T. F. Hlavac.
The Prothorax of Coleoptera: Origin, Major Features of Variation.
Psyche 79(3):123-149, 1972.

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PSYCHE
Vol. 79 September, I 972 No. 3
THE PROTHORAX OF COLEOPTERA:
ORIGIN, MAJOR FEATURES OF VARIATION1
BY T. F. HLAVAC~
Biological Laboratories, Harvard University, Cambridge, Massachusetts
The unique evolutionary success of the order Coleoptera is a. re- sult of great size combined with enormous biological diversity. The huge number of species (ca. 280,000) is arrayed across a broader ecological spectrum than that of any other group of terrestrial ar- thropods. Four adaptive zones have been extensively occupied: sur- face. substrate, aquatic, aerial. Higher categories of beetles are, with exceptions, very broad, overlapping adaptive radiations. This evolutionary conlplexity is associated with great structural variation and a small number of basic adaptations, particularly in the Iocomo- tory system.
Because the Coleoptera are a series of replicated experiments in ecological differentiation, the group may be used for studying a suite of problems in the evolution of adaptation. And, because beetles are such a large, diverse and ubiquitous group of insects, they should have a place as subjects for developing and refining modern syste- matic methodologies. Work at these two superficially different levels has been hindered or made unfeasible (q. v. Brundin 1972: 72) by the lack of a firm foundation of comparative morphology. "Most work on comparative structure of beetles suffers from one 'A preliminary version of this work was submitted as part of a Ph.D. thesis to the Biology Department, Harvard University. 'I thank Drs. R. A. Crowson, H. E. Evans, J. F. Lawrence, E. Mayr, and E. 0. Wilson for many useful comments on the manuscript. Work on
thoracic morphology has been supported by NSF grants GB 19922 (Reed C. Rollins, Harvard University, Principal Investigator), GB 12346 (P. J. Darlingtun, Jr., Harvard University, Principal Investigator), and GB 31173 (F. M. Carpenter, Harvard University, Principal Investigator). Manuscript received by the editor October 1,1972



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124 Psyche [September
or more of the following limitations: small and/or poorly chosen sample; superficial analysis of raw data, or none at all; impoverished treatment of adaptive phenomena. There are few even moderately comprehensive studies of structural variation; see Arnett ( 1967) for list and brief descriptions. And, many glaring problems have not been dealt with. For example, prothoracic characters have long been used in subordinal diagnoses, yet detailed comparisons of representatives of each taxon have not been made.
Since ecological differentiation and thoracic adaptation are so in- timately related, understanding the locomotory system is particularly important for the future development of beetle systematics. Due to
size and diversity of the group, work on this functional complex is neither easy nor quickly accomplished. The only feasible compromise is to select a natural subunit of this system for detailed study. The prothorax is the obvious, initial choice. Prothoracic structure and mechanics are simple as compared to those of the two pterothoracic segments where both ambulatory and flight functions are combined. Differences in size, structure and f~inc- tion in the prothorax are readily perceived and correlated with phys- ical demands of various environments. Furthermore, details of pro- thoracic mechanisms are commonly diagnostic of higher categories. The interplay between adaptive phenomena and historical clevelop- ment in the prothorax is considered below at 2 levels: origin of the coleopterous prothorax and variation within the two biologically diverse suborders.
The generalizations and evolutionary hypotheses presented here are based on dissection of over 600 selected genera, and external ex- amination of many others. Raw data, primarily drawings, group diagnoses, and discussions of variation within major taxa, will be presented elsewhere as will results of a current study of pterothoracic structure.
The walls of the rigid, cylinder-like prothorax of Coleoptera are always formed by the notum dorsally, by the sternum ventrally, and, in some forms, the pleuron forms distinct lateral walls (figs. 1-9). The trochantin, a small, sometimes movable sclerite, is attached to the sternum and pleuron and along with the latter articulates with the coxa, the basal leg segment (figs. 2, 7 Tn). The coxa also
rests on and sometimes mechanically articulates with the posterior section of the sternum - the cryptosternum (fig. 7 CrS) .



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19721 {Hlavac - Prothorax of Coleoptera 1 25 Almost without exception, the pleuron is divided into an external section and an internal, invaginated region, the endopleuron, that is concealed by the notum (figs. 2, 7, 9, PI, EndPl). The rim of the external portion may be completely folded over, broadly so anteriorly and posteriorly to form flanges, parts of paired articulation-collars and quite narrowly so ventrally forming half of membranous sternal and trochantinal attachments and part of the coxal articulation (figs. 2, 4, g ANFL, APLFL, PNFL, PPLFL, RFM, AF, VF, PF). These anterior notal, sternal, and sometimes pleural flanges pro- duce a complete articulation-collar or socket which encloses the pos- tenor aspect of the head, cervical membrane, and, if present, cervical sclerites. Likewise, the posterior notal flange, and sometimes a pleural Figs. 1-5. Prothoraces of Archostemata and Myxophaga. Figs. 1, 2. Lateral external and internal views of Priacma serrata (Ar-
chostemata, Cupedidae).
Figs. 3, 4. Same of Pryopteryx britskil (Myxophaga, Torridinicolidae). Fig. 5. Lateral view of Hydroscapha natans (Myxophaga, Hydrosca- phidae).
Margins of enclosed structures indicated by dashed lines.



