yes Identity and phylogenetic significance of the metapostnotum in nonaculeate Hymenoptera [1/1, 1 ed.]

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Identity and Phylogenetic Significance of the Metapostnotum in Nonaculeate Hymenoptera JAMES B. WHITFIELD,1 NORMAN F. JOHNSON, AND MICHAEL R. HAMERSKI Department of Entomology, Ohio State University, Columbus, Ohio 43210

Ann. Entomol. Soc. Am. 82(6): 663-673 (1989)

ABSTRACT The metapostnotum is present and distinguishable in members of nearly all examined families of Hymenoptera; its anatomy and associated structures are described and figured for representatives of nonaculeate families. On the basis of the evidence from the metapostnotum and associated structures the Orussidae are the extant sister-group of the apocritan Hymenoptera, and the Cimbicidae are convergent with respect to metapostnotal structure with the Orussidae and Apocrita. Parallel reductions in the metapostnotum and associated muscles have occurred in the Ichneumonoidea, Chalcidoidea, Proctotrupoidea, Cynipoidea, and Ceraphronoidea; in all of these groups, at least some species have relatively unreduced structures. KEY WORDS Insecta, musculature, comparative anatomy, phylogeny

IN THE PAST 15 YEARS, concepts of the phylogeny

and higher taxa of the Hymenoptera have changed considerably. It has long been suspected that the Symphyta, considered in a broad sense, and the "Parasitica," or nonaculeate Apocrita, are at best paraphyletic groups, but convincing evidence to support a single alternative classification has been lacking. The work of Brothers (1975), using the more extensive knowledge of the aculeate Hymenoptera, produced a new classification of Aculeata that, although not universally accepted, forms a solid base from which future work can proceed (Carpenter 1986). Unfortunately, the extensive phylogenetic treatment of Hymenoptera by K6nigsmann (1976,1977,1978a,b), drawn mainly from literature sources, made only too clear that a mature classification of the nonaculeate Hymenoptera would require far more new data, as well as careful comparative studies of previously unanalyzed character systems. In recent years entirely new or previously underused sources of phylogenetic data have been tapped, including thoracic and metasomatic musculature (Daly 1963, 1964; Gibson 1985; Johnson 1988; W. R. M. Mason, personal communication), skeletal structures of the pleural and metathoracic regions (Brothers 1976; Masner 1979; Saini & Dhillon 1980; Shcherbakov 1980, 1981), comparative behavior (Farish 1972, Iwata 1976), the structure of the venom glands of adult females (Robertson 1968), and the fossil record (Rasnitsyn 1969, 1980). Findings from these studies often have challenged or contradicted earlier classifications at the superfamily, infraorder, and suborder levels. 1 Current address: Department of Biology, University of Missouri-St. Louis, St. Louis, Mo. 63121.

Brothers (1975, 1976) found the metapostnotum to be an important phylogenetic character system for the Aculeata; Saini & Dhillon (1980) extended Brothers' survey through other groups of Hymenoptera, but with little emphasis on the actual structure of the metapostnotum and very little representation of the parasitic higher taxa. Saini's (1986) survey is more complete for Symphyta, but it still omits critical parasitic taxa such as Orussidae, Megalyridae, Evanioidea, Stephanidae, and Proctotrupoidea. The traditional ignorance of the comparative anatomy of the metapostnotum and associated sclerites and musculature is surprising, because it is in this region of the body that much of the morphological change characterizing the apocritan has taken place. In its plesiomorphic condition (Snodgrass 1927), the hymenopteran metapostnotum is a transverse, medially continuous tergite. It articulates or is fused with the metepimeron laterally. The metapostnotum is closely associated with the metanotum anteriorly and the first abdominal tergum posteriorly (propodeum in the Apocrita), but is not completely fused with them. Internally the metapostnotum bears the third phragma; this is usually marked externally by a groove or sulcus. The third phragma provides the posterior points of attachment of the paired metathoracic longitudinal flight muscles (2ph-3ph), which attach at their anterior ends to the second phragma (Brothers 1976). Snodgrass (1910, 1927) believed that the metapostnotum is composed of the enlarged precosta and antecosta of the first abdominal segment. Because of its double-walled structure, Matsuda (1970) concluded that the third phragma is of more complex origin (i.e., it is derived from more than the antecosta). The structure of the pamphiliid meta-

