and makes a loosely packed bed into which she repeatedly lays eggs (Klopffleisch 1973). The process of supplying burrows with plant matter has been described as one step of a seven-stage phylogenetic process that may be considered as the emergence of social behavior in insects (Alexander 1961). Females ofAnurogryllus arboreus have a minute ovipositor and lay eggs on the floor of their underground chamber. A female brings in plant material and then remains with the hatched larvae in the burrow until she dies, i.e., when they have reached the second or third instar (Alexander 1961, West and Alexander 1963). Mole crickets, which are even more adapted to subterranean life, have no ovipositor at all. Gryllotalpa gryllotalpa lays her eggs into a nest the size of a hen's egg which has firm, smooth walls (Harz 1960). The female guards the nest, licks the eggs from

W. Loher and M. Dambach

80

2c

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Fig. 2.12 Modes of egg deposition in six American Oecanthus tree crickets. 1, 0. etcclamationis egg in oak wood. 2, 0. niveus (as 0. angustipennis): a, egg punctures and egg positions in apple wood; b, egg punctures in bark (pl, healed over puncture from previous year; p2, recent egg puncture vvithout egg plug; p3, recent egg puncture with plug); c, enlarged longitudinal section of bark and wood, showing egg and plug. 3, 0. quadripunctatus: a, egg punctures in wild carrot; b, enlarged section of same. 4, 0. pini: a, egg punctures in pine; b, enlarged section of same. 5, 0. nigricornis: a, egg punctures in raspberry; b, enlarged section of same. 6, 0. latipennis: a, egg puncture in grape; b, enlarged section showing eggs in goldenrod. All longitudinal sections in the bottom row have the same scale. (After Fulton 1915.)

time to time, and cares for the larvae until their second molt. On the other hand, Brachytrupes achatinus has a short ovipositor with which she thrusts the eggs into the soil at the end of the burrow (Ghosh 1912). Mter hatching, larvae and adult stay together for only 2-3 d. Females of Myrmecophilinae, which live as inquilines in ant nests and also have minute

Reproductive Behavior

81

ovipositors, lay few, but large, eggs directly in the nest. The eggs pass down an ovipositor that is considerably modified. The dorsal valvulae have a lobe-shaped extension vvith numerous filiform hairs, which in Myrmecophilus americanus are spread over the whole surface (Cappe de Baillon 1922), whereas in M. acevorum the hairs cover only the ovipositor tip (Schimmer 1909). Apparently these ovipositors function as tactile sensory organs, similar to cerci. A correlation is thus emerging among brood care, changes in egg-laying behavior, and modification of external oviposition apparatus. Conclusions This chapter shows the amazing diversity of behavior patterns and strategies, all geared to achieve reproductive success in the face of evolutionary pressures to which crickets had, variously, to adapt. While most insect orders and families display an array of sexual expressions, behavioral analysis in crickets has the advantage of dealing with a mainly acoustic group whose calling songs allow reliable classification and where song emission accurately reflects the behavioral and physiological state of the male and gives the experimenter a tool to acoustically manipulate both sexes. This is especially important for evaluating responses of the silent males. Although acoustic information transfer is the primary mode of cricket communication, other means have been shown to lead to reproductive success. It is therefore imperative to investigate the conditions under which sound production and reception have been lost and replaced with other means of intraspecific communication. These include chemical, vibratory, and visual signals. Such mechanisms, which have developed at the expense of, or in addition to, the acoustic mode, are still largely unexplored. As to behavioral complexity, crickets are mostly solitary and have not reached the intricate sophistication of social insects. However, in terms of parent-offspring relations, some burrowing crickets have come a long way: spending much time in a subterranean, altered environment, provisioning a burrow with edible material and defending it, and providing brood care for the first lmvaJ stages as exemplified by Anurogryllus arboreus females. These adaptations parallel evolutionarily many of the elaborate behavioral expressions common in social insects (Alexander 1961). In general, cricket behavior lends itself well to studies of contemporary problems in ethology, and ecologists and physiologists have contributed with an enormous amount of data, posing and answering why and how. Nevertheless, research on the evolutionary origin of cricket sexual behavior in its multifaceted expressions, and the many functional mecha-

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W. Loher and M. Dambach

nisms that have been shaped to execute these expressions, requires continuing discussion and collaboration in field and laboratory research.

Acknowledgments We are grateful to Thomas Moore, Thomas Walker, and Rex Dadd for critically reviewing the manuscript. We thank Christina Jordan for her expert drawings of Figures 2.1 and 2.6. This work has been supported by NIH Grant R01-HD-03619.

CHAPTER THREE

Temporal Organization of Reproductive Behavior WemerLoher

Throughout their evolution crickets have been exposed to daily alternations of light and darkness and accompanying changes in temperature and humidity. These are a consequence of the earth's position relative to the sun and its rotation around its longitudinal axis. The resulting 24-h periodicity has shaped the behavior and physiology of crickets such that they have developed a rhythmicity similar to that of the environment. The reproductive behavior of the Australian field cricket, Teleogryllus commodus, for example, demonstrates how the various behavioral activities involved are organized in time (Loher and Renee 1978). At dusk a mature male standing at the entrance of his burrow emits a loud calling song, often for hours without interruption. During stridulation the male constitutes a stationary sound source, to which receptive females are attracted. On arrival of a female mutual antenna! contact ensures sex-identification (Renee and Lober 1977) and after a brief courtship mating proceeds, during which a spermatophore is transferred to the female's genitalia. The postmating period is characterized by the male guarding the female, which serves to prevent her from eating the spermatophore before its contents have fully migrated into her spermatheca. He also drives away challenging males and keeps her for further matings. During the guarding phase the female's behavior gradually changes; her locomotory activity slows down, and the calls from nearby males are no longer attractive. Yet courtship from her mate is still effective, and several matings may take place between the same pair. Field studies have shown that mated females usually lay their eggs shortly after mating in or near the male's burrow (Evans 1983). From the above scenario it is evident that some behavior patterns must appear at a certain time to satisfY the following conditions: Within an individual mutually exclusive patterns have to be temporally separated so as not to interfere with one another, whereas cooperative patterns must 83

W. Loher

84

be released in concert or sequence to make functional sense. Between the individuals of opposite sex complementary patterns instrumental in bringing partners together for mating should be released at about the same time. In short, it seems that behavior patterns are displayed at a time that is both safe and optimally beneficial to the insects concerned. These criteria are met through the operation of an endogenous "clock" mechanism that oscillates at a period of approximately 24 h and rhythmically releases these behaviors at the "right time." Such rhythms are called circadian (from circa, "about," and dies, "day"). Since most insect clocks do not run at an exact 24-h period, they have to be synchronized with the environmental cycle. The most prominent entraining agents, or zeitgebers, are the time cues of dusk and dawn, light-dark and dark-light transition, and the periodic alternation of temperature. A stable phase angle relation is then established between the phase of the environment and that of the behavior concerned. That a rhythm entraining to the light -dark (LD) cycle is indeed regulated by an internal clock mechanism can be proven by exposing a cricket to continuous light of constant intensity (LL) or continuous darkness (DD) and eliminating all other possible zeitgebers. In such a situation a clock will "freerun" at its own period, which is somewhat longer or shorter than 24 h, as demonstrated by the time course of the behavioral event (Aschoff 1960). In contrast, daily periodicities of behavior that depend on and are imposed by regular environmental fluctuations disappear when the external conditions are kept constant. In the first part of this chapter, experimental evidence is given for the circadian nature and influence of low temperatures on premating ·behavior; there then follows a comparison of singing activity under laboratory and field conditions. It is further shown that the mating act itself releases a temporally organized sequence of postcopulatory patterns, of which one component is possibly circadian. Oviposition is demonstrated to be under circadian control as well. In the second part, site and nature of the clock mechanism in crickets and in related insects are discussed. Temporal Arrangement or Behavior Circadian Coordination of Premating Behavior In the male premating behavior of Teleogryllus commodus three patterns have to interact: calling, walking, and spermatophore formation (considered here as a behavioral event). Calling and male walking are mutually exclusive and ideally should be displayed in opposite parts of an LD cycle. The incompatibility ofthe two behaviors is further suggested by the observations that calling males do not move around, and that walking males do not emit the calling song. In T. commodus the production of a spermatophore has to be finished before a male begins to sing so that he is ready for mating should a female be acoustically lured to his burrow.

Temporal Organization of Reproductive Behavior

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Calling and male locomotory activity. In a laboratory environment with a 12:12 LD cycle acoustically isolated males begin to call3-1 h before the light -dark transition, and then stridulatory activity continues throughout most of the night. The abrupt light -dark change may inhibit singing for 5-50 min, whereas a dark-light change sometimes revives short, sporadic bouts of stridulation (Figs. 3.1 C and 3.2). The LD transition from the previous day provides the cue for resetting the clock, which then releases singing before the next onset of darkness (Renee 1976). Constant environmental conditions provoke freerunning activity, and its period is measured as T. In LL, single males exhibit a free running calling pattern at T = 25.3 ± 0.5 h (Fig. 3.1 A), and in DD at T = 23.5 ± 0.15 h (Fig. 3.1 B). In all cases so far investigated individual differences are evident, but all T LL values are longer and all Ton values are shorter than 24 h (Loher 1972). Gryllus campestris males show three qualitatively different singing patterns in LD: they are light-active, dark-active, or both. When LL follows LD, the light-active males freerun, but their rhythm experiences a shift of180°, so that the activity is cophasic with the subjective night. The dark-active and the light-and-dark-active males start singing in LL from their darkactivity phases exhibited during LD. The durations of Tu. and T 00 are

86

W. Loher

,_

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Double-plotted simultaneous recordings of calling and walking rhythms of one Teleogryllus commodus male under two laboratory light conditions. In LD 12:12 (upper 17 rows of recordings for each behavior) the two behaviors are essentially antiphasic, but some alternations do occur in both parts of the cycle. In LL (lower 18 rows of recordings for each behavior) the two activities freerun with the same period. Light regimes for 48-h periods of recordings diagrammed above, showing number of rows of recordings at that regime; each row progresses 1 d forward, so the left record in each double-plotted row is identical to the right record from the row above, thus revealing the progression of activity through 24-h periods; row 18 of recordings indicated by dashes at left.

Fig. 3.2

usually longer and shorter than 24 h, respectively, and the light -active singers remain silent during the night. Honegger (1981a) suggested that daytime calling developed from night activity. ln field studies the natural distribution of calling activity in G. campestris has been confirmed (Rost and Honegger 19871.

Temporal Organization of Reproductive Behavior

87

The mutually exclusive character of calling and walking in T. commodus males becomes apparent when the two behaviors are monitored from the same cricket (Fig. 3.2). In LD, calling begins 3-1 h before light-out, and most singing happens in the dark part of the cycle, except during the last 2 h. Walking1 in contrast1 is predominant during the hours oflight and the last 2 h of the night, thus following the indicated temporal separation of the two behaviors. It is, however, also obvious that during both parts of the cycle, singing and walking do interdigitate to some extent, and then short bouts of both activities occur in alternation, but not at the same time during the same phase. In LL, both rhythms freerun; singing begins shifting from where it started during the last LD cycle; the walking rhythm splits into two components that can be traced back to the portions of previous day activity and the last 2 h of night walking (Fig. 3.2). Spontaneous splitting is rare, and usually walking in LL is expressed as a uniform band of activity. But these so-called atypical patterns provide information concerning the existence of multiple clocks, as outlined below. A comparison of singing and walking periods shows that they are similar (T = 25.5 h) thus corroborating earlier findings on T. commodus (Sokolove 1975). In males of G. bimaculatus, LD 12:12 entrainment releases locomotor activity in D with a major peak immediately following the LD transition. In LL and DD, T values of 25.49 ± 0.49 hand 23.91 ± 0.23 h, respectively, were obtained (Tomioka and Chiba 1982a). Males of Acheta domesticus in LD 12:12 walked either right from the onset of darkness, began walking up to 3 h later, or walked before the LD transition (Cymborowski 1973). In DD, these essentially night-active males exhibited locomotory periods that were either longer or shorter than 24 h. Recordings in LL have not been tried (Cymborowski 1981). The actograms in Figure 3.2 show the amount of calling and walking activity to be approximately equal in aT. commodus male, but this is by no means always so. Some males stridulate during a large portion of a 24-h day and walk very little. In contrast, a male may call for only 2-3 h and then locomotor activity is overwhelmingly in evidence during the rest of the cycle (Sokolove 1975). It seems that a balance between the amount of singing and walking is maintained, and that as soon as one activity is extensive the other shortens, suggesting the availability of only a limited amount of energy per 24 h which is divided between these two main behaviors. This interpretation presupposes that energy expenditure during the execution of walking and calling is about equal. Metabolic studies on the house cricket seem to support such a view: the metabolic rate of a male at rest is 3.65 ± 1.1 cal · g- 1 · h·\ it rises equally during walking or singing by a factor of 5-6 times (B. Renee, unpublished; see chapter 2, this volume). A comparison of these two activities from the same insect makes terms such as diurnal, crepuscular, or nocturnal obsolete. A cricket that performs various kinds of locomotor activity during most of the day and

88

W. Lober

continues to sing at dusk and through a good part of the night is clearly all three.