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126 Psyche [September
flange as well, form an incomplete collar enclosing part of the meso- thoracic rim and some intersegmental membrane (fig. 8). The fold forming the anterior sternal flange may continue dorsally to join with a pleural or notal rim fold and then extend ventrally to form a membrane enclosing joint with the trochantin. Below the trochantin, the sternum generally bears a shelf-like, poorly pigmented, concave region, the cryptosternum, which supports and is concealed by the coxa (figs. 7, g CrS). The cryptosternum also bears a pair of invaginations or apophyses close to the posterior margin. Frequently, the sternum is evaginated medially, forming a projection which may extend between, behind, and sometimes above the coxa. Sometimes a second, smaller sternallar projection is present as well (fig. 7 Spj, SLpj). A complete posterior collar is commonly formed from union of either of these projections with notal or pleural projections (figs. 333 43,467 48,k 64).
SUBORDINAL DIFFERENCES AND THE HYPOTHETICAL STEM CONFIGURATION
There are major differences in pleural size, structure, motility and in its trochantinal attachment among the four suborders of Coleop- ten. Subordinal configurations can be diagnosed as follows. ARCHOSTEMATA. Pleuron large, rigid, forming lateral wall of segment. Trochantin motile, external. Anterior pleural flange exter- nal, small and enclosed, or absent with internal anterior fold. Ster- nal joint membranous to solidly fused (figs. I, 2, I I, 12, 14). MYXOPHAGA. Pleuron variable in size (figs. 3-5), rigid, fused to trochantin. Anterior pleural flange external or enclosed (figs. 3-5). Sternal joint membranous.
ADEPHAGA.
Pleuron, a prominent part of body wall, rigid. Tro- chantin, small, motile, enclosed along with coxal articular region by pleural and sternal cowlings. Anterior pleural flange absent, ante- rior fold internalized by union of noltal (and sternal flanges (figs. 8, 9. I 3, 61-65). Sternal joint fused.
POLYPHAGA. Pleuron greatly reduced in size and fused to tro- chantin; this highly variable compound structure may be motile and contribute to coxal movement, and may be completely enclosed. No- tun1 and sternum attached anterior to pleuron to form body wall. Anterior pleural fold and zone of fusion between pleuron and tro- chantin present in a few primitive groups. Sternal joint membranous to solidly fused (figs. 6, 7, I 5-1 7, 23-55).



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19721 ~Hlavac - Pro thorax of Coleoptera 127 In addition, rim-fold joints between body wall sclerites and mov- ing parts are very widely distributed within the Coleoptera. Prothoracic configurations of extant forms can be derived from a single hypothetical stem form diagnosed as follows. HYPOTHETICAL STEM CONFIGURATION: Pleuron large, rigid, forming distinct part of body wall, with broad anterior and posterior flanges; attached by rim-fold joints to notum and sternum. Endo- pleuron present. Trochantin external, motile. A complete anterior collar and partial posterior collar enclose most of the intersegmental Figs. 6-9. Prothoraces of Polyphaga and Adephaga. Figs. 6, 7. Lateral external and internal views of Omalium marginaturn (Polyphaga, Staphylinidae). [See text and captions of Figs. 25-55 for ex- planation of Fig, 6A, B].
Figs. 8, 9. Same of Amphiwa insolens (Adephaga, Amphizoidae).



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19721 li/avac - Prothorax of Coleoptera 129 membrane. Peri-coxal .and trochantinal membrance enclosed by cowl- ings on pleuron, sternum and trochantin (fig. 24). Evidence for the primitiveness of individual characters is obtained from both extant and fossil forms.
The tripartite body wall and anterior collar is a major feature of the archtypical coleopterous prothorax. This arrangement is geo- metrically the simplest way of producing a sclerotized cylinder bear- ing paired sockets, i.e., through anterior development and folding of dorsal, lateral and ventral elements without shift in relative position. Pleural size and structure of three suborders (Archostemata, Ade- phaga, Myxophaga) is similar to that of the stem configuration but differ from it in having a reduced or internalized anterior flange. A relatively small, external, anterior pleural flange is present only in some extant members of the Archostemata and Myxophaga. In other members of both groups, the flange is even further reduced and enclosed by overlap of notal and sternal elements (figs. 1-5, I I, 12). In one group of Archostemata (Cupdidae, Ommadinae) and in all Adephaga, the anterior flange is absent, and a small anterior pleural fold is internalized, frequently by membranous connection of a lobe- like expansion of the sternal flange with the notal flange (figs. 8, 9, 13, 14). In some Adephaga and Myxophaga notal and sternal flange rims may overlap but are not connected
(figs. 3, 8). The Myxo-
phaga and Adephaga then overlap the Archostemata at opposite ends, of this morphocline.
In many Mesozoic Coleoptera of dubious subordinal position, the notum and sternum are widely separated by the pleuron (e.g., Pono- marenko 1969; figs. 74, 102). No internal evidence is available. However, since the posterior rim of the head is clearly enclosed by Figs. 10-19. Morphology of anterior section of the pleuron and surround- in,g structures ; sclerites slightly disarticulated. Figs. 10-15. Internal views.
Fig. 10. Hypothetical stem configuration. Fig. 10A. Section through pleuro- sternal joint.
Fig. 11.
Priacma serrata (Archostemata, Cupedidae, Cupedinae) , Fig. 12. Cupes concolor (Cupedidae, Cupedinae) . Fig. 13.
Amphisou insolens (Adephaga, Amphizoidae) . Fig. 14. Tetraphalerus wagneri (Cupedidae, Ommadinae). Fig. 15. Generalized Polyphagon.
Figs. 16-18.
External views of Polyphaga.
Fig. 16. Peltastica turberculata (Derodontidae) . Fig. 17. Sarabandus robustus (Helodidae) . Fig. 18. Megarthrus robustus (Staphylinidae). Fig. 19. External view of Pr'iucma; sections included above, heavily stippled.