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postnotum (see below) appears to support this latter view, but a full resolution of this issue requires analysis of insects outside of the Hymenoptera. The correct identification of the metapostnotum in Hymenoptera can be made using several lines of anatomical evidence. It is continuous or articulates laterally with the metepimeron anterior to the propodeal spiracle. It provides the posterior sites of attachment for the 2ph-3ph muscles (usually by means of the third phragma). It bears the third phragma (when developed). It is usually demarcated from the first abdominal tergite (or propodeum) by the metapostnotal-propodeal suture. When specimens are dissected and examined carefully, the metapostnotum can be positively identified, at least laterally, in virtually all groups of Hymenoptera. Nevertheless, its identity in many groups has been ignored or obscured by incorrect terminology, especially in Apocrita (Brothers 1976), although notable exceptions exist (Richards 1956, Riek 1970). Our work provides a more extensive survey of the identity, anatomy, and distribution of the metapostnotum and associated structures among the nonaculeate hymenopteran families. We supplement the survey with a discussion of the possible evolutionary significance and phylogenetic trends in metapostnotal structure, and their implications for the classification of Hymenoptera. Materials and Methods For all taxa except Syntexis, Megalyra, Megischus, and Ibalia, material preserved in 70% ethanol or Dietrich's solution was used. Thoraces were cracked between the mesopleuron and metapleuron and separated, causing the dorsal surface to split between the scutellum and metanotum. The relative positions of various muscles and sclerites were noted, then the metanotum was carefully dissected away from the anterior edge of the metapostnotum, which it usually overlapped. For the dry-preserved taxa listed, the same procedure was used following maceration in 10% potassium hydroxide (KOH). For some individuals musculature was removed by maceration in 10% KOH; in others, the attached muscles were left intact. Some (especially smaller) specimens were then dehydrated and critical-point dried. Muscle attachments on the second phragma were noted from the wet dissections, as were all structures on the larger individuals. Additional techniques for examination of these features are given by Hamerski (1984). A listing of taxa examined is presented in Table 1. Voucher material of the dissections is maintained by the second author. Results The metapostnotum is present and identifiable in at least some members of all examined groups

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Table 1. Taxa in which the metapostnotum has been examined (all females). For those taxa marked by an asterisk only a single specimen was examined; for all others three specimens were dissected Family Symphyta Xyelidae Pamphiliidae Tenthredinidae Argidae Cimbicidae Cephidae Anaxyelidae Siricidae Xiphydriidae Orussidae Apocrita Megalyridae Ceraphronidae Megaspilidae Stephanidae Evaniidae Aulacidae Gasteruptiidae Trigonalidae Ichneumonidae Braconidae Pelecinidae Heloridae Proctotrupidae Diapriidae Scelionidae Platygastridae Ibaliidae Figitidae Cynipidae Torymidae Pteromalidae Eulophidae Chalcididae Eurytomidae

Genus and species Xyela sp. Pamphilius excavatus (Norton) Dolerus sp., Tenthredo sp., Macrophya sp. Schizocerella pilicornis (Holmgren)* Trichiosoma triangulum Kirby* Cephas cinctus Norton Syntexis libocedrii Rohwer* Tremex columba (L.) Xiphydria sp. Orussus terminalis Newman Megalyra fasdipennis Westwood* Ceraphron sp. Megaspilns sp. Megischus sp. Prosevania fuscipes (Illiger), Zeuxevania sp.* Aulacus spp., Pristaulacus strangaliae Rohwer* Gasteruption spp. Orthogonalys pulchella (Cresson) Ichneumoninae undetermined sp., Megarhyssa sp., Scambus sp. Atanycolus sp., Aleiodes sp., Macrocentrus sp. Pelecinus polyturator (Drury) Helorus anomalipes Panzer* Exallonyx sp. Trichopria sp., undetermined Belytinae sp., undetermined Diapriinae sp. Scelio sp., Trissolcus sp. Isocybus sp., Platygaster sp. Ibalia leucospoides (Hochenwarth)* undetermined sp. Andricus sp. Torymus sp. 3 undetermined spp. Sympiesis sp. Spilochalcis sp.* Eudecatoma sp.

of Hymenoptera (with the possible exception of Ceraphronoidea), although its structure and extent are extremely variable among the families. In many species, it is reduced. Below is a brief description of the structures in each family (Symphyta) or superfamily (most Apocrita). Symphyta Xyelidae. The metapostnotum in Xyela is a continuous transverse strip, narrower medially, partially fused to Tl, but separate from the metanotum (Fig. 1). Internally it bears the third phragma, represented by two shallow lateral braces that nearly reach the midline. Externally the position of the phragma is marked by a transverse inflection in the surface of the metanotum at about its midlength. The 2ph-3ph muscles attach to the anterior faces of the metapostnotum as flattened bundles and extend anteriorly to attach dorsally along the

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PN3

PN3

Fig. 1-6. Dorsal view of metanotum, metapostnotum, and anterior two abdominal tergites. (1) Xyela sp., (2) Macrophya sp., (3) Cephus cinctus, (4) Syntexis libocedrii, (5) Tremex columba, (6) Xiphydria sp. N3, metanotum; PN3, metapostnotum; Tl, first abdominal tergite; T2, second abdominal tergite. The stippled regions are membrannns. nous.