Spermatophore formation and calling. Emission of the calling song signals the ability to mate and the possession of a spermatophore in most gryllines. Regardless of whether spermatophore production takes place when the male is alone or in the company of a female, the behavioral procedure always consists of the same steps, displayed in the following sequential order: (1) the abdominal tip is pressed vertically against the ground; (2) a few minutes later the boat-shaped ninth abdominal sternite is lowered, while between the two erect ventral phallic lobes a white mass appears, soft and rodlike, which is squeezed from the ejaculatory duct; (3) the new spermatophore protrudes well beyond the abdominal tip, adopting within the next 15 min its final droplike shape and beginning to solidifY; (4) 30 min after protrusion of the spermatophore it is actively withdrawn and disappears slowly in the phallic cavity, where it is stored until used (Lober and Renee 1978). How, then, is spermatophore formation connected with the calling song'? Since the regular onset of the calling periods offers stable phase reference points to which presence or absence of a spermatophore could be related, isolated crickets were inspected for spermatophores. This was done at irregular intervals, one to three times a day, interspersed with days where no inspections were made so as not to introduce artificial periodicities. The results from many insects were pooled to obtain indications of temporal relation between spermatophore presence and singing. Figure 3.3 summarizes the data obtained from 105 males exposed to LL, DD, or LD. At the onset of stridulation about 95% of the males carried a spermatophore, and 4 h before that time 75-82% possessed one. Prior to 4 h preceding calling, fewer than 50% of the males had spermatophores. During the 4 h after the singing period, only about 40% still kept a spermatophore, while the other males must have lost it. The histograms summarize the same data and re-emphasize the rhythmicity of spermatophore production in all three light regimes. Spermatophore production always preceded singing by 1-4 h, and the two behaviors occurred in sequence. The majority of isolated males (83%) produced only one spermatophore per 24 h. However, stridulation is not absolutely contingent on the presence of a spermatophore, once it is formed. Spermatophore removal during calling does silence the male for a few minutes, but then stridulation continues and no spermatophore is produced until the next calling period, which may be 20 h later. Similarly, preventing the production of a spermatophore by removing the accessory glands or cutting the abdominal nerve cord does not inteifere with the periodicity of calling. Thus a clock mechanism links two autonomous processes into a functionally meaningful sequence (Lober 1974). Female walking and male calling. In T. commodus females sexual behavior and rhythmicity in walking appear between the third and seventh day after the imaginal molt, although in special circumstances mat-

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fig. 3.3 Possession of spermatophores in Teleogryllus commodus males (N=105) under varying laboratory light conditions. A, In LL. B, In LL also, but the ommatidial ne1ves bet\,yeen the compound eyes and their respective lobes had been cut, thereby surgically blinding these males. C, In LD 12:12. Light regime and hours diagrammed above; shaded area, stridulation times; percentages in A show relative numbers of males with spermatophores 4 h before and after the singing period (slanting lines) and for 8 h before the slanting line on the left; slanting lines in B mark 4-h periods before and after the singing period; closed circle, individual with spermatophore present; open circle, individual with spermatophore absent. Corresponding histograms summarize the same data, by 2-h sampling periods, and show percentages of spermatophores present during the freerunning periods (A, Bl and during a 24-h cycle in steps of 2 h (C); percentages ;vith spermatophores present also shown below C by 2-h sampling periods. (Modified vvith permission from Journal of Insect Physiology, vol. 20, W. Loher, © 1974, Pergamon Journals Ltd.)

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W. Loher

ing with older males can occur at the age of 4 h, when the couple is in a confined space (Loher and Edson 1973). One of the main functions of female locomotion is to find a male for mating, and his calling provides directionality to reach that goal. Figure 3.4 shows a representative actogram of female locomotor activity in different light regimes. Whereas -r 00 = 23.5 ± 0.32 hand TLL = 25.25 ± 0.32 h, a detailed study oflocomotion in LD revealed three types of walking rhythms: 1. Most or all activity was restricted to the dark part of the cycle, beginning immediately with the LD transition (28 females). 2. The main activity was concentrated in darkness, but sporadic bouts appeared during the light part of the cycle (17 females). 3. Activity was clearly bimodally distributed, with peaks observed in both parts of the cycle (4 females). If the main function of premating behavior is to promote mate finding, then a strong temporal relation between its two major features, male calling and female walking, should be expected. Both behaviors do in fact occur predominantly during darkness, even when crickets are separated and in acoustic isolation. The temporal linkage is further confirmed by comparing the endogenous rhythmicity of the two patterns. Nohvithstanding individual variability, the periods are surprisingly similar. As to the chances that those few females exhibiting part oftheirwalking activity during daytime might hear a singing male, a survey of the temporal distribution of calling under laboratory conditions in LD from 469 males showed that 91.5% of the males followed the common stridulatory schedule, beginning 3-1 h before the onset of darkness and singing during the night. Only 1.5% called exclusively during the day, and 7% of the crickets sang several hours during the day and also during the night. These data indicate that day-walking females may find day-singing males (Loher 1979a). Similar reflections are valid for G. campestris, where males in a population sing either exclusively during the night or day or during both pa11s of the cycle. Day-singing males seem to have a better chance to attract females, because within their preferred time slot for singing they would have to compete with only a limited number of callers (Honegger 1981a), although they run greater risks from day-active predators and parasites. According to a large field study in which both female walking and male calling of G. campestris were directly recorded, the timing of these two activities did not coincide (Rost and Honegger 1987): whereas males called either during the night or day or during both parts of the cycle, female walking occurred mostly during the light hours, and all copulations happened then, preceded by courtship activity. This discrepancy can be explained if one considers the low nightly temperatures, which dropped from ca. 16°C at dusk to 13°C at 2300 and further to 10° after 0200 in the morning. At such low temperatures females no longer walk and are no longer phonotactically attracted to singing males for matings. On the other hand it must be surmised that calling in males produces enough

Temporal Organization of Reproductive Behavior

91

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What causes the rise of JH concentration in mated females? Four groups of females of the same age were treated as follows: two groups were mated and two remained virgin; one virgin and one mated group received sand for oviposition, whereas the other two groups did not. Females without sand as substrate laid very few eggs. Eggs were collected daily for 1 wk, and at the end ofthe experiment hemolymph was collected from each group, and the eggs that were stored in the ovaries were counted. The group of mated females on sand produced about twice as many eggs as each of the other three groups (Fig. 4.10), whose egg output did not significantly differ among the groups. JH concentration in the hemolymph of the mated group with sand substrate was five times as high as in the mated group that had been denied oviposition. But the JH titer of the mated egg-laying group was seven times as high as that of the'virgin group that lacked spermathecal prostaglandins stimulating egg release, and even 20 times as high as in the virgin group that had also been deprived of sand (Loher et al. 1987). In conclusion, females need both insemination and an appropriate egg-laying substrate to empty their ovaries of stored eggs; only then are theCA stimulated into full JH synthesis. Ovaries storing eggs impede JH production. The results of nerve transections, ovariectomy, and allatectomy suggest a humoral feedback mechanism between the CA and the ovaries, with two as yet unknown factors involved. Both are possibly derived from the ovarial tissues: one may inhibit the CA, whereas the other could have a stimulatmy influence.

Effect ofjuvenile hormone Ill on phonotaxis and sexual receptivity. A sexually mature female cricket is phonotactically attracted by the calling song of a conspecific male and moves toward the sound source (see Chapters 10 and 11). Her copulatory readiness is indicated by mounting

140

W. Loher and M.D. Zaretsky T. commodus:

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Structure and Function of the Endocrine System

141

the male and engaging in copulation. It has been proposed that, inAcheta domesticus, the female's receptivity and degree of directionality in her approach are under the influence of the CA. When supernumerary CA from receptive donors were implanted in unreceptive females, they became receptive within 1 d after the transplantation. Further work showed that topical application of either JH III or methoprene, a synthetic juvenoid, on 1-d-old immature females enhanced sexual maturation as indicated by copulatory readiness and phonotactic directionality on days 2 and 3. Ordinarily, these behaviors only appear 4 d after the imaginal molt (Stout et al. 1976). A more compelling prooffor the active participation of the CA in sexual behavior was furnished by allatectomy in sexually responsive females: angular orientation and directness in the phonotactic response, and also sexual receptivity, declined within the next 10 d but reappeared when JH III was topically applied. However, receptivity and phonotaxis do not always match (Koudele et al. 1987). Despite these findings, allatectomy does not always abolish receptivity in females, but longer courtship sequences may be necessary to induce mating in allatectomized individuals than in controls (Renucci et al. 1985). The reasons for these discrepancies in A. domesticus are unknown.

Control of pigmentation. Body coloration of larval and adult Gryllus bimaculatus is determined by a black and a yellow pigment. Implantation of extra CA into penultimate- or last-instar larvae induces a breakdown of cuticular melanin after the next molt, and last-instar larvae or adults become light-colored. On the other hand, allatectomy during the last two larvel ins tars leads to black body coloration following another molt (Roussel1966). Ecdysteroids In the larval stages of crickets, the prothoracic gland (PTG) is the principal source of ecdysteroid hormones. It is still unclear whether the gland produces 20-hydroxy-ecdysone (20-0H-ECD) and ecdysone (ECDJ or whether 20-0H-ECD is synthesized from ECD in other tissues. At the time of the imaginal molt the PTG atrophies and ecdysteroid synthesis is primarily taken over by the ovaries and possibly by the testes (Hagedorn 1983). Ecdysteroids are sometimes conjugated to more polar moieties, and such compounds are practically inactive (Hoffmann and Hetru 1983, Bulenda et al. 1986).

Occurrence ofecdysteroids in preimaginal stages. In freshly laid eggs of Acheta domesticus, up to 97% of the ecdysteroids are biologically inactive conjugates (Renucci and Strambi 1981), a fact that has been confirmed by Whiting and Dinan (1988). In Gryllus bimaculatus, according to Hoffmann et al. (1981), large amounts of free ecdysteroids are found in the eggs, but conjugates have not been measured. In that species eggs laid by 12- and 23-d-old females contain the highest concentrations of ECD

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W. Loher and M. D. Zaretsky

and 20-0H-ECD, which coincides with the highest ovarian ecdysteroid titer at that time. It is not known whether these hormones have a function at that early stage, and no information is available on their concentrations during later embryogenesis. In locusts, for instance, peak titers of free ecdysteroids in embryos are correlated with cuticle deposition (Lagueux et al. 1979). Body extracts from third-instar larvae (that stage lasts 75-80 h) of G. bimaculatus yielded two peak concentrations of ECD and 20-0H-ECD. The first maximum showed values of 300 ng-equiv. of ECD and 650 ngequiv. of20-0H-ECD, and the second 500 and 1300 ng-equiv., respectively. These peak ecdysteroid levels were correlated with DNA synthesis in several tissues. Incorporation of thymidine into cell nuclei was correlated with the second ecdysteroid peak, but no causal relationship could be observed (Romer and Eisenbeis 1983). In locust larvae ECD is the only hormone synthesized in the ventral glands, which are homologous to the PTG in crickets (Him et al. 1979); it is then converted to 20-0H-ECD in the fat body and in the Malpighian tubules (Feyereisen 1977).

Ecdysteroid titers and reproduction. Acheta domesticus females synthesize free ecdysteroids in their ovaries. After ovariectomy these hormones disappear from the hemolymph, suggesting that the ovaries are the sole site of synthesis (Renucci and Strambi 1981). Analysis of ovaries and hemolymph show that their ecdysteroid titers run in parallel and relate to oocyte development as follows: growth from the previtellogenic oocyte to the final egg size takes 72 h, and the eggs from the first set are laid 112 h after ecdysis. However, ecdysteroid synthesis begins only at 48 h (i.e., at a time when the terminal oocytes are already half-grown). By 80 h the titers rise to 38 ng/ovary and 43 ng/100 j.Ll hemolymph. When oviposition approaches, the titers decline rapidly, but rise again after egg laying (Renucci and Strambi 1981) .It is therefore quite clear that ecdysteroids are not responsible for the appearance of vitellogenins in the hemolymph, since they were already present before ecdysteroid synthesis had begun. These results agree with earlier findings that in the absence of the ovary and its steroids, vitellogenesis takes place unimpeded (Bradley and Edwards 1978). Undoubtedly then, vitellogenin synthesis is directly related to JH synthesis, which already in the previtellogenic stage of the terminal oocytes experiences an initial boost as discussed earlier (Renucci and Strambi 1983). The ratio between free and conjugated ecdysteroids changes according to age of adult A. domesticus females. On the first day after ecdysis, free ecdysteroids make up 65% of the total; by oviposition time on day 5 they have declined to 18%, and the percentage of the conjugated form rises proportionally (Renucci and Strambi 1981). These mutual changes suggest an efficient regulatory mechanism: according to the biological state of a cricket, titer changes can be made by converting biologically active free ecdysteroids into the inactive conjugated form, and vice versa. In view of