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1 30 Psyche [September
the prothorax in these forms, it is reasonable to infer the existence of a tripartite anterior collar. In other fossil beetles, the notum and sternum extend in front of the ~leuron. This could represent either a stage of anterior ~leural reduction as in figs, 3, 4, 12, or a distinct noto-sternal joint. The evidence is ambiguous. Based on the geometry of major sclerites, and structural variation in extant and fossil forms, a three element body wall and anterior collar is taken to be primitive for Coleoptera. The distribution of extant forms evolved though variable, possibly, parallel reduction of the anterior flange.
A separate, motile trochantin occurs in the Archostemata, Ade- phaga and in other holometabolous orders. The trochantin and pleu- ron are fused in the Myxophaga and Polyphaga. A separate tro- chantin is doubtless a primitive trait in Coleoptera. The almost universal presence of membrane enclosing rim-fold joints between major sclerites, and moving parts within both extant and fossil forms is evidence for the primitiveness of these articula- tions within the Coleoptera. A rigid rim-fold joint is produced by medially bending the edges of two sclerites to form a pair of flat- tened, normally horizontal articulation surfaces which may bear tongue-groove devices (fig. IoA). Attachment membrane extends between the margins of the two sclerites and is enclosed. In all ex- tant forms, the body wall elements are connected with mebranous rim-fold joints (frequently in primitive members of higher taxa) or are solidly fused together, sometimes with a distinct internal carina and often without the slightest vestige of a suture (figs. 9, 59, 50). Rigid rim-fold joints seem to be universally present in fossil Coleop- tera, as well.
Except in extreme surface grade polyphagans, membrane around the coxa and trochantin is enclosed by loose rim-fold joints or both structures may be entirely enclosed by cowlings, see below. There is no obvious membranous band between moving parts in the fossil Coleoptera depicted by Ponomarenko ( 1969). It is assumed, then, that rigid and loose rim-fold joints are primitive characters in the Coleoptera.
Except for the Polyphaga, Recent suborders and early fossil Cole- optera are similar to the stem configuration in basic organization. The major differences between the Adephaga, Archostemata, and Myxophaga are either simple modifications of structural details (re- duction of anterior pleural flange, trochantinal fusion) or adaptations for improving structural integrity (enclosure of coxal articular re- gion and trochantin in the Adephaga). The grea,t differences between



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19721 Hlovac - Prothorax of Coleoptera I3 1 Polyphaga and other Coleoptera are integral parts of a unique pleuro- coxal mechanism.
In all Polyphaga, the trochantin and pleuron are fused together; in a few members of apparently primitive groups (Staphylinoidea, Eucinetoidea) the structure of a distinct internal zone of fusion be- tween the two sclerites indicates union of a pair of rim folds (figs. 7, 24). This compound structure is frequently movable and can contribute to coxal rotation and/or flexation. Since the pleuron is a moving part, it can not contribute to the rigidity of the segmental wall, and is greatly reduced in length and width. In most cases, pleural height is so reduced that the coxal apex is concealed by the notal rim-fold. A rigid segment is obtained by anterior attachment of notum and sternum. Evidence that this specialized mechanism has evolved from a configuration with a tripartite body wall is found in what are probably vestiges of the anterior pleural fold between the notum and sternum in members of these presumed primitive groups of Polyphaga (Eucinetoidea, Staphylinoidea, Derodontidae) (figs. 16-18). The major muscle powering pleural motility in the Poly- plhaga is also found in other suborders, but its function, in these groups, given rigid external pleural walls, is problematical (figs. 2, g MI^) . The Polyphaga can therefore be derived from the hypo- thetical configuration through modification of the pleuro-coxal mech- anism resulting in the acquisition of pleural motility. It is generally concluded that the Holometabola is a strictly mono- phyletic taxon, but see Matsuda ( 1970: 215). The Coleoptera and the other major orders are believed to