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2ph-3ph costa relatively dorsally on the second phragma. The first tergite (Tl) is split medially. Pamphiliidae. In Pamphilius excavatus, the metapostnotum is a transverse strip separate from Tl and the metanotum. It is continuous medially anterior to the third phragma but divided into two lateral wedge-shaped sclerites posterior to the phragma. Whether this is an indication that the posterior portion is of similar origin to Tl, which is also medially split, and the anterior of different origin cannot be determined. The third phragma is represented internally by two well-developed lateral braces that nearly reach the midline. Externally it is marked by a transverse inflection of the metanotum at its midlength. The 2ph-3ph muscles attach to the third phragma as a flattened bundle and to the second phragma (along the 2ph3ph costa) subdorsally. Tenthredinidae. The metapostnotum is present as a transverse, medially continuous tergite, separate from both Tl and the metanotum (Fig. 2). The third phragma is reduced over much of its width, appearing as sublateral protuberances internally and as an inflection of the metapostnotal surface externally. The 2ph-3ph muscles are usually reduced in size relative to the preceding groups and attached to the anterior faces of the sublateral lobes and nearby on the phragma. Tl is split medially. Argidae. The metapostnotum is a transverse, medially continuous, but weakly narrowed tergite, partly fused to Tl sublaterally, but separate from the metanotum. The third phragma is represented by two shallow, thin braces extending laterally from near the midline. The phragma is marked externally by an inflection of the metapostnotal surface posterior to the midlength of the tergite. The 2ph3ph muscles attach as a flattened bundle to the anterior faces of the third phragma and extend anteriorly to attach to the 2ph-3ph costa near the dorsal edge of the second phragma. Tl is split medially. Cimbicidae. The metapostnotum is extremely narrow medially and is virtually hidden in external view, completely fused to Tl, and separated from the metanotum. The third phragma is represented internally by shallow, thin braces extending laterally from near the midline. The third phragma appears to be near to or at the posterior margin of the metapostnotum which is discernible in dorsal view only laterally. The 2ph-3ph muscles attach as a flattened sheet to the anterior faces of the metapostnotum and anteriorly to the second phragma near its dorsal margin. Tl is not split medially but shows some ridging medially that may mark a line of fusion. Cephidae. The metapostnotum is split medially, matching Tl in this respect (Fig. 3). The third phragma is essentially absent, and the poorly developed 2ph-3ph muscles attach to the inner surface of the metapostnotum posteriorly and anteriorly to the paired lateral portions of the 2ph-3ph

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costa on the second phragma. The metapostnotum and Tl, although not fused, act as a single hinged unit. Anaxyelidae. As in the next two families the metapostnotum and Tl are medially split (Fig. 4). The metapostnotum is not fused to Tl (unlike Siricidae), and the third phragma is essentially absent (unlike Siricidae and Xiphydriidae, but as in Cephidae). The poorly developed 2ph-3ph muscles attach directly to the inner surface of the metapostnotum. Siricidae. The metapostnotum is split medially (as in Cephidae) so that it folds medially along with the split Tl (Fig. 5 and 10). It is fused to Tl and laterally bears a well-developed third phragma. The third phragma is represented by thin braces extending from near the midline and is produced laterally into small rounded flanges. The 2ph-3ph muscles are absent. The muscles that attach as tight bundles to the anterior faces of the lateral lobes of the third phragma are instead the t3-3ph muscles, which arise anteriorly on the metanotum. Xiphydriidae. The metapostnotum and Tl are medially split (Fig. 6), and the metapostnotum is at least weakly fused sublaterally to Tl. The third phragma bears a pair of small sublateral rounded lobes to which the strong t3-3ph muscles attach (Fig. 11). Externally the position of the third phragma is marked by an inflection near the midlength of the tergite. Orussidae. The condition of the metapostnotum and associated structures is very similar to that of many nonaculeate Apocrita (e.g., Fig. 8). Neither the metapostnotum nor Tl is split medially. The metapostnotum is immovably fused to Tl and strongly narrowed medially (Fig. 7). The third phragma appears to be at or near to the posterior margin of the metapostnotum and is represented internally by very shallow ridges extending laterally from either side of the midline (Fig. 12). Externally, its position (and probably also that of the posterior margin of the metapostnotum) is marked by a crenulate groove extending the width of the fused tergites (Fig. 7). The 2ph-3ph muscles appear to be very short and strong, attaching laterally to the third phragma and extending anteriorly to the 2ph-3ph costa on the second phragma at the dorsal (anterior) margin of the cenchri. Apocrita