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the cricket's inability to synthesize cholesterot the precursor of ecdysteroids, a conversion seems economical, because it precludes titer regulation by excretion and the effort to produce new hormones, should high levels be required. There may be still other modes of titer regulation. Ecdysteroid titers have been measured in ovaries, hemolymph, fat body, muscles, and in whole-body extracts during the entire adult lifespan of G. bimaculatus (Hoffmann et al. 1981). Radiolabeled ECD injected into crickets accumulated preferentially in the ovaries (Bulenda et al. 1986). In general, the fluctuations of free ecdysteroids in these tissues run parallel and reach their highest concentrations at the time of heaviest ovarian weight (Hoffmann et al. 1981), whereas low levels in whole-body extracts have been observed before and during egg laying (Shabab and Romer 1982). Body extracts from G. bimaculatus males revealed relatively low hormonal levels of ca. ZOO ng/g equivalents of ecdysteroids or less, with two notable exceptions. First, on days 8 and 12 after ecdysis the titer soared to 2900 and 3370 ng/g equivalents, respectively, and these peaks lasted for only 1 d each (Shabab and Romer 1982). Second, similar values, but on days 10 and 20, were found for extracts of testes, fat body, muscles, and abdomens (Hoffmann and Behrens 19821. The significance of these sudden hormonal surges is unknown. Further studies with G. bimaculatus females have shown an important correlation between ecdysteroid production and environmental temperature, similar to that in JH production. Crickets were reared at a temperature cycle of Z4:1Z°C (16:8 hJ vs. a constant Z0°C, both in an LD 16:8 photoperiod. The free ecdysteroid titer in ovaries and fat body from crickets exposed to periodic temperatures was five times as high and reached its maximum 10 d earlier than from crickets held at constant temperature. Maximum values were obtained, when the ovarian weight and the oviposition rate were highest. The accelerated and increased fecundity combined with ecdysteroid peaks suggest an involvement of these hormones in egg grov.rth and development. That possibility was strengthened by results of ecdysteroid injections eve1y other day into females exposed to constant temperature during the first 20 d of imaginal life. Doses of Z or 5 fLglinjection caused a significant accelerated increase in ovarian fresh weight and oviposition rate (Behrens and Hoffmann 1983). The question of whether ecdysteroids act independently or via theCA has been addressed in studies ofAcheta domesticus, where injections of 20-0H-ECD into newly emerged adults increased both the growth of terminal oocytes and the number of eggs laid by virgin and mated females. Allatectomy during the last larval instar and injection of 20-0H-ECD into the adult did cause some oocyte growth beyond the previlellogenic stage (Chudakova et al. 1982). Participation of ecdysteroids in oogenesis of Teleogryllus commodus would also agree with results that show that females have large quantities of ECD and 20-0H-ECD in the ovaries and hemolymph, and although allatectomized females contain no JH III in the

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body they can still produce ca. 20% of a normal egg load (W. Lober, unpublished). In G. bimaculatus males neither temperature fluctuations nor ecdysteroid injections increased the rate of spermatophore production (Behrens and Hoffmann 1983). This may mean that ecdysteroids have nothing to do with the reproductive physiology of males, but the role of enormous amounts of ecdysteroids in the testes still must be explained (Hoffmann and Behrens 1982). Ultrastructural work, bioassays, tissue extractions, and results from in vitro cultures have implicated the oenocytes of G. bimaculatus as an additional ecdysteroid source (Romer 1974, Romer et al. 1974, Romer and Eisenbeis 1983). But at least in A. domesticus females the reproductive organs must be the only site of ecdysteroid synthesis because after ovariectomy these hormones disappear (Renucci and Strambi 1981). In light of some apparent striking physiological differences between closely related species, more data are necessary from one and the same species, before comparisons become meaningful.

Conclusions One of the prerequisites for understanding the mechanisms by which neurosecretory cells (NSCs) control different functions is their morphological and physiological identification, so that their inputs and outputs can be studied in many specimens and from several species. The large number of NSCs in the brain of crickets and other orthopteran insects limits the achievement of such goals. Nevertheless, anatomical studies on single brain NSCs have yielded both diversity and similarity in their morphology, which may reflect functional specificity (Zaretsky and Lober 1983). The large number of medial NSCs implies an organization into functional units consisting of several cells interacting with each other. Other functional units might consist of neighboring NSCs with very similar morphologies, such as those found in Teleogryllus commodus among medial NSCs with axons joining the NCC I (M.D. Zaretsky, unpublished). Such cells might not interact with one another, but could serve as parallel channels of release of neurohormones. The evidence available suggesting the sites of neurohormonal release indicates that the axons of the medial and lateral NSCs that leave the brain do so in the separate tracts of NCC I and NCC II, respectively. Axons of the medial NSCs project to the anterior CC and to the esophageal nerve ofT. commodus, and together with those from the lateral NSCs they innervate the CA (Moore and Lober 1988). In Acheta domesticus only the lateral NSCs have been implicated as innervating theCA (Strambi et al. 1986), as in Schistocerca (Strong 1965a, 1965b; Mason 1973). Neurosecretory cells may release their products into the CNS, where they could modulate the activity of nonsecretory neurons or of other

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NSCs. Local NSCs, observed in both the PI and the tritocerebrum, undoubtedly release neurohormones into the brain itself. The ultrastructurally defined deutocerebral neurohemal area in a cricket brain is another site for neurohormonal release by cerebral NSCs (Geldiay and Edwards 1973, Geldiay and Karcali 1980). The loop cells ofthe PI (Zaretsky and Loher 1983) and the PNCs of the subesophageal ganglion (Weinbormair et al. 1975) both have significant arborizations in the ventromedial deutocerebrum to release their neurohormones directly into the hemolymph, as might other NSCs. There are ultrastructural and cytological correlations among the MNSCs, for example, the presence oflarge spherical nuclei in type I cells that also contain large droplets formed by the aggregation of neurosecretory granules in high concentration. These observations suggest further experiments to elucidate the mechanism by which neurohormones are synthesized. Another prerequisite for understanding the control of functions by NSCs is the chemical identification of their neurohormones. Immunohistochemical methods and radioimmunoassays are both powerful analytical tools for identification of NSCs and their products, as has been successfully demonstrated in studies on proctolinergic cells of Periplaneta americana (Bishop and O'Shea 1982). But our knowledge of the cellular and network physiology of NSCs in crickets has not kept pace with anatomical and chemical studies, incomplete as they are. We still lack information on sensory and central input on neurosecretory cells and their target organs, particularly in relation to behavior. Only combined anatomical, chemical, and electrophysiological studies will permit an insight into the functional role of the cerebral neurosecretory system during molting, reproduction, or sexual behavior. But in order to combine brain function with these events, its connection and correlation with the CA, the PTG, the gonads, and their corresponding hormones have to be established. In all except the last larval stages of crickets, juvenile hormone (JH) III and ecdysteroids are both active at the same time, and the primary function of JH III is to retain lmval status. Attempts to further elucidate the role of JH III in larvae by means of allatectomy early or late in the seventh instar have led empirically to reproducible results. The outcome was, however, completely unpredictable when allatectomy was performed at midpoint of the ins tar, and a spectrum of anatomical, physiological, and behavioral changes was obtained. These results might have been associated with individual differences in rates of JH synthesis, JH esterase activity, and affinity changes of JH binding proteins, resulting in a differential JH: ecdysteroid ratio. The production of some eggs by females of T. commodus allatectomized during the last lmval instar deserves comment. These results are inconsistent with the current dogma that in orthopterans the CA are indispensible for egg production (Engelmann 1983). Since whole-body

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extracts from allatectomized females do not contain JH CW. Loher, unpublished), one explanation could be that ecdysteroids are involved and are responsible for the production of ca. 20% of a normal egg load found in allatectomized females. If one considers the simultaneous action of these two hormones during juvenile development, then a further cooperation in the adult stage to assure egg production seems to be merely the variation of a common theme. Ecdysteroids have been found in many tissues of Gryllus bimaculatus, especially in the ovaries (Hoffrnann et al. 1981, Bulenda et al. 1986), and repeated injections of ecdysteroids increased both egg mass and oviposition rate (Behrens and Hoffmann 1983). Likewise, the ovaries of T. commodus also contain these hormones (W. Loher, unpublished). In spite of possible involvement of ecdysteroids in egg production of crickets, the main responsibility for that function rests still with JH III. However, no complicated time schedules are expected, such as switch-on and switch-off mechanisms ofthe CA, as is the case in locusts and certain cockroaches (Tobe 1980), because in all likelihood these glands are never turned off. This can be deduced from the asynchrony in oocyte growth, suggesting continual hormonal presence, and from the daily oviposition rate. Crickets have, therefore, a simpler type of reproduction biology than more specialized orthopterans, and crickets may reveal patterns of nervous and endocrine organization that are typical for many hemimetabolous species.

Acknowledgments We thank John Edwards and Thomas E. Moore for a critical review of the manuscript. This work has been supported by NIH Grant R01 HD 03619.

CHAPTER FIVE

Vision and Visually Guided Behavior Hans-Willi Honegger and Raymond Campan

The wealth of knowledge about auditory sense and associated acoustic behavior in crickets contrasts with the scarce information on their vision and visually guided behavior. Although many cricket species possess welldeveloped compound eyes and ocelli, their role in behavior has been neglected. Except for the importance of light received through the compound eyes which acts as one of the zeitgebers for circadian rhythms (see chapter 3), knowledge of cricket vision limps far behind that of the welldocumented form, color, and movement perception capabilities of honeybees, flies, and ants. This chapter summarizes what is presently known of cricket vision and its role in nature. We discuss crickets' evaluation of objects such as dark borderlines and deal with aspects of visual acuity, shape discrimination, and specific strategies used in following objects with the antennae. We then review the structure and function of the compound eyes, the optic lobes (lamina, medulla, and lobula), and some of their projections to the central brain in the processing of visual information and include recent study results on polarized light and color perception. Visually Guided Behavior in the Field Negative Phototaxis Many cricket species avoid bright light and are active only at dusk and at night. Acheta domesticus, for example, is essentially nocturnal (Lutz 1932) and spends the day hidden under a shelter, a tendency oflarvae and females more than of males (Remmert 1960, Fraenkel and Gunn 1961). Federhen (1955) stressed the point that A. domesticus is guided by negative phototaxis in searching for resting sites. Moreover, negative phototaxis is coupled with thigmotaxis in finding suitable shelter (Kieruzel 1976). In the field many cricket species prefer holes or caves with specific 147

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humidity levels (Chopard 1938). Chao (1975) reported that freshly hatched larvae of the two Chinese mole crickets, Gryllotalpa unispina and G. af ricana, flee the sun immediately. It is surprising, therefore, that one can catch crickets in light traps-for instance, Nemobius sylvestris in France with white light (Chopard 1938) or the mole crickets Scapteriscus acletus and S. vicinus in Florida and Gryllotalpa unispina and G. africana in China with ultraviolet light (Ulagaraj, 1975, Chao 1975). In Florida trapping of mole crickets was more successful when ultraviolet light was combined with sound (Beugnon 1981). Ocelli and Phototaxis Early experiments on Gryllus bimaculatus led Jander and Barry (1968) to the hypothesis that phototactic behavior is partially controlled by the ocelli. In their model the median ocellus is thought to measure the brightness of light, and the two lateral ocelli are considered to act as antagonists or synergists to the compound eyes in phototropotactic orientation. But the ocelli may even evaluate the horizon, as has been shown in other insects. Borders as Visual Cues Adults and juveniles of Nemobius sylvestris live among leaf litter in oak and pine tree forests of the temperate region in the Old World. Their patchy distribution and density is higher along forest borders and paths than within the forest (Morvan et al. 1976, 1977, 1978). Movements of marked crickets have revealed daily migrations between forest borders (preferred habitat early in the day) and open fields close by (preferred habitat during afternoon and evening) (Lacoste et al. 1976, Beugnon 1980). These observations have guided field experiments summarized by Campan et al. (1987), as reported below. N. sylvestris adults were released individually during the day from the center of a circular platform positioned some meters away from a forest border, and their walking courses were recorded (Beugnon 1983, Beugnon et al. 1983). As shown in Figure 5.1 A, the main walking course of 100 individuals was toward the west when the sun was visible and the tree line at the capture site was oriented in a north-south direction. When the san1c individuals were again put on the platform, now 20 rn away from the former forest border but closer to a tree line oriented in a south-east direction, the crickets moved toward this closest border (Fig. 5.1 B), which was now the area reflecting the least light. N. sylvestris adults captured SO km away from the experimental area and inside a forest were released on the platform facing the north-south border of the same forest from the east. Like residents, the experimentally introduced crickets also moved toward the west (Fig. 5.1 C). This finding emphasizes that the closest forest border is chosen, even by crickets that may never have encountered a marked borderline before. This orientation behavior remained basically the same, although with

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an increased individual scatter when visible terrestrial cues were obscured by a cylindrical wall around the platform and only blue sky was visible. Resident crickets walked toward the west (Fig. 5.1 D), but nonresident crickets from a location other than the release point walked randomly (Fig. 5.1 E). When blue sky was obscured by clouds or by artificial cover and terrestrial landmarks were abolished, residential crickets also failed to orient themselves. These results suggest the following about N. sylvestris adults: (1) they are visually guided by the closest tree border with the lowest reflection of light; (2) no local knowledge of the habitat is required because residents and nonresidents orient similarly; and (3) resident crickets remember direction and also use celestial cues, such as the pattern of polarized light, when direct vision of sun and trees is obscured; crickets not familiar with the habitat walk randomly under these circumstances. Celestial orienta-

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tion is time-compensated: it takes into account the different positions of the sun and the related change of polarized light pattern throughout the day (Campau et al. 1975, Lacoste et al. 1976). These observations demonstrate that crickets use a combination of cues for visual orientation similar to that described in other insects (Wehner 1981). Development of Visual Orientation Three-week-old larvae of N. sylvestris return to the forest border as accurately as adults that must be 1-2 y old (Beugnon et al. 1983). Although the larvae already use celestial cues, each individual has its own preference of route to the forest; but if only celestial cues are visible, each larva displays an individual compass-direction preference, only a few of which lead to the forest. However, 9-wk-old resident larvae preferred a distinct common compass direction, with minor individual differences. This finding is strong evidence that details of the habitat are learned and that celestial cues are used by residents; it is confirmed by the fact that even laboratory-reared 5-mo-old larvae do not orient like field-reared larvae when first exposed to a forest border. Thus, orientation to both terrestrial and celestial cues seems to be learned during the second month of larval life in nature. Influence of Gravity Visual cues are not the only ones guiding N. sylvestris under natural conditions (Mieulet 1980, Beugnon et al. 1983). With the platform positioned close to the forest border but inclined 50° in different positions near the border, 10-mo-old resident larvae oriented correctly as long as terrestrial cues were visible. However, with terrestrial cues obscured, the direction of inclination determined their course; crickets always walked downhill. This gravitational orientation depends on age. One-mo-old larvae did not orient to gravity when visual cues were obscured, possibly because they have fewer clavate hair sensilla on the cerci (Horn and Faller 1985; chapter 7, this volume). But after 4 mo in their natural habitat, all crickets walked downhill when tested on a tilted platform. The gravityrelated orientation persisted through adulthood. Surprisingly, 10-mo-old larvae captured in another forest, where they lived on a flat surface, failed to use gravity for orientation when tested under the conditions described for residents. Forest Paths as Visual Cues When N. sylvestris adults were positioned on a platform in the middle of a forest path and released about 3m from both tree borders, they oriented to nearby tree trunks (Campan and Gautier 1975). Crickets captured during the day at the western edge of the path and released in the middle of it walked toward the west; those captured at the eastern edge of the path walked toward the east. A detailed analysis showed that orientation toward the tree trunks varied over the day and was lowest around 1500,

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when the crickets began to feed (Lacoste et al.1976).At night (2100) all runs were random, as one might expect. Paths present both terrestrial and celestial cues. With terrestrial cues obscured, the animals still chose their directions according to capture site direction, suggesting memory of relative direction. Again, development of orientation behavior was seen under path conditions. Two-week-old larvae either did not orient or oriented poorly toward tree trunks and showed no sign of compass orientation. Although they walked toward trees 1 wk later, only at age 10 wk had they "learned" compass orientation.