Unlike many of the Symphyta, in Apocrita the first abdominal tergite (Tl, propodeum) is never divided medially, but forms a single, often strongly convex and declivous posterior wall of the mesosoma. Posterior to the metapostnotum, the propodeum is often the anterodorsal plane of attachment of muscles involved in the metasomatic articulation. The metapostnotum also is never truly divided medially, although it is often depressed medially and reduced to a thin strip anterior to the meta-

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t3-3ph C

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8 Fig. 7-12. 7 and 8. Dorsal view of metanotum, metapostnotum, and anterior abdominal tergites. (7) Orussus terminalis, (8) Megalyra fasciipennis. 9-12. Anterior, partly internal view of metapostnotum, third phragma, and third phragmal muscles. (9) Trichiosoma triangulum, (10) Tremex columba, (11) Xiphydria sp., (12) Orussus terminalis. N3, metanotum; PN3, metapostnotum; Tl, first abdominal tergite; T2, second abdominal tergite; 3ph, third phragma; 2ph-3ph, stub or location of attachment of longitudinal muscles 2ph-3ph; t3-3ph, stub of longitudinal muscles t3-3ph. postnotal-propodeal suture. Even more often it is hidden in dorsal external view by the overhanging metanotum. In some cases (especially Chalcidoidea), it is absent. In many apocritans, the metapostnotum is often difficult to identify externally because of the loss of definition of the metapostnotal-propodeal suture, or confusion in coarse sculpturing patterns. The descriptions for the apocritan taxa that follow are primarily intended to aid in the recognition and extent of the metapostnotum in these groups so that homologies can be established for systematic studies. Megalyridae. The metapostnotum and third phragma are similar to that of Orussidae, although the form of the first abdominal tergum in the two groups is very different (cf. Fig. 7 and 8). Laterally

the metapostnotum is subtriangular and well developed, extending forward around the lateral edges of the metanotum. Medially the metapostnotum is a thin strip, not readily visible externally. The third phragma runs internally along the anterior margin of the groove that externally marks the line of fusion between the metapostnotum and propodeum. The 2ph-3ph muscles are present and attach sublaterally as flattened sheets to the third phragma. Stephanidae. Megischus has a large third phragma (Fig. 25), similar in overall size to that of Aulacidae and Gasteruptiidae, but their structure differs substantially. The phragma extends posteriorly at an angle from the inner surface of the ridge marked externally by the metapostnotal-propodeal suture, as a thin but broad bracing sheet of roughly

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uniform width across the dorsum of the segment. Maceration in potassium hydroxide made examination of the 2ph-3ph muscles difficult, but they are apparently present, although reduced in size relative to the phragma. The large phragma may play more of a role as the anterior point of attachment of metasomatic levator muscles. Externally, the metapostnotum is a narrow strip anterior to and including portions of the broadly crenulate metapostnotal-propodeal suture (Fig. 13). Evaniidae. The metapostnotum is very narrow, especially medially, and is fused to the propodeum but delineated from it by a narrow sulcus (Fig. 15). The third phragma extends across the entire width internally but is largest laterally, where it forms shallow braces (Fig. 27). The 2ph-3ph muscles are apparently absent. The form of the third phragma and absence of the 2ph-3ph muscles contrasts to the structure of the next two families with which evaniids are usually grouped. Aulacidae and Casteruptiidae. The metapostnotum is broad laterally, narrow medially, and delimited from the propodeum by a crenulate groove (Fig. 14). The anterior portions of the groove are apparently of metapostnotal origin, as internally the groove bears the third phragma. Unlike all other taxa save the Gasteruptiidae, the third phragma is represented by a thin bracing ridge, developed submedially into two large, flattened lobes angled posteriorly from the interior of the crenulate groove (Fig. 26). The 2ph-3ph muscles attach to the anterior faces of the flattened expansions, but are not as large as the lobes would lead one to expect. In shape, the third phragma more closely resembles the second phragma than it does the third phragma of other Hymenoptera. Trigonalidae. Orthogonalys is essentially similar to the Megalyridae and Ichneumonoidea. The metapostnotum is broadest laterally, narrow medially, separated from the propodeum by a crenulate external groove (Fig. 16), and internally bears a laterally developed third phragma. The phragma is smaller than in Megalyra and is virtually absent medially. The flattened 2ph-3ph muscles are present and insert on the lateral portions of the phragma. Ceraphronidae and Megaspilidae. Interpretation of the metapostnotum and associated structures is very difficult in these groups. Aside from the small size of the beasts, the metanotum and metapostnotum are apparently fused together with the propodeum so that the limits of the individual structures are unclear. The propodeum has a horizontal anterior and a declivous posterior portion; a crenulate furrow is sometimes present anteriorly (absent in Fig. 17) that may correspond to the metapostnotal-propodeal suture. Anterior to this, then, would lie the fused metanotum and metapostnotum. There is no evidence of a third phragma associated with the interior of the furrow. In some specimens, weak muscles arising on the posterodorsal face of the second phragma appear to

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attach to the "propodeal" wall in the vicinity of the furrow. These may be reduced 2ph-3ph muscles. Ichneumonidae and Braconidae. The metapostnotum is broad and subtriangular laterally, very narrow medially, and completely fused with the propodeum in Megarhyssa. Its line of demarcation from the propodeum is represented by a groove and change of angle of the cuticular surface anterior to the propodeal spiracles. The third phragma is medially represented by a thin internal ridge but is expanded laterally into shallow braces onto which the well-developed 2ph-3ph muscles attach. In the Ichneumoninae, the anatomy is similar (Fig. 23) except that the 2ph-3ph muscles are smaller and the metapostnotum is less strongly fused to the propodeum (and, with some effort, could actually be separated from it). Among ichneumonids, the third phragma apparently is at the posterior margin of the metapostnotum and along the anterior margin of the metapostnotal-propodeal suture. In Atanycolus (Braconidae), the metapostnotum is similar to the Ichneumonidae although reduced in size, even laterally (Fig. 24). The 2ph-3ph muscles are well developed, contrary to the findings of Alam (1951) for another braconid, Stenobracon; they attach to the anterior faces of the lateral, broader portions of the third phragma (Fig. 29). The 2ph-3ph muscles are apparently absent in Macrocentrus. In both genera, the metapostnotum is represented medially only by a thin ridge anterior to the metapostnotal-propodeal suture and is virtually or entirely hidden in dorsal external view. For many other braconids, the metapostnotum was even less obvious externally, but was always present. Proctotrupoidea Pelecinidae, Heloridae, Proctotrupidae, Diapriidae, Scelionidae, and Platygastridae. In Pelecinus, the metapostnotum is reduced to a narrow band anterior to and partially including the broad, deep crenulate groove separating it from the propodeum (Fig. 18). The third phragma is reduced to two small, lateral, subtriangular flanges along the posterior margin of the metapostnotum. There was no evidence of the 2ph-3ph muscles. In Helorus, the metapostnotal-propodeal suture is broad, deep, and crenulate; anterior to it the metapostnotum is narrowly triangular laterally and extremely narrow medially (Fig. 19). The third phragma is reduced to a thin transverse ridge, most obvious laterally, onto which very small 2ph-3ph muscles attach at the lateral ends. In Exallonyx (Proctotrupidae), the metapostnotum is laterally a subtriangular extension of the metepimeron, medially a narrow strip anterior to and partially including the broadly crenulate metapostnotal-propodeal suture. The third phragma is

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PN3

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Fig. 13-20. Metapostnotum and propodeum (abdominal Tl), dorsal view. (13) Megischus sp., (14) Pristaulacus strangaliae, (15) Zeuxevania sp., (16) Orthogonalys pulchella, (17) Megaspilus sp., (18) Pelecinus polyturator, (19) Helorus anomalipes, (20) undetermined belytine diapriid. PN3, metapostnotum; Tl, propodeum; T2, first metasomatic tergite (second abdominal tergite).

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27

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29 Fig. 21-29. 21-24. Metapostnotum and propodeum, dorsal view. (21) Isocybus sp., (22) Ibalia leucospoides, (23) undetermined ichneumonine ichneumonid, (24) Atanycolus sp. 25-29. Anterior, partly internal view of metapostnotum and third phragma. (25) Megischus sp., (26) Gasteruption sp., (27) Zeuxevania sp., (28) undetermined belytine diapriid, (29) Atanycolus sp. PN3, metapostnotum; Tl, propodeum; 3ph, third phragma; 2ph-3ph, stubs of longitudinal muscles 2ph-3ph.

represented only by a shallow ridge along the inflection corresponding to the suture, but sublaterally does receive small, slender 2ph-3ph muscles that arise from the costa on the second phragma. The muscles are small enough that their ends on

the third phragma could be misinterpreted as membrane fragments. In the Belytinae (Diapriidae, three species examined), the metapostnotum is medially a very thin, transverse band anterior to the metapostno-