Orientation inside a forest. Reflected light inside a forest is randomly distributed throughout the year, with the sky poorly visible in summer. In only one experiment, when 10-mo-old larvae were released from the platform within the forest, no significant orientation was found. Comparative Aspects InN. sylvestris the habitat determines visually guided behavior through specific terrestrial and celestial cues. This appears true also for Pteronemobius heydeni and P. lineolatus swimming to the shore of a lake or a river bank (Beugnon 1985, 1987). Details of each particular habitat are apparently learned during larval life in relation to the complexity of the immediate surroundings. It remains to be shown to what extent the results reported for forest crickets hold for other cricket species, especially those living in open habitats. Studies suggest that during the day all ground crickets choose spots with the least reflected light when searching for a burrow or shelter.

Antennal Tracking of Visual Targets Honegger (1981b, 1984) and Kammerer et al. (1987) described visually guided behavior in Gryllus campestris which involves antenna! tracking of a target moving in a horizontal plane in front of a cricket (Fig. 5 .2). During such tracking the head is fixed and does not make saccadic movements. Tracking is initiated by a black disk on a white background (or a white disk on a black background) if the target subtends a visual angle of at least 21°. During tracking, the antennae scan the leading or trailing edge of the object. As disk diameter becomes smaller, tracking becomes less precise and habituation increases (Honegger et al. 1985). On the other hand, large disks of a visual angle of up to 48° elicit a precise tracking response. Vertically oriented rectangles are preferred over horizontally positioned ones, even if the surface area of vertical rectangles is smaller than that of horizontal rectangles. For tracking, the distance of objects does not seem crucial. It can be initiated by objects as far away as 120 em, provided they are large enough (visual angle 21°). Targets farther away have not been tested.

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In competition experiments a close (6.6-cm) and a distant (11.6-cm) black vertical stripe (13° visual angle with a gap of 35° visual angle) against a white background were simultaneously moved around a cricket. No preference was expressed for either of the stripes. Instead the antennae tracked the leading edge of the combined stripes, when they moved from medial to lateral, and their trailing edge when they moved in a lateral to medial direction. When a pattern of five stripes (pattern frequency 26°) was used as a stimulus, antennae similarly tracked the leading or trailing edge of the whole pattern, depending on the direction of movement (H.-W. Honegger, unpublished). Tracking precision increases during and between visual target presentation when low-amplitude vibrations of 250-600 Hz are applied through rods holding a test animal in place (Honegger et al. 1985). This indicates that multimodal input maintains longer-lasting and more precise tracking response. Targets on the substrate in the vicinity of a cricket which move horizontally through its visual field produce substrate vibrations (see chapters 6 and 7). Antenna! tracking may be useful for further inspection of visually recognized approaching or passing walking targets.

Perceptual Capabilities of Cricket Eyes Visual acuity is defined by (1) the angular sensitivity (i.e., the minimal visual angle a target subtends in an eye for detection against the background) and (2) the resolution threshold (i.e., the minimal visual angle between targets for distinction from one another). In Nemobius sylvestris, visual acuity has been measured by choice tests in the laboratmy. Adult crickets facing a pair of visual targets of different sizes and complexities of internal structure oriented toward the more complex one, for instance, a checkerboard with smallest-size squares. When the size of these squares was progressively reduced, the animal reversed preference and oriented toward the less complex target because the smallest squares could no longer be resolved by the visual system. The

Fig. 5.2 (continued) angle shaded) from the longitudinal axis (l.a.) of the cricket; abscissa, time in s. Angular tracking of antennae is superimposed in relation to same axis. The target moves with an angular speed of 20°/s from right (R, upper left) to left (L, lower right). Note that the antennae track the trailing or leading edge of the target with rapid saccadelike movements; the right antenna (upper LI-ace) stops moving at about 30° Land remains pointing in that direction; the left antenna starts tracking from its prior resting position at about 45° L shortly after the leading edge of the target crosses 0°. Antenna! movements were analyzed by on-line Apple Ile computer sampling data every 20 ms. Heary horizontal lines result from ovedaps of 50 data points per second of the nonmoving antennae. (C from Kammerer et al.1987, courtesy of Journal of Neuroscience Methods.)

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minimal resolution angle of 4°30' corresponds to the anatomical interommatidial angle in N. sylvestris. The angular sensitivity measured in N. sylvestris by intracellular recordings from single retinular cells fits the interommatidial value (Jeanrot 1977). However, with a different target adults can orient toward dark vertical bars of 1 o visual angle width. Thus the threshold of bar detection is smaller than the anatomical and physiological resolution of ommatidia (Camp an and Medioni 1963, R. Camp an, G. Beugnon, and F. Mieulet, unpublished). Jander (1968) reports a visual acuity angle of 4° for Acheta domesticus and Oecanthus pellucens. Perception of Form and Visual Scanning Visual form perception requires scanning with the head to detect real or relative movements of the target with respect to the retina. N. sylvestris adults were tethered and allowed to rotate a small polystyrene sphere underneath their legs. This restraint did not change or interfere with form perception as measured in freely moving crickets (Fig. 5.3 A) (Campan et al. 1976). In freely moving crickets with the head fixed to the thorax the response remained, but in head-fixed, tethered crickets it disappeared. Thus relative motion between visual target and retina is necessary. However, tethered animals with the head fixed to the thorax regained form perception when the target was illuminated by a flickering light below a frequency of6 Hz (Fig. 5.3 A). Tethered crickets with the head free scan by moving the head (Lambin 1984). InN. sylvestris visual scanning with the head is discontinuous and jerky, interrupted 2-3 times per second, resulting in a (flicker) frequency of 2-3 Hz. The angular width of the head oscillations, normally up to 5°, and occasionally up to 10°, covers at most 1-2 rows of ommatidia (Fig. 5.3 B). Freely walking N. sylvestris adults do not move the head with respect to the thorax, but the whole body oscillates around the long axis with an Fig. 5.3 Measures of visual scanning in adults of both sexes of Nemobius sylvestris. A, Orientation toward a black vertical bar as expressed by an orientation index (1 of inferred longitudinal body axis orientation (ratio of the orientations toward the target minus orientations in the opposite direction divided by the total number of orientations: maximum values, +1 to -1). All tested under normal room illumination (50-Hz flicker) unless otherwise indicated. Fi,FrH, tethered ·crickets walking with the head free; Fi,wH, tethered crickets with the head waxed to the thorax and with light flickering at 4Hz, 50 Hz (center), or 6Hz; Fr, freely moving animals; Fr,wH, freely walking crickets with the head waxed to the thorax; bars represent standard error of means. B, Distributions of angular values (D of 478 head oscillations in six tethered crickets during orientation walks. C, Speed during consecutive bouts of one cricket walking toward a black target, measured from cinematic film frames: ordinate, speed evaluated by distance gained between consecutive frames; abscissa, elapsed time. D, Distribution of angular values (D of 88 whole-body oscillations vvith respect to fixated edge of the target during unrestrained walking by 16 cinematically filmed crickets, one walking sequence each. (A modified from Campan et al. 1976, B and C modified from Lambin 1983, D modified from Lambin 1984). 0 )

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angular amplitude of less than 5°, comparable to that of the head in a tethered cricket (Fig. 5.3 D) (Lambin 1984). Walking is also discontinuous and meandering (Fig. 5.3 C) in the range of2-3 Hz. AnA. domesticus adult walks similarly on a spherical treadmill when it tracks a target visually, and this walking mode differs from tracking target acoustically (Weber et al. 1987). Fixation of Targets A detailed analysis of body position in relation to the visual target during walking for N. sylvestris adults showed that the animal fixates on the edge of a black target with a pm1icular region of the eye. The region is located ca. 20° laterally and close to the e:xtemal edge of the binocular field of view (Fig. 5.4) (Lambin 1984). Some evidence of functional heterogeneity of the retina was demonstrated in behavioral experiments, where tethered N. sylvestris adults faced a black target positioned precisely within the visual field. A particularly effective area for form perception centers around 30° lateral to the sagittal plane and extends with decreasing response to about 90° (Fig. 5.4 C) (Jeanrot et al. 1981). With the head fixed and the target illuminated by a flickering light of 4Hz, the area reduced to 70°. These results indicate that crickets may use a fixation area around the edges of the binocular field of view for form perception. Perception of Relative Distance Adults of N. sylvestris which can choose between two black targets presented at the same visual angle but at different distances always prefer the closer target. This indicates that the visual system can discern relative distance. With pairs of targets (visual angle 11° X 56°) whose angular widths were much larger than the visual acuity threshold, two types of choice experiments have been performed: (1) vertical bars were positioned at 5 and 10.9 em from the head (short-distance test); (2) the bars were positioned 52 and 130 ern from the head (long-distance test).

(continued) edge of the target from g (inset); g, angle to the edge toward which the cricket walks. The angle increases after a 6-s run because an antenna touched the target; then the animal turned toward the midline of the target. B, Histograms of distribution of angles between body axis and target edges as cricket walks toward the target (left graph) or along one edge (right graph). White bars represent angular deviations toward the g edge; black bars toward the opposite (d) edge. C, Diagram of visual field and fixation areas in polar coordinates for a tethered adult (see Fig. 5.8); angular distance between any two concentric circles in diagram is 20°; limits of visual field of the right eye indicated by heavy contour; black areas represent regions of eye where the target is projected during walking orientation; head free, left side of diagram; head fixed, right side of diagram. (A and B modified from Lambin 1984, C modified from Jeanrot et al. 1981.) Fig. 5.4

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In short-distance tests freely walking cricket larvae (first to sixth instar) discriminated between targets if the visual angle varied between 5° and 45°; adults discriminated between targets at visual angles between 10° and 45 a (Fig. 5.5 A). These values correlate with the extension ofthe binocular field of view. In long-distance tests, discrimination failed in first- and second-instar lmvae (Fig. 5.5 B) and was limited in fifth- or sixth-instar larvae to 10-20° and to 15-25° angular gap in adults. Both discriminating larvae and adults preferred the proximal target. Tethered crickets with the head allowed to move discriminated between distances in short-distance tests like freely walking animals (Fig. 5.5 C). Discrimination failed when the head was fixed to the thorax in tethered crickets, even with the target illuminated by flickering light (Fig. 5.5 D). Thus relative movement between insect and target is a necessary condition for the perception of relative distances. A flickering light is unable to mimic relative movements in perception of relative distance (Goulet et al.1981J. Distance discrimination does not occur during antennal tracking by tethered Gryllus campestris, as stated earlier. Form Discrimination Jander (1965) studied spontaneous preferences of N. sylvestris between two black rectangular targets of equal width but different heights. The crickets always chose the shorter rectangle. They preferred a height of10° of visual angle, and their response decreased to targets of heights between 25 and 40°. A preference comparable to that of N. sylvestris is also reported by Jander (1968, 1971) for A. domesticus, G. bimaculatus, and G. campestris. Jander hypothesized that form perception and discrimination involves a detector system with an excitatory center and an inhibitory surround, but so far this model has not been tested. Vertical bars elicit antennal tracking of G. campestris more reliably than horizontal bars (H.-W. Honegger, unpublished). According to Jander (1968, 1971), N. sylvestris and A. domesticus prefer horizontal stripes, while 0. pellucens prefers vertical to horizontal black-and-white stripes. He speculates that this preference reflects different visual conditions in nature. Oecanthus pellucens is a tree cricket and lives on shrubs, whereas the other species are ground crickets. However, it is doubtful whether differences in life style can explain these preferences. Recently Atkins et al. (1987) confirmed A. domesticus preference for horizontal black-and-white bars in arena and treadmill experiments. Later tests with N. sylvestris (Campan 1978), however, did not confirm lander's results: the animals preferred vertical over horizontal bars (Fig. 5.6 C). Form Discrimination and Learning An avoidance conditioning paradigm was first developed by Robert (1967) for A. domesticus and by Venet (1971) for G. bimaculalus, using light and dark areas, and boLh authors gave first hints that crickets could learn to discriminate form of visual targets. Later, more detailed analyses of

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Fig. 5.5 Relative visual perception of distance in walking adults and juveniles of Nemobius sylvestris as evaluated by presentation of a pair of vertical bars with identical visual angles but at different distances from the animal . Angular distances (a.d.) of the inner edges of the two bars (abscissa) were consistently changed by to• from 10-90° of visual angle; radial distances in relation to the animal remained constant (either S and 10.9 em or 52 and 130 em). 10 (ordinate) represents the ratio of the number of orientation responses of crickets toward the nearest target minus those toward the more distant target divided by total number of orientation responses (maximum values +1 to -1). Age of crickets indicated by symbols shown in inset at A: Ll-2 (triangles), larval instars 1 and 2; L5-6 (squares), larval instars 5 and 6; Ad. (circles), adult. A, Freely moving crickets; closer stripe is 5 em away. B, Same, but closer stripe 52 em away. C, Tethered animals with free head; closer stripe 5 em away. D, Tethered adults with head fixed and light flickering al 4Hz; closer stripe 5 em away. (Modified from Goulet et al. 1981.)

form discrimination by leaming were carried out inN. sylvestris (Campan and Lacoste 1973) and inA. domesticus (Kieruzel and Chmurzynski 1982), using spontaneous preference tests in addition to discrimination learning paradigms. The crickets were raised in terraria divided into two parts (Fig. 5.6 A). The wall in each compartment was covered with one of two visual targets used in the test. The cricket's approach to only one of the targets was rewarded by food, water, and shelter. Figure 5.6 demonstrates the change in preference before and after training. The strong spontaneous scototactic tendency of N. sylvestris could be reversed by training (Fig. 5.6

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Fig. 5.6 Diagrams of tests of discriminative learning of visual cues in Nemobius sylvestris adults. A, Rectangular training terrarium partially separated into two compartments; food (f), water (w), and shelter (s) were offered together as rewards in front of alternative patterns; crickets were released at the center of the arena. During conditioning, patterns presented on opposing walls were either a broad white versus a broad black vertical stripe (B), or horizontal versus vertical blackand-white narrower stripes (C). Before and after training, similar patleins were presented on the dotted wall of a circular arena (B, C). Half-circles show the percentile choices of naive and trained crickets to alternative targets or the dotted arena background. B, White (rewarded pattern during training) versus black broad vertical stripe. Arrows in B and C emphasize rewarded pattern during training, in results of tests after training. C, Horizontal (rewarded pattern during training) versus vertical multiple narrower black and white stripes. !Modified from Campan and Lacoste 1973.)

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B). Daily repetitions of scototactic tests over 40 d without reward led to a drastic habitatuation of the response (Campan 1978). When choices of vertical bars versus horizontal bars were rewarded alternately, a clear discrimination between visual targets was revealed (Fig. 5.6 C). Thus learned discrimination and modified preference of visual targets was shown for each pair of form stimuli tested. This demonstrates that at least N. sylvestris can widen the spectrum of form perception by learning. Behavioral Evidence for Polarized Light Perception Brunner and Labhart (1987) showed in G. campestris that polarized light may be an orientation cue. Tethered crickets, walking on an airsuspended sphere, responded spontaneously to the e-vector of polarized light presented from above (Fig. 5.13 A). This vector orientation controls both strength and direction of turning but is itself independent of intensity, except near the absolute threshold. The absolute threshold intensity is extremely low (ca. 3 X 10 7 quanta· cm-2 • s-1 at 433 nm), which could enable the cricket to perceive polarized light at intensities comparable to those present on a moonless night. Spectral sensitivity studies gave the best fit with the blue receptor between 380 and 500 nm (Herzmann and Lab hart 1987). Covering different regions of the compound eyes revealed that the response is mediated by an anatomically and physiologically specialized area, the dorsal rim area. Beugnon and Campan (1988) further reported that G. campestris adults in the field orient back to their burrows better under blue sky than overcast sky and that their homeward orientation is displaced if a polarizing filter above them is rotated. Further details of this general senso:ry capacity are discussed later in this chapter.

Structure and Function of the Visual Pathway Compound Eye Structure The compound eyes of Gryllus campestris and G. bimaculatus are ellipsoid, with a tilt in their longitudinal axis of 47" when compared with the horizontal body axis of a walking animal, whereas the shape of Nemobius sylvestris compound eyes is roughly spherical. The number of ommatidia varies among different species: 160-190 in Cycloptiloides canariensis (Egelhaaf and Dambach 1983), ca. 1,159 inN. sylvestris (Goulet et al. 1981), 3,100 in G. campestris (Honegger 1980), 3A80 in Acheta domesticus (Hocking 1964), and up to 7,000 ommatidia in G. bimaculatus (Burghause 1979). Moreover, regularity of shape as well as constancy of size varies among different eye regions in different species. In N. sylvestris, the ommatidiallenses are rather uniform over the whole eye, whereas in G. campestris the ommatidia in the posterior and lateraventral parts are larger in diameter than those in the center of the eye. Within the frontal region, where new ommatidia are generated during postemb:ryonic development, the lenses are similar in diameter to those

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in the center but are flatter (Fig. 5.7 F). Lenses are also flatter and of smaller size in the dorsal rim area, with a smooth appearance in G. campeslris lFig. 5.7 DJ. In N. sylvestris the ommatidia are arranged in a hexagonal fashion, except in lateral areas where they have a more or less tetragonal organization (Lamb in 1984). This arrangement allows definition of three axes !Fig. 5.7 B). This pattern is less obvious inN. sylvestris larvae, and it changes during larval development. Gryllus campestris adults exhibit a hexagonal array only in the center region (H.-W. Honegger, unpublished!. Visual Fields Figure 5.8 illustrates the visual fields, measured by the pseudopupil technique, of different European species. Pseudopupils can be seen in all regions except the dorsal rim area, which contains blue receptors exclusively (Labhart et al. 1984). The determined visual fields are similar in the five species. All cricket eyes have a blind region that extends from ventrorostral to ventrocaudal. The interommatidial angle-the angle between optical axes of two neighboring ommatidia-differs from species to species because of the different numbers of ommatidia per surface area. Most ommatidia in cricket eyes have biconvex corneal lenses and a crystalline cone that is subdivided into four sectors (typical for the eucone type). Each ommatidium contains eight retinular cells, which form a fused rhabdom. The ommatidia are isolated by pigment cells. Those in the dorsal rim area show the greatest specialization (Burghause 1979, G. bimaculatus; Egelhaaf and Dambach 1983, Cyclopti/oides canariensis). The corneas are flat, the lenses barely subdivided (Fig. 5.7 D); and the large, fused rhabdom is formed by only six retinular cells with an orthogonal microvillar arrangement and without screening pigment cells. This dorsal rim area in G. campestris is specialized for polarized light vision, as it is in honeybees (Labhart 1980). Because N. sylvestris has a dorsal rim area (Fig. 5.7 B) and the capacity to use blue sky for compass orientation, one might expect to find a similar retinular cell organization and reception of the pattern of polarized light.

Fig. 5. 7 Scanning electron micrographs of compound eyes of Gryllus campestris (A, C-F) and Nemobius sylvestris IBJ adults. In A and B, up is posterior, down is anterior, ventral is left in A and right in B, with respect to normal head position. In B, the three axes ilines a, b, c) show ommatidia! arrays in rows; in the lower left corner a part of an antenna] socket can be seen; note also the larger size and lower numbers of ommatidia; the smooth-appearing dorsal rim area shows above c. C-F, enlargements of areas indicated in A by same capital letters; Dis dorsal rim area; note different shapes of corneal surfaces and different diameters of ommatidia. Scale:A, 120 J.Lm; B, 100 J.Lm; C-F, 20 J.Lm. (B courtesy ofM. Lambin;A and C-F from H.-W. Honegger.)

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Fig. 5.8 Total visual field of both eyes in polar coordinates for Gryllus campestris (Gc), Nemobius sylvestris (Ns) (see Fig. 5.4 C), Gryllus bimaculatus (Gb), Acheta domesticus (Ad), and Oecanthus pellucens (Op) adults, determined by pseudopupil method. Vertical figure axes indicates animals' sagittal planes, central points represent animals' vertical axes (viewed from above), horizontal lines (0-0) indicate horizontal planes, up is posterior. Lateral limits of the heavy lines are the lateral limits of each monocular field, areas enclosed by overlapping left and right monocular fields are binocular fields; each inner circle indicates 90°, each outer circle 180° in any direction. Circular planes represent planar projections of spheres. Extent of monocular fields similar in all species. Binocular fields differ slightly among the four ground-dwelling crickets shown, but size and form of binocular field of the vegetation-dwelling cricket (Op) are strikingly different. Blind spaces spread from the anteroventral to the posteroventral side; thus, the visual fields of these crickets are not completely spherical. (Courtesy of M. Lambin.)

Binocular Overlap G. campestris and G. bimaculatus have an unusually large binocular field of view, especially in the dorsal eye, which extends to the rostral and caudal parts of the visual field (Fig. 5.8). Such large binocular fields are in strong contrast to those of flying insects that orient on individual small targets, as do predators. Development of Compound Eyes During larval development inN. sylveslris the cornea lenses increase in diameter and in thickness, accompanied by a lengthening of the cones and the retinular cells. The faster growth of retinular cells in the longitudinal axis of the eye reduces interommatidial angles from about 12° in the first larval instar to about 4° in adults (Fig. 5.9 A). The total visual field does

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not change in the horizontal plane, whereas the binocular field of view enlarges (Fig. 5.9 B) (Goulet et al. 1981). Thus the spatial limits of binocular triangulation improve during postembryonic development, according to the models of Chmurzynski (1963) and Burkhardt et al. (1973) (Fig. 5.9 C).

Retinal Physiology Retinular cells of Nemobius sylvestris, when recorded intracellularly, exhibit the typical three-component response of an insect photoreceptor (Baumann 1975): a fast on-transient, followed by a lower plateau, then a rapidly decreasing off-transient (Lambin and Jeanrot 1982) (Fig. 5.10 A). Lall et al. (1985), using the ERG-method, found green and ultraviolet sensitivity in the compound eyes of Gryllus firmus, and Labhart et al. (1984) identified green- and ultraviolet-sensitive receptors in the dorsal area of the eye of G. campestris. The photoreceptor cells within the dorsal rim area are blue-sensitive (lamda max= 435 nm) (Fig. 5.10 B), and they have an extremely wide visual field, varying between 10° and 35° or more. These cells show high polarized light sensitivities (PS) at an average of 8.3 ± 5.9 (Fig. 5.10 C). A similarly high polarized light sensitivity is reached for off-axial stimuli. The polarized light sensitivity of receptors in the dorsal area is low: 2.6 ± 0.8 in green cells and 2.6 ± 0.9 in ultraviolet cells. These cells have narrow acceptance angles of 5.9 ± 2.2°. The responses of the blue receptors in the dorsal rim area have greater latency than the responses of green receptors in the dorsal eye area (Labhart et al. 1984, T. Labhart, personal communication) but do not show a phasic-tonic response as do ultraviolet and green receptors (Zufall et al. 1988). The latter authors confirmed the results ofLabhart et al. (1984) and also showed that the retina of G. bimaculatus has ultraviolet (lamda max= 332 nm), blue (lamda max = 445 nm) and green receptors (lamda max = 515 nm), with the blue receptors confined to the dorsal rima area. After green-light adaptation the green sensitivity of ultraviolet receptors strongly decreased, but the ultraviolet sensitivity of green receptors after ultraviolet adaptation did not. This result suggests a positive electric coupling between ultraviolet and green receptors but not vice versa (Zufall et al. 1988). Optic Lobe Structure As in most insects, the optic lobes of crickets consist of three major retinotopically organized neuropiles: lamina, medulla, and lobular complex, separated by the first and second optic chiasmata (Fig. 5.11). In Gryllus campestris, Teleogryllus commodus, and a Brachytrupes species from Tunisia, the second optic chiasma is expanded to form a long tract, separating the medulla from the lobula (Fig. 5.11 A). In G. bimaculatus and Nemobius sylvestris, as in many other insects, the lobula borders directly

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on the medulla, indicating a distinct relationship between the length of the chiasma and the width of the head (H.-W. Honegger, unpublished). In G. bimaculatus, the short primary visual fibers originating from the retinular cells terminate in the lamina neuropile, according to unpublished data from L. Williams. Six distinct morphological types have been encountered (R1-R6). Four lie at relatively shallow depths, whereas two penetrate the outer two-thirds of the lamina. Long, primary visual fibers, with characteristic arborizations within the lamina, pass through this layer and project to the outer medulla (L. Williams, unpublished), as in other insects (Strausfeld and Nassel1980). Zufall et al. (1988) stained the retinal receptors of G. bimaculatus with Lucifer yellow after recording. All ultraviolet receptors recorded apparently project into the median medulla, like the long visual fibers of other insects. The axons of green receptors terminate in the lamina, with compact terminal branches. Those of blue receptors project deep into the lamina, with numerous fine terminal branches. The axon of one blue receptor projected into the outer medulla; in contrast to ultraviolet receptors, it had brushlike fine branches in the lamina. Within the optic lobes of G. bimaculatus 10 different monopolar cells were traced with the cobalt sulfide technique. Five of them terminated in the outer medulla. Some of these monopolar cells have rather narrow fields of arborizations, and they probably match the spread of receptor arborizations. The so-called giant monopolars branch tangentially to wider areas of the lamina. A special feature within the lamina is the nan·ow termination of a centrifugal cell. Its cell body is located between lamina and medulla, and the projections within the outer medulla resemble those of long primary visual fibers. Projections of monopolar neurons and those of some of the centrifugal T -shaped cells (T1) determine the columnar organization of the outer medulla. InN. sylvestris (Fig. 5.12), as in G. bimaculatus (based on unpublished data from Golgi impregnations and cobalt fills derived by L. Williams), both monopolar cell fibers and primary visual long fibers project to the serpentine layer of the outer

Morphological and functional characteristics of compound eyes in Nemobius sylvestris adults and larvae. A. Change in number of ommatidia (left) and in interommatidial angle in degrees lrightl, by larval instar (larval instar 10 = adult). A slight sexual dimorphism is indicated for adults. !Modified from Goulet et al. 1981.) B, Growth ofinterocular space (length in mm), total visual field (width in degrees), and binocular field (width of region of overlap in degrees), relative to larval instar (upper scale, larval instar 10 = adult), and in relation to growth of femur length (in mm, lower abscissa scale). C, Change in binocular triangulation with age (larval instar 10 = adult). EO, distance (in mm) from front of head to point of convergence of the medial ommatidia! view of both compound eyes in the horizontal plane; E oo, distance from front of head to point of convergence of view from the least convergent ommatidia of both eyes. (Modified from Burkhardt et al. 1973.)