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tal-propodeal suture, laterally of variable size and subtriangular (Fig. 20). The third phragma is reduced to a thin, shallow ridge internally along the anterior margin of the interior of the metapostnotal-propodeal suture (Fig. 28). The 2ph-3ph muscles are apparently absent. In the Diapriinae, the metapostnotum is sometimes larger laterally than in Belytinae, and in one Brazilian species weakly developed 2ph-3ph muscles are present and attached to the anterior face of the shallow third phragma. The metapostnotum in most species is not visible medially in external view, being overlapped by the metanotum. In larger Scelioninae, the metapostnotum is laterally narrowly transverse-triangular, medially represented by only a thin ridge anterior to the metapostnotal-propodeal suture. The third phragma is present as a continuous internal thin brace, more strongly developed laterally, onto which weak, sheetlike 2ph-3ph muscles attach. In Isocybus (Platygastridae), the metapostnotum is reduced to a thin strip over most of its width, separated from the propodeum by the metapostnotal-propodeal suture (Fig. 21). The third phragma is not or only weakly developed as a ridge along the inflection of the suture, but weak, sheetlike 2ph-3ph muscles are present and attach anteriorly to this ridge. These muscles are often absent in smaller scelionids and platygastrids. Cynipoidea Ibaliidae, Figitidae, and Cynipidae. In Ibalia, the propodeum is small and very coarsely sculptured, making external identification of the metapostnotum difficult. It is triangular laterally and represented medially by only a thin band anterior to the deep but somewhat irregularly defined metapostnotal-propodeal suture. Internally, the suture bears at its anterior edge the well-developed third phragma, a pair of sublateral, flat, transverse lobes extending into the body cavity (Fig. 22). The 2ph3ph muscles are reduced to exceedingly tiny, fragile bundles attached to the extreme lateral ends of these lobes and are probably nonfunctional.

Chalcidoidea Torymidae, Pteromalidae, Eulophidae, Chalcididae, and Eurytomidae. The metapostnotum in Torymus is an extremely narrow strip anterior to the thin metapostnotal-propodeal suture and is not obvious even laterally in external view. Nevertheless, it bears a shallow internal ridge that sublaterally receives the sheetlike 2ph-3ph muscles arising from the transverse costa on the second phragma. The metapostnotum, third phragma, and 2ph3ph muscles vary considerably among the Chalcidoidea, from the proportionally reduced but full set of structures as in Torymus to having no sign

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of any of them, especially among the smaller representatives of the superfamily. Discussion Using only the features of the metapostnotum and associated structures, it is possible to characterize several major established lineages of Symphyta. One of these is the Siricoidea sensu stricto (Anaxyelidae, Xiphydriidae, and Siricidae) joined by the Cephidae (contrary to the classifications of Ross [1937], Benson [1938], and Rasnitsyn [1980]). These four families share the apparently derived state of having a medially divided metapostnotum. The Orussidae appear to be, at the closest, a sistergroup to all four combined, as Orussus has the plesiomorphic undivided metapostnotum and the apparently apomorphic (for Hymenoptera) undivided first abdominal tergite. Xyelidae and Tenthredinoidea (Tenthredinidae, Argidae, Cimbicidae) primarily share plesiomorphies, and little can be said from our analysis about their relationships. A major exception is the Cimbicidae, which, only on the basis of the structures discussed here, would have to be grouped with the Orussidae. Both families share the typical apocritan combination of a medially undivided first abdominal tergite strongly fused to a narrow, medially reduced metapostnotum. A close relationship between these two families is contradicted by a number of other character systems, including the inversion of the male genitalia and larval proleg configuration (Benson 1938, Richards 1956). It is possible that reduction of the metapostnotum in the two groups is a convergent and inevitable result of the strong fusion and sclerotization of the abdominal attachment. Of all the symphytan groups, the Orussidae most closely approximate the Apocrita in the apomorphies of the metapostnotum and associated structures. This similarity supports a sister-group relationship between the Orussidae and Apocrita, as suggested by Ashmead (1896), Rohwer & Cushman (1917), Brues (1921), Tillyard (1927), Lanham (1951), Bradley (1958), Robertson (1968), Iwata (1976), Rasnitsyn (1980), Gibson (1985), and Johnson (1988), based on a host of other characters. There is no evidence from this study for a sistergroup relationship between the Apocrita and Cephidae as suggested by Borner (1919), Ross (1937), Oeser (1961), Malyshev (1969), and Konigsmann (1977), or between the Cephidae + Siricoidea (including Orussidae) and Apocrita. Most likely, the Siricoidea are paraphyletic if the Orussidae or the Apocrita are removed (Bischoff 1926, Cooper 1953, Gibson 1985). This leaves a major systematics problem even if one recognizes orussids as a separate suborder (Rohwer & Cushman 1917) or transfers them to the Apocrita (Rasnitsyn 1980). There is an apparent synapomorphy for Xiphydriidae + Siricidae, the loss of the 2ph-3ph muscles along with the unique shape of the posterior attachment for

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the t3-3ph muscles. This contradicts the findings of Gibson (1985), whose data suggest a sister-group relationship between Xiphydriidae and Orussidae + Apocrita. The anatomy of the metapostnotum and associated structures supplies little evidence of the patterns of relationship among the nonaculeate Apocrita. The basal lineages of each superfamily are relatively plesiomorphic, but within each numerous taxa have undergone reductions. In most groups, the primary reductions have occurred in the medial dorsal length of the metapostnotum, the breadth of the third phragma, and the development of the 2ph-3ph muscles. These reductions have apparently occurred independently in the Ichneumonoidea, Proctotrupoidea, Ceraphronoidea, Chalcidoidea, and Cynipoidea. Several striking apomorphies can be noted within the Apocrita. In the Stephanidae, the third phragma is expanded and larger than in any symphytan examined. Its structure, however, is not particularly unusual except that it is not narrowed medially. The Aulacidae and Gasteruptiidae have two large submedial third phragmal lobes, unlike other hymenopteran known to us. These structures are not found in Evaniidae, which are often grouped with the Aulacidae and Gasteruptiidae into the Evanioidea (e.g., Riek 1970, Richards 1977, Rasnitsyn 1980). This is not a critical test of the hypothesis of relationship, but is consistent with the suggestion that Evaniidae is not closely related to the other two families (Townes 1950, Bradley 1958, Gibson 1985). Finally, the structure of the metapostnotal region in the Ceraphronoidea is quite distinctive. In no other taxon are the metanotum, metapostnotum, and propodeum fused into a single unit with the associated loss of the third phragma. Acknowledgment We thank Sydney A. Cameron and Howell V. Daly for comments on the manuscript. This material is based upon work supported by the National Science Foundation under grant BSR-8516579. References Cited Alam, S. M. 1951. The skeleto-muscular mechanism of Stenobracon deesae Cameron (Braconidae, Hymenoptera)—an ectoparasite of sugarcane and juar borers of India. I. Head and thorax. Aligarh Muslim University Publications (Zoological Series) Indian Insect Types, Aligarh, India. Ashmead, W. H. 1896. The phylogeny of the Hymenoptera. Proc. Entomol. Soc. Wash. 3: 323-336. Benson, R. B. 1938. On the classification of sawflies (Hymenoptera: Symphyta). Trans. R. Entomol. Soc. London 87: 353-384. Bischoff, H. 1926. Uber die systematische Stellung der Orussidae. Proc. Int. Congr. Entomol. (Zurich) 2: 134-144. Borner, C. 1919. Stammesgeschichte der Hautfliigler. Biol. Zentralbl. 39(4): 145-186. Bradley, J. C. 1958. The phylogeny of the Hyme-

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noptera. Proc. 10th Int. Congr. Entomol. (Montreal 1956) 1: 256-265. Brothers, D. J. 1975. Phylogeny and classification of the aculeate Hymenoptera, with special reference to Mutillidae. Univ. Kans. Sci. Bull. 50: 483-648. 1976. Modifications of the metapostnotum and origin of the propodeal triangle in Hymenoptera Aculeata. Syst. Entomol. 1: 177-182. Brues, C. T. 1921. Correlation of taxonomic affinities with food habits in Hymenoptera, with special reference to parasitism. Am. Nat. 55: 134-164. Carpenter, J. M. 1986. Cladistics of the Chrysidoidea (Hymenoptera). J. N.Y. Entomol. Soc. 94: 303-330. Cooper, K. W. 1953. Egg gigantism, oviposition and genital anatomy: their bearing on the biology and phylogenetic position of Orussus (Hym.: Siricoidea). Proc. Rochester Acad. Sci. 10: 38-68. Daly, H. V. 1963. Close-packed and fibrillar muscles of the Hymenoptera. Ann. Entomol. Soc. Am. 56: 295-306. 1964. Skeleto-muscular morphogenesis of the thorax and wings of the honey bee, Apis mellifera (Hymenoptera: Apidae). University of California Publications in Entomology, Berkeley, 39. Farish, D. J. 1972. The evolutionary implications of qualitative variation in the grooming behavior of the Hymenoptera (Insecta). Anim. Behav. 20: 662-676. Cibson, G. A. P. 1985. Some pro- and mesothoracic structures important for phylogenetic analysis of Hymenoptera, with a review of terms used for the structures. Can. Entomol. 117: 1395-1443. 1986a. Evidence for monophyly and relationships of the Chalcidoidea, Mymaridae, and Mymarommatidae (Hymenoptera: Terebrantes). Can. Entomol. 118: 205-240. 1986b. Mesothoracic skeletomusculature and mechanics of flight and jumping in the Eupelminae (Hymenoptera, Chalcidoidea: Eupelmidae). Can. Entomol. 118: 691-728. Hamerski, M. R. 1984. An examination of thoracic characters to assess their potential for inference of relationships among some Hymenoptera. M.S. thesis, Ohio State University, Columbus. Iwata, K. 1976. Evolution of instinct: comparative behavior of Hymenoptera. Amerind Press, New Delhi, India. Johnson, N. F. 1988. Midcoxal articulations and the phylogeny of the order Hymenoptera. Ann. Entomol. Soc. Am. 81: 870-881. Konigsmann, E. 1976. Das phylogenetische System der Hymenoptera. Teil 1: Einfuhrung, Grundplanmerkmale, Schwestergruppe und Fossilfunde. Dtsch. Entomol. Z. 23: 253-279. 1977. Das phylogenetische System der Hymenoptera. Teil 2: "Symphyta." Dtsch. Entomol. Z. 24: 1-40. 1978a. Das phylogenetische System der Hymenoptera. Teil 3: "Terebrantes" (Unterordnung Apocrita). Dtsch. Entomol. Z. 25: 1-55. 1978b. Das phylogenetische System der Hymenoptera. Teil 4: Aculeata (Unterordnung Apocrita). Dtsch. Entomol. Z. 25: 365-435. Lanham, U. N. 1951. Review of wing venation of the higher Hymenoptera (Suborder Clistogastra) and speculations of the phylogeny of Hymenoptera. Ann. Entomol. Soc. Am. 44: 614-628. Malyshev, S. I. 1969. Genesis of the Hymenoptera and the phases of their evolution. Methuen, London (English translation). Masner, L. 1979. Pleural morphology in scelionid