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Micrographs of sections through the optic lobe of a Gryllus campestris adult. A, Whole array of retina, optical ganglia, and part of the central brain. C, calyx of mushroom body; La, lamina; Lo, lobula; M, medulla. Noncrossingfibers of the first optic chiasma can be seen between La and M; the long tract between M and Lo is the second optic chiasma. Arrow indicates approximate position of serpentine layer in the medulla (cf. Fig. 5.12). Plane of section is oblique with respect to frontal and horizontal planes, posterior to the top. Bodian stain; scale, 400 f.Lm. (Courtesy of F. Huber.) B, Cross-section through second optic chiasma (position indicated by vertical black bar in A). Tracheae and large-diameter axons can be seen in the anterior (lower) part of the chiasma; these noncrossing axons bypass the lobula. Distinct compartments of fiber groups are separated by glial cells. Scale, 20 f.Lm. C-D, Enlargements of parts indicated in B. In D, axons 2-3 f.Lm in diameter are surrounded by populations of tiny axons of less than 1 f.Lm in diameter. Total number of axons in this section, approximately 15,000. Scale, 5 f.Lm.

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medulla. This layer is also occupied by terminals of fibers originating from cells in the contralateral optic lobes in G. campestris (Honegger and Schiirmann 1975). The inner part of the medulla is defined by the arborizations of certain lobular T-shaped cells (LT1) . Some cells contribute to the columnar organization of the medulla, particularly expressed in the parallel alignment of one centrifugal cell (C1), collateral arborizations in the outer medulla and within the first optic chiasma of another T -shaped cell (TZ), and arborizations in the outer medulla and the distal arborizations of (Mil) and (Tm1). The tangential a rborizations of other cells define the limits of the three major medullary layers. This is best demonstrated for the serpentine layer by wide-field neurons (W1) and (WZ) and projections from the brain (st). Most of the cells that were traced projected proximally to the second optic chiasma, while some lobular T -shaped (LT1) neurons terminated in the lobula. G. Boyan and L. Williams (personal communication, 1986) traced an interneuron, which responded to sound, from the lateral protocerebrum into the m e dulla. This result might

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Fig. 5.12 Diagrams of 17 different cellular elements in the medulla of a Nemobius sylvestris adult, as revealed by Golgi impregnations and cobalt fills. The medulla can conveniently be divided into three layers: outer medulla, serpentine layer, and inner medulla (Strausfeld and Nassel 19801. Cl, centrifugal cell with cell body below the inner surface of the medulla and with numerous projections to the lamina which are aligned along the axis of fibers in the fkst optic chiasma; LTla, LT1-3,lobularT-cells (T-shaped cells) with arborizations in the inner medulla and lobula; Mil, Mi2, medullary intrinsic neurons; st, projection from the brain into the serpentine layer; T2, medullary T-cell with diffuse projections in the lamina and two parallel arborizations in the outer medulla; Tm 1-6, Lransmedullary neurons, each distinguishable on the basis of their arborization patterns; Wl, VV2, wide-field neurons with diffuse arborizations (arborizations ofWl neurons always spread in the serpentine layer; W2 arborizations delimit the proximal part of this layer). (Courtesy of L. Williams.)

explain the earlier results of Honegger (1978), who recorded medullar units with both visual and acoustic inputs in G. campestris. In G. campeslris, the second optic chiasma contains ca. 15,000 fibers, of which 11,000 have a diameter of1 J.Lm or less (Fig. 5.11 B-D) (Honegger and Schiirmann 1975). These fibers include both afferents and efferents. One efferent fiber could be traced to the ventral intermediate tract (VIT) in the brain, and it may even miginate from a cell located in the ventral nerve cord (L. Williams, personal communication, 1986). The lobula is undivided in N. sylvestris, G. campestris, and G. bimaculatus, is smaller than in acridine grasshoppers but has a similar structure (Gouranton 1964, O'Shea and Williams 1974). InN. sylvestris the lobula

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contains a lobula giant movement detector neuron (LGMTJ with a structure similar to that studied in detail in locusts (O'Shea and Williams 1974). Projections to the Central Brain Tracing fibers from the optic lobes to the central brain in Gryllus campestris reveals projections to the proto- and deutocerebral areas. Visual fibers from the medulla, and probably the lobula, project predominantly into the ipsilateral nonglomerular neuropile of the protocerebrum (Honegger and Schiirmann 1975). This is also the case in Nemobius sylvestris (L. Williams, personal communication, 1986). Projections from the optic lobes were also discovered in certain glomerular neuropile areas, such as the central body complex, the protocerebral bridge, and within the calyces of the corpora pedunculata. The latter discovery agrees with anatomical findings (Mobbs 1984) in the honeybee brain and with immunohistochemical results in the cockroach and the honeybee (Klemm et al. 1984). Moreover, Honegger and Schiirmann (1975), by cobalt fills from the optic lobe, traced a group of neurons with cell bodies located in the pars intercerebralis and with branches penetrating the protocerebral bridge. The two optic lobes on either side are connected with each other by means offour defined fiber tracts. Visual fiber projections are also found in the caudal parts of the ipsilateral deutocerebrum, but there is no evidence of fibers that descend to the ventral nerve cord. First-order ocellar interneurons that seem to connect ocellar regions directly with the lobula and medulla have been traced in Acheta domesticus and in Periplaneta americana (Koontz and Edwards 1984). They also branch in the lateral protocerebral neuropil areas. Fibers stained in the ocellar nerves of G. campestris by Honegger and Schiirmann (1975) are thought to belong to this group. Ocellar efferents have been reported by Kondo (1978, cited in Koontz and Edwards 1984). This strengthens the point that inputs from the ocelli and the compound eyes converge in the protocerebral bridge, the lateral protocerebral areas, the ocellar regions, and the optic lobes. The functional importance of this convergence is unknown. Physiology of Visual Neurons Optic Lobes Extracellular recordings within the lamina neuropile of Nemobius sylvestris exhibited summed potentials, which were reversed in sign when the angle of incident light was moved more than 20° away from the spot eliciting the maximal response. Furthermore the multicellular response decreased with increasing stimulus size and intensity, indicative of a kind of lateral inhibition within the lamina (Lambin and Jeanrot 1982).

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Dingle and Fox (1966) found visually activated, sustained, and transient on- and off-units within the brain ofAcheta domesticus. Honegger (1978) reported similar responses from medullary units of Gryllus campestris, and Lambin and Jeanrot (1982) from units within the medulla and lobula of N. sylvestris. Some of these units in G. campestris were motionsensitive, with a preference for movement in specific directions. Neither the anatomy of the units nor the structure of the receptive field is known. Polarization Opponent Interneurons Most medullar neurons observed in G. campestris show an intensitydependent change in their activity (Honegger 1978). Another type of medullar neuron with sustained response is indifferent to intensity variations and receives antagonistic input from polarization-sensitive photoreceptor cells of the dorsal rim area with orthogonally arranged microvilli (Labhart 1988). This is the first demonstration of polarization-opponent (POL) intemeurons (Fig. 5.13 B). These POL neurons exhibited spontaneous activity in the dark or when stimulated with nonpolarized light, and their responses increased or decreased according to 90° changes in e-vector orientation. Three different cell types, each with maximal responses to one of three different e-vector orientations, have been found. Their independence from variation in light intensity prevents intensity gradient interference with the change of e-vector signals, thus enhancing contrast and modulation of the vectors of polarized light (Labhart 1988). Labhart discussed how these POL neurons could be used to evaluate the polarization pattern of the sky. Receptive Fields From some of the medullary units with sustained responses, r·eceptive fields were measured (Honegger 1980). Outstanding is a sustained off-unit with a center-surround antagonism (Fig. 5.14). Its large receptive field covers both monocular and binocular regions of the eye. If the left and the right unit were to convey information concerned with binocular vision, one would expect a subsequent postsynaptic cell to he maximally excited. The antagonistic center-surround organization could assist in selecting optimal target size in the antero-posterior axis. Multimodal Aspects A sustained on-unit had a receptive field with a rather flat sensitivity profile. As soon as a target moved into its receptive field, the activity of the unit was suppressed. Antennal movements caused an increase in its discharge, independent of the location of the visual target. This again indicates that nonvisual information invades the medulla in G. campestris (Honegger 1980). Indications suggesting convergence of visual with nonvisual input are also known for central brain neurons because many local brain neurons process multimodal input (Schildberger 1984a). Such convergence is not

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Fig. 5.13 Diagram and oscillograms demonstrating polarization sensitivity in Gryllus caiTipestris compound eyes. A, Diagram of circular walking of one tethered cricket under open-loop conditions in response to stationary e-vectors of polarized light; turning tendency varies inversely with slope of the trace. The tethered cricket walked on an air-suspended polystyrene sphere and was exposed to a polarized, wide-field, projected stimulus centered at the zenith. Note that the cricket changed its turning tendency when the e-vector of the polarized light was changed by 90° and resumed the identical turning tendency when the original e-veclor was presented again. (From Brunner and Labhart 1987, courtesy of Physiological Entomology.) B, Responses of one polarized opponent medullar interneuron to 1-s polarized white-light pulses (indicated by bars below traces) with different e-vectors (indicated by orientation of bars in circles at right). The spontaneously active unit is strongly excited by polarized light with an e-vector of 45° with respect to the long axis of the dorsal rim area of the compound eye, and inhibited by rotation of the e-vector orientation by about 90° (here, at 125°). (Courtesy ofT. Labhart.)

surprising if one takes into account the neuroanatomical evidence for massive visual and nonvisual input into the central brain. Descending Visual Neurons InA. domesticus Palka (1969) recorded activity extracellularly from descending units in the neck connectives as a target moved across the retina ipsilateral to the connective from which activity was recorded. Since its receptive field was ipsilateral, Palka called this unit a descending ipsilateral movement detector (DIMD). Its morphology is not established. In a moving cricket the excitation caused by a moving target was suppressed. Shading the eye or movements of large targets also caused inhibition. The

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Fig. 5.15

DIMD responded preferentially to small targets. No descending contralateral movement detector (DCMD) has yet been found in A. domesticus (Palka 1972). Further descending visual units have been extracellularly recorded in the neck connectives of G. bimaculatus (Richard et al. 1982, 1985). Cobalt fills revealed that the cell body was located in the lateral or medial portion of the protocerebrum (Fig. 5.15 A) and that the axon coursed contralaterally down the cord, with arborizations in all thoracic ganglia. All neurons had distinct receptive fields restricted to the cricket's binocular field of view, to only the monocular field of view, or to both binocular and monocular visual fields. The fields covered areas ranging from 20 to 200° of visual angle, with the smallest fields in the binocular area. Eighty percent of the

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receptive fields overlapped near or within the zone of the lateral limits of the binocular field, where visual targets elicited strong orientation responses inN. sylvestris (Fig. 5.15 E) (Lambin 1983). Most descending units behaved like low-pass filters; that is, they responded only within certain ranges of low-frequency stimulation (Fig. 5.15 D), a finding that corroborated with behavioral data (Campan et al. 1976). In G. bimaculatus, Tomioka and Yamaguchi (1984) extracellarly recorded descending neurons, again in neck connectives, which responded to visual, auditory, and mechanical stimulation. Even more interesting, their responses differed when compared during rest or in tethered flight. About 45% of these descending neurons were activated by light flashes or moving targets in a restrained but resting animal. During tethered flight, their response increased together with "spontaneous activity," while the other neurons responded to light only during flight. None of the morphologies of these neurons are known. Comparative Aspects When compared with the better understood system of descending visual neurons in locusts, the following conclusions can be drawn about these neurons in crickets. A. domesticus possesses a descending neuron resembling the descending contralateral movement detector neuron of Schistocerca gregaria (O'Shea and Williams 1974, O'Shea et al. 1974), but in the cricket this neuron sends fanlike collaterals into the deutocerebrum (Rosentreter and Schiirmann 1982), branches not found in the locust. The tritocerebral giant interneuron (TCG), whose morphology and function were first described inS. gregaria by Bacon and Tyrer (1978), has a morphological homologue in G. bimaculatus (Bacon 1980). The TCGs of both crickets and locusts receive visual input. However, input from windsensitive hair sensilla that control the TCG in the locust is replaced in the cricket by input from the antennae (Bacon 1980). This last comparison indicates that different connectivities exist in the TCGs of crickets, compared with those oflocusts, very probably reflecting different evolutionary influences in both groups.

Conclusions The investigation of the visual system and of visually guided behavior in crickets is still in its infancy. Vision clearly plays a major role in cricket behavior, but in only a few species has the visual pathway been studied to the level of single neurons, and these studies were often not accompanied by behavioral investigations. At the behavioral level it has been partially demonstrated that different habitats with specific visual cues strongly influence the orientation strategy of crickets. For some visual capabilities, such as recognizing different targets and relative distances, learning seems to be involved.