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wasps (Hymenoptera: Proctotrupoidea)—an aid to higher classification. Can. Entomol. I l l : 1079-1087. Matsuda, R. 1970. Morphology and evolution of the insect thorax. Memoirs of the Entomological Society of Canada, 76. Oeser, R. 1961. Vergleichend-morphologische Untersuchungen iiber den Ovipositor der Hymenoptera. Mitt. Zool. Mus. Berlin 37: 1-119. Rasnitsyn, A. P. 1969. Origin and evolution of lower Hymenoptera. Trudy Paleontologicheskogo Instituta, Akademiya Nauk SSSR No. 123 (in Russian). 1980. Origin and evolution of Hymenoptera. Trudy Paleontologicheskogo Instituta, Akademiya Nauk SSSR No. 174 (in Russian). Richards, O. W. 1956. Hymenoptera. Introduction and keys to families. Handbooks for the Identification of British Insects 6, Royal Entomological Society of London. Riek, E. F. 1970. Hymenoptera, pp. 867-959. In The insects of Australia. Melbourne University Press, Carlton. Robertson, P. L. 1968. A morphological and functional study of the venom apparatus in representatives of some major groups of Hymenoptera. Aust. J. Zool. 16: 133-166. Rohwer, S. A. & R. A. Cushman. 1917. Idiogastra, a new suborder of Hymenoptera, with notes on the immature stages of Oryssus. Proc. Entomol. Soc. Wash. 19: 89-98. Ross, H. H. 1937. A generic classification of the Nearctic sawflies (Hymenoptera, Symphyta). Illinois

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Biological Monographs 15. University of Illinois Press, Urbana. Saini, M. S. 1986. Comparative studies on the metapostnotal modifications in suborder Symphyta and Hymenoptera Parasitica. J. Entomol. Res. 10: 8590. Saini, M. S. & S. S. Dhillon. 1980. Metapleural transformations with respect to the propodeum and metapostnotum in Hymenoptera. Fla. Entomol. 63: 286291. Shcherbakov, D. E. 1980. Morphology of the pterothoracic pleura of Hymenoptera. 1. Groundplan. Zool. Zh. 59: 1644-1653 (in Russian). 1981. Morphology of the pterothoracic pleura of Hymenoptera. 2. Modifications of the groundplan. Zool. Zh. 60: 205-213 (in Russian). Snodgrass, R. E. 1910. The thorax of the Hymenoptera. Proc. U.S. Natl. Mus. 39: 36-91. 1927. Morphology and mechanism of the insect thorax. Smithsonian Miscellaneous Collections 80(1), Washington, D.C. Tillyard, R. J. 1927. The ancestry of the order Hymenoptera. Trans. Entomol. Soc. London 1927: 307318. Townes, H. K. 1950. The Nearctic species of Gasteruptiidae (Hymenoptera). Proc. U.S. Natl. Mus. 100: 85-145. Received for publication 26 January 1988; accepted 25 July 1988.