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At the neuronal level the visual pathway organization is similar to that in other insect groups. Present knowledge is insufficient for researchers to conclude whether structure correlates with particular behavioral pattems. We are still unable to assess both to what extent comparable neuronal structures serve similar or different functions and the significance of differences in neuronal structure among species. Visual field organization partly reflects the special tasks crickets perform in their complex visual surroundings. The demonstration of polarized light reception and of polarization opponent neurons in the optic lobes of crickets should increase our understanding of orientation to celestial cues. We know already that in the visual pathways of crickets multimodal information is processed. This may be the basis for extracting and evaluating combinations of environmental cues at this level, but as yet we have no real understanding about how such a system might operate. This chapter shows that the visual capacities of crickets are as complex as those of other insects, and we hope it will encourage further research on cricket vision and visually guided behavior. Acknowledgments We greatly appreciate the cooperation of Guy Beugnon, Michel Lamb in, Veronique Preteur, and Daniel Richard, who allowed us to use their results. The work of Lester Williams reported herein supported by a grant of the Centre national de recherches scientifique (CNRS) toR. Campan, and the research of H.-W. Honegger was supported by grants of the Deutsche Forschungsgemeinschaft (DFG) (Ho 463/4-12,14-6 and 14-8). J. Kien critiqued a first version of the manuscript. We thank U. Klepsch for her tireless typing, and we owe special thanks to the three editors for their great help in bringing the text to its final form.

CHAPTER SIX

Vibrational Responses Martin Dambach

The vibratory sense-that is, the perception of the manifold mechanical stimuli operating at close range between an organism and its environment-is of particular interest in the life of crickets. Crickets, with the exception of certain species adapted to special biotopes, live basically in two habitats: (1) on the ground in crevices or selfmade burrows (Gryllinae), (2) on trees, bushes, and plant stems, which are favored by tree crickets (Oecanthinae). For perception of, and communication with, vibratory signals, contact with the substrate or with conspecifics is required. Characteristic features of this communication channel are that the signaling distance is generally short, ranging from centimeters to a few meters when mediated by ground or air. Contact vibrational signal exchange between two individuals naturally requires direct contact between their bodies or appendages. Furthermore, vibrations are usually not perceived by a single sense organ but often involve several organs of different structure and organization; their ranges of perception overlap or complement one another. These characteristics need detailed definitions to provide a basis for the various categories of mechanical stimuli commonly called "vibrations."

Types of Mechanical Oscillations Vibratory signals, rhythmically changing mechanical stimuli, are identified by Mark! (1983, p. 333) as follows: "If a mechanical exteroceptive sense is neither touch, nor hearing, nor perception of gravity, angular acceleration or medium currents-then it must be [perception of] vibration." The term vibration can be applied to three relatively distinct types of mechanical oscillations that serve as a signal or stimulus for contact vibration, substrate vibration, and near-field medium motion. 178

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Contact Vibration Here signal transmission is established by direct physical contact between sender and receiver without the aid of another signal-carrying medium. These stimuli differ from normal tactile stimuli by their rhythmicity. Receptors adapted to receive contact vibration stimuli include various types of phasically responding hair receptors, campaniform sensilla, or chordotonal organs arranged between segments or within joints which provide information about position of body segments or tension in muscles (see chapter 7). Substrate Vibrations as Boundary Vibrations in Solid Materials Here the vibratory stimulus is transmitted to the receiver through a solid medium (ground or plant matter). It is produced when a sender (emitter) sets the medium locally in motion. The physics of the propagation of mechanical waves through solids and among solid or air boundaries are complicated, but have been well explained in a recent review (Marld 1983). The different types of boundary waves can be characterized by the direction of propagation of wave, speed of propagation, and attenuation properties of the conducting material. A stimulus adequate to excite the vibration receptors of an insect standing on solid substrate has motion components mainly perpendicular to the substrate surface (Marld 1983l. In all tree- and bush-dwelling insects torsional and bending waves transmit vibration through vegetation (plant stems, branches, etc.) (Michelsen et al. 1982, Keuper and Kuhne 1983). It is noteworthy that in almost all laboratory investigations concerned with sensory or central neurons and behavioral responses to vibratory stimuli, more or less the same stimulating technique has been employed. The experimental vibratory stimulus is a sinusoidal oscillation generated by an electrodynamic vibrator applied to a preparation or an animal via a connecting rod or platform. Calibrations are generally made with an accelerometer. These vibratory stimuli are characterized by the frequency of the sinusoidal oscillation, which can also be modulated into pulses (parameters: pulse duration, pulse repetition rate, rise and fall times of the pulse), and by stimulus intensity, which can be expressed either in terms of acceleration (m/s 2 ), velocity (m/s), or displacement (m). Near-Field Motion of Medium This is a special type of rhythmic, mechanical signal whose perception is neither "true" hearing nor "true" vibratory reception. An object that oscillates in an elastic medium, such as air, generates and radiates a farfield compressional sound wave when the vibrating body is large compared to the sound wavelength. Below a critical size-frequency limit (i.e., when the emitter works at a low frequency) the resulting air oscillation is a form of near-field sound. Most of the energy appears as local pressure

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gradients rather than as propagated longitudinal pressure waves. Therefore, motion of the medium (particle displacement) becomes the dominant parameter. The effective communicative range on the basis of these near-field medium-oscillation signals is relatively low. It is determined by specific characteristics of the emitting structure and the receptor (Tautz 1979). Receptors adapted to receive this type of medium-vibration are the filiform hair sensilla on the cerci (see chapter 7).

Behavioral Responses to Substrate Vibration The evolution of a highly sensitive system for the detection of substrate vibration in crickets allows its use in different behavioral contexts. First, substrate vibration can participate in intraspecific communication, either as a separate channel or in combination with auditory information. Second, it can be part of a general warning system, since substrate vibrations generated by an approaching predator elicit an alert or escape reaction. Through natural selection, sensitivity and/ or reaction time should have improved on the receiver side. Third, the detection of self-produced vibrations during different motor activities of the cricket (e.g., stridulation, walking) could be used for proprioceptive feedback control. Substrate vibration for intraspecific communication and orientation is well documented in tettigoniids (Kalmring et al. 1983, Latimer and Schatral1983). Tettigonia cantans can localize a sound source from acoustic cues alone. When, however, additional vibratory stimuli transmitted by a singer via the substrate are provided, the receiver prefers to follow a vibratory track toward the sound source. This strategy may be of advantage in environments where a singer is concealed by vegetation and an approaching female is confronted with a choice on her way to the sound source. Whether tree crickets use similar mechanisms remains to be investigated. However, in the large group of ground-living cricket species, substrate vibrations, as far as we know, are relevant as communicative signals only at close range. Long-range communication based on such signals is not expected because of the limited transmission properties of soil. The use of substrate vibration signals in predator-prey relationships is indicated by a commonly observed phenomenon in crickets: a singing ground cricket suddenly stops singing (silencing response) when a 1mman obsmver or any other large object approaches. Vibrations of the ground, caused by footsteps, provide an appropriate warning signal because the singer stops even if the source of disturbance is not seen. In field experiments in a meadow occupied by singing males of Gryllus campestris, an iron ball weighing 3 kg was dropped from a height of 1 m (M. Dambach, unpublished) while calling songs of selected singers were individually recorded with small microphones placed nearby. At a distance of 7 m the silencing reaction was elicited. The percentage of silencing re-

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spouses increased as the stimulus was brought progressively closer to the singer (80% silencing responses at a distance of 5 m, 100% responses at 3 m and closer). A falling iron ball is certainly not a biological stimulus, but with this method vibration stimuli can be reproduced and quantified under field conditions. In a subsequent experiment on similar natural substrate, the attenuation of such a shock signal was measured in relation to distance. Calibrated accelerometers were mounted in the soil at different distances from the point of impact. The spectral content of the signal passing the three recording sites was calculated (von Dahlen 1981), and a frequencydependent attenuation with transmission distance was found (Fig. 6.1). At a distance of several meters, only frequencies lower than 400Hz remain strong enough to stimulate the vibratory receptors. The ground, therefore, acts as a low-pass filter. Thus it can be concluded that the silencing response is based mainly on stimuli containing frequencies below 400 Hz. These results in the field were confirmed in the laboratory: male crickets singing on a platform were stimulated with defined vibratory shocks (Fig. 6.2) and von Dahlen found two distinct types of behavioral responses. The first was elicited with stimulus frequencies below 400 Hz and was identical to the silencing response described in field behavior: the male stopped singing for several seconds, usually lowered its tegmina, and changed leg position. This makes sense because cessation of singing makes it difficult, if not impossible, for a listening predator to localize prey. Changes in leg position may optimize sensitivity for signals or prepare the animal for escape. The second type, response to vibration with frequencies above 400 Hz and up to 3 kHz, is characterized by a pause or delay in the chirp rhythm of the male (delay response): when the stimulus is applied within the interchirp pause, the subsequent chirp is shifted in time IFig. 6.2). Stimulation during the chirp, or up to 50 ms before its expected onset, had no effect. It was possible to determine the thresholds for both types of responses using sinusodial vibration stimuli (von Dahlen 1981). Within the frequency range of 50-500Hz, the silencing response exhibited a slight habituation. Dishabituation was avoided by increasing the stimulus strength slowly to the values eliciting a delay response (Fig. 6.2). The threshold cmves for the delay response coincided largely with the electrophysiologically measured thresholds of the suhgenual organ (cf. Figs. 6.2 and 6.4). This suggests that the subgenual organ receives the sensory input for the delay response. Sensory input pathways responsible for the silencing response have not yet been clearly determined. Possible candidates are subgenual organ receptors connected to low-frequency interneurons (LF1 type, Kuhne et al. 1985; cf. Fig. 6.6), campanifonn sensilla of the trochanterofemoral joint, and, perhaps, even a chordotonal organ in the distal tibial region (see chapters 7 and 13). While the biological significance of the silencing response is quite obvious as a strategy to reduce predator success, it is still difficult to relate the delay response of singing males to a particular behavioral context.

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However, this response may reflect properties of the song-generating neuronal network and its control by external and self-generated vibratory stimuli. Sensory Basis of Substrate Vibration Perception Sense Organs The subgenual organs have long been suspected as sensors for substrate vibration (Eggers 1928), but clear proof came only with the advent of electrophysiological recordings (Autrum 1941a, Autrum and Schneider 1948). These scolopophorous organs are located below the femorotibial joints within the proximal parts of all legs (see chapter 13). Their gross anatomical structure varies in different insect orders, as revealed by comparative studies in Blattodea, Dermaptera, Saltatoria, Phasmida, Lepidoptera, and Hymenoptera (Friedrich 1929, Autrum and Schneider 1948). The most elaborate subgenual organs are found in Blattodea (cockroaches) and in Saltatoria (which includes crickets). Within the genus Gryllus each

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Fig. 6.3 Sense organs within the proximal part of the left foretibia of Gryllus campestris. Frontal view of a longitudinal section shows arrangement of tracheae, nerves, and sense cells. CS, campaniform sensilla; LN, leg nerve; LT, large tympanal membrane location; MTR, main trachea; SGO, subgenua! organ; SN, subgenual nerve; ST, small tympanal membrane position; STR, small trachea; TAN, tarsal nerve; TIN, tibial ne1ve; TN, tympanal nerve; TO, tympanal organ. (Combined from Michel 1974 and Eibl 1978, courtesy of Zeitschrift fiir Morphologie der Tiere and Zoomorphologie, respectively.)

organ contains ca. 25 sense cells arranged in rows like the ribs of a fan, which-together with accessory cells-are suspended across the hemolymph canal of the tibia (Fig. 6.3). In the Saltatoria Ensifera (crickets, katydids, etc.), where tympanal organs are developed in the foretibiae, the subgenual organs are adjacent to them and separating both sense organs is anatomically difficult. Middle and hindlegs lack typical tympanal organs but possess homologous structures of reduced development and subgenual organs (Eibl1978; chapter 13, this volume).

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Each tibia is also equipped with a group of 14-15 campaniform sensilla arrayed in a characteristic pattern ncar a cuticular boundary that externally marks the location of the subgenual organ (Eibl 1978). The courses and central projections of the primary sensory fibers of these tibial sense organ complexes show a high degree of correspondence within the three pairs oflegs (Eibl and Huber 1979). The receptors project only to the ipsilateral half of their segmental ganglion, with a clear separation between tympanal elements (especially the serially homologous fibers to the mesothoracic and metathoracic ganglia) projecting to a crescent-shaped neuropile; and nontyrnpanal tibial elements arborize outside the crescent. Six groups of campaniform sensilla, located near the trochanterofemoral joints of all three pairs of legs in Gryllus campestris, also seem involved in the reception of substrate vibration signals (von Dahlen 1981). Electrophysiology of Vibration Receptors Vibration sensitivity in sensory cell complexes of insect legs has been reported in various insects with or without subgenual organs, such as termites and cockroaches, leaf-cutting ants, tettigoniids, and crickets (Autrum 1941a, A utrum and Schneider 1948, Howse 1964, Markl1970, Schnorbus 1971, Dambach 1972a). In all of these studies electrical activity was recorded with metal electrodes implanted into the nerves of excised legs or vvith wire electrodes surrounding the leg nerves of freely standing animals (Dambach 1972a) while the legs were vibrated at various frequencies and intensities. The threshold for a stimulus-correlated neural response was determined to evaluate the maximum sensitivity of the whole leg. It was shown that the sensitivity differed for each pair oflegs (Autrum 1941a, Dambach 1972a). Furthermore, these multiunit recordings localized the sources of sensory input by selective destruction of the sense organs. In crickets the threshold curves for sinusoidal vibrations differ for the three leg pairs in well-defined and separated maxima of sensitivity (Fig. 6.4). Maxima of sensitivity were found between 400 and 500 Hz for the hindlegs, between 700 and 1,000 Hz for the middle legs, and between 800 and 1,000 Hz for the forelegs. All stimulus intensities were measured as acceleration. If a hindleg was artifically damped with an additional mass, the maximum sensitivity did not shift. This means it cannot be affected by resonance properties of the entire leg, suggesting that resonances of the subgenual organ itself determine the maxima of vibration sensitivity (Dambach 1972a). The difference in sensitivity between isolated legs and those in intact, free-standing individuals is surprisingly small (compare the curve from Autrum and Schneider [1948] with curves from Dambach [1972a] in Fig. 6.4!. Therefore contact between the legs and the vibrating substrate transmits the stimulus, and the sensitivity of the subgenual organ is scarcely affected by articulations or by vibrating properties of the entire leg. Recordings from single sensory neurons, most likely representing sin-

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Fig. 6.4

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' 180 spikes/s: Nolen 1984). Is neural activity in Int-1 sufficient to elicit negative phonotaxis in tethered flying crickets even in the absence of ultrasound? Figure 11.19 shows that it is. It was necessmy to use anode-break-excitation to excite Int-1 at a high spike rate; but when it was so excited, EMG activity was

359

Evasive Acoustic Behavior and Its Neurobiological Basis

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Fig. 11.15 Intracellularly recorded responses of prothoracic auditory Interneuron-1 in a Teleogryllus oceanicus adult female when stimulated by signals similar to echolocation cries of bats, presented at different pulse repetition rates. Ultrasonic stimuli consisted of 0.5-ms clicks (pulses) at 30 kHz with a soundpressure level of 80 dB. A, 10 pulses/s. B, 50 pulses/s. C, 100 pulses/s. D, 200 pulses/s. E, 300 pulses/s. There were at least 10 s between stimulus trials. (From Moiseff and Hoy 1983, courtesy of Journal of Comparative Physiology.)

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?.___ __....,.,.~•.------- 30kHz Fig.11.16 Experimental preparation used in necessity and sufficiency tests of the role of prothoracic auditory Intemeuron-1 (Int-1) in ultrasound-stimulated steering behavior in tethered flying Teleogryllus oceanicus adult females. A minimally dissected animal was mounted ventral side up with the prothoracic ganglion exposed for microelectrode recording and stimulation, and myogram electrodes were inserted into abdominal dorsal longitudinal muscles (DLM, steering muscles). The animal was stimulated by a standard 30-ms excitatory tone of 30kHz at 85 dB, played from the right, which elicited a burst of spikes in the right INT-1 and action potentials in the contralateral (left) DLMs. The ipsilatei'al (right) DLMs were not responsive to this ultrasound stimulation protocol. Calibrations below: ordinate, 50 mV; abscissa, 50 ms. (From Nolen and Hoy 1984; Science 226:992-994, © 1984 by the AAAS.)

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4 Fig. 11.17 Simultaneous activity in prothoracic auditory Intemeuron-1 (Int-11 and abdominal dorsal longitudinal muscles (DLMI in tethered flying Teleogryllus oceanicus adult females stimulated by ultrasound. A, Control, negative phonotactic steering response in a "fictively flying" animal (flight muscles active); the ipsilateral (with respect to sound source) Int-1 was strongly excited, followed by muscle spikes in the contralateral DLM. B, Response of Int-1 to ultrasound depressed by direct injection of a hyperpolarizing electrical stimulus ( -14 nA) preceding presentation of sound; down-pointing arrow indicates unset of hyperpolarization, up-pointing arrow indicates release from hyperpolarizing stimulus; hyperpolarization strongly diminished Int-1's spike rate-although excitatory postsynaptic potentials (EPSPs) persisted throughout the auditory stimulationand abolished contralateral DLM response. C, Control trial immediately following the hyperpolarization trial in B, both Int-1 and the contralateral DLM have regained their responsiveness following sound stimulation. Experimental preparation as in Figure 11.16; 1, intracellular recording from the left Int-1; 2, extracellularly recorded muscle spikes from right DLM; 3, similar myograph from left DLM; 4, auditory stimulus pattern used in all eJ~.'f)eriments, 330-ms, 30-kHz tone played from the left side at a sound-pressure level of 82 dB; voltage calibration above only for Int-1 records. (From Nolen and Hoy 1984, Science 226:992-994, © 1984 by the AAAS.)

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> overlapping those of types AN1 and ANZ. Furthermore BNCZs rarely exhibit a single distinct projection area; their tenninal structures do not allow identification of input (presynaptic) and output (postsynaptic) regions. Extrinsic glomerular neurons (EGN) respond to auditory stimuli and to stimuli of other modalities (Schildberger 1984a) and arborize in the glomerular neuropile of a mushroom body or of the central body complex. Their widespread additional branches indicate that they may contact ascending, descending, and/or local brain neurons. Generally EGNs exhibit structural polarity: in the calyces ofthe mushroom bodies they show knoblike endings; the terminations within the mushroom body lobes are fine and dense. Ultrastructural investigation ofEGNs revealed presynaptic terminals in the calyces and postsynaptic terminals in the lobes (Schiirmann 1986).

Descending brain neurons. Figure 14.3 shows examples of descending brain neurons (DN) with a soma in the brain and an axon descending to the ventral nerve cord. Such neurons have arborizations within the brain, mainly in a postero-ventral region, that overlap with projections from most other cell types previously described. Their axons course either ipsilaterally (Fig. 14.3 DN1) or contralaterally (Fig. 14.3, DNZ), in relation to their somas, down to the ventral cord. Anatomical Aspects of Central Auditory Pathways Despite limited knowledge about the number of central auditory neurons in crickets and their morphology, a picture emerges which allows some anatomical interpretation of the central auditory pathway. Auditory information arriving from the ears is transmitted to several prothoracic neurons, and their morphologies indicate that it is transferred to the head ganglia as well as to subsequent ventral cord ganglia. Within the prothoracic ganglion, the two intrasegmental ON1s and ONZs connect both ganglionic hemispheres, indicating binaural processing. Within the brain, ascending auditory neurons are apparently connected via BNC1s and

Central Auditory Pathway Behavioral Correlates

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BNC2s to descending neurons, possibly even including pathways that involve mushroom bodies and the central body complex. There is further anatomical evidence that ascending neurons in the brain may transfer information more directly to descending cells. This would shorten the response time, a prerequisite for rapid behavioral actions such as predator avoidance. A physiological indication for a loop via the brain is given in tethered flying Teleogryllus oceanicus that do not exhibit abdominal flexions to sound after the connectives between the pro thoracic ganglion and the head ganglia have been cut (see chapter 11). Furthermore, Nolen and Hoy (1984) found that their Int-1 neuron (probably homologous to AN2) seems both necessary and sufficient for eliciting the abdominal motor response. A loop pathway including the subesophageal ganglion has not been reported in crickets, although some arborizations of type AN1, AN2, and TN1 neurons indicate that contacts may exist there. In locusts Boyan and Altman (1985) have described neiVe cells that respond to sound, having cell bodies in the subesophageal ganglion and axons ascending to the brain. But for none of the neurons descending from the prothoracic ganglion or from the brain are the target cells known. Processing of Auditory Information by Central Neurons We concentrate first on identified auditory neurons in the prothoracic ganglion and in the brain, and describe their responses related to sound frequency, intensity, and direction. Later we focus on neuronal correlates for song recognition and sound localization. Sound Frequency Cricket ears are sensitive within a broad range of sound frequencies (at maximum from 2 to 100kHz) which goes far beyond the range of frequencies produced in the songs of males (see chapters 8 and 13). Ultrasound frequencies are perceived and provide information about acoustically signalling predators such as bats (see chapter 11). But crickets even hear the noises produced by stridulating katydids and grasshoppers, as well as by timballing cicadas, sharing the same habitats. For intraspecific communication often only certain small frequency bands are used. In the calling songs of most cricket species the frequency range covers 2-8 kHz (see chapter 8), but Otte and Alexander (1983) give audiospectrographs of calling songs for three Australian species of Gryllotalpa (Gryllotalpinae) whose center frequencies are from 1.5 to 1.7 kHz and for 16 species in seven other genera (representing subfamilies Gryllinae, Mogoplistinae, and Nemobiinae) which are from 8.6 to 9.9 kHz; courtship sounds of crickets commonly contain wider bands of frequencies and higher center and upper frequencies than their calling songs. In Gryllus campestris and G. bimaculatus the threshold for phonotaxis is lowest between 4 and 5 kHz, and it increases with increasing sound frequency, accompanied by a

432

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change in the angle oftracking (see chapter 10). In Teleogryllus oceanicus sound frequencies above 15 kHz elicit negative phonotaxis (see chapter 11).

A first step of frequency analysis and discrimination is carried out in the ear, and different receptors encode different frequencies (see chapter 13). The question arises whether frequency discrimination of the ear stays the same, is enhanced, or even reduced, within the central auditory pathway. Prothoracic auditory neurons exhibit two types of frequency tuning (Fig. 14.4). Low-frequency neurons, such as types AN1, ON1, ON2, and DN1, have their lowest thresholds around the carrier frequency of the calling song (4-5kHz); and high-frequency neurons, such as types AN2 and TN1, have lowest thresholds above 10kHz. Among the ascending AN1s two classes can be separated among different animals, those with sharpest tuning between 4 and 8kHz at all intensities; and others that respond more evenly even up to 20 kHz. There is some indication that variability in tuning of AN1s derives from varying inhibition in response to sound frequencies above 10 kHz. In some individuals this high-frequency inhibition is overridden by excitatory input at high sound-pressure levels. A physiologically different low-frequency neuron in G. campestris (called AN3 by Boyd et al. 1984) exhibits an additional inhibitory sideband below 4kHz. A shrupening of tuning by inhibition in the low- or highfrequency range, or both, has not been obseJVed in DN1 and ON2. TYPe AN2s in some individuals of G. campestris and G. bimaculatus show larger differences in their tuning cuJVes, although greatest sensitivities are most commonly obseiVed above 10 kHz. Many of the AN2s exhibit a bimodal tuning cuiVe, peaking at 15 kHz and also at 5 kHz. This latter peak varies widely in sensitivity and is even missing in some individuals. It has been suggested that AN2s may exist in more than a single mirror-image neuronal pair, although unequivocal morphological evidence is still missing. In Acheta domesticus two functionally different high-frequency neurons of the AN2 morphological type have been reported (Stout et al. 1985), and recently an L-shaped type has been described as most sensitive to frequencies of 4-5 kHz with a decrease in sensitivity at higher frequencies (8-15kHz) of about 15-20 dB (Stout et al. 1988). A clear example of an inhibitory effect at 5kHz is given by Hutchings and Lewis (1984) for an ascending neuron (probably homologous to AN2) in Teleogryllus oceanicus, on the basis of two-tone stimulation. In this neuron simultaneously presenting a 5-kHz sound suppressed a strong excitatory response to high-frequency sound. The behavioral importance of this suppression in T. oceanicus is not yet understood because the lowfrequency component is necessary for phonotaxis, even though higher song harmonics increase the fidelity oflocalization and orientation (Latimer and Lewis 1986). Local and descending brain neurons repeat, to some extent, the tuning obseiVed in prothoracic neurons. Again, two classes can be distinguished: a low-frequency class (sharply tuned around 5 kHz) and a class with

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broader tuning (Fig. 14.4 c, d). But even at their best frequencies, brain neurons in general have thresholds about 15-25 dB higher than the prothoracic neurons. So far there is no indication of sharper tuning of auditory neurons in the brain.

Behavioral relevance of two-tone suppression. Hutchings and Lewis (1984) provide evidence that two-tone suppression functions to sharpen pattern copying in ascending neurons ofT. oceanicus, because inhibition limits the duration of the excitatory response to 5-kHz syllables in the corresponding neuron and enhances copying the temporal rhythm. In Gryllus bimaculatus a local brain neuron was identified best tuned to the courtship song carrier frequency (around 16 kHz). It is suppressed in its response to a 16-kHz tone as soon as a 5-kHz tone is presented (Boyan 1981). The behavioral significance is not clear. Two-tone interactions may even play a role in the switch from positive to negative phonotaxis (see chapter 11). Sound Intensity Acoustic communication in the field functions only within certain distances; these are determined by the loudness of the sender's signal, by attenuation of the signal when travelling through the habitat, by climatic factors, and by the sensitivity of the individual's ears and auditory pathway. The intensities covered by the auditory system range from soundpressure levels about 35-100 dB. The loudness of the calling song in some ground crickets decreases from 100 dB at the source to about 80 dB at a distance of 1 m, and minimum hearing threshold is reached at a distance of about 10m, measured in grassy vegetation (Popov et al. 1974). Encoding of sound intensity by auditory receptors and central neurons is best described by their intensity-response function (Fig. 14.5). The ascending neuron known to receive excitatory input predominantly from low-frequency auditory receptors (AN1) most typically exhibits a linear relationship, from 35 to 40 dB (5-10 dB above threshold) to more than 90 dB at the calling song carrier frequency of 5 kHz. This range covers all calling song intensities encountered in nature. When stimulated with sound frequencies above 10 kHz, and with increasing sound intensities, the number of spontaneously occurring action potentials in AN1 is reduced, indicating some form of inhibition delivered through a highfrequency pathway. JYpe AN2s receive excitatmy input from both low- and high-frequency receptors, but the slope in their intensity-response function differs for the two frequency bands; it is much steeper in the high-frequency range. Furthermore, AN2 responses to high-frequency stimuli do not saturate until sound-pressure levels of 90 dB (Fig. 14.5). Encoding of sound intensity by low-frequency (5kHz) local brain neurons (BNCl) differs from AN1s; their response curves are flatter and saturate at higher sound intensities than those of AN1s. The slopes at 15-kHz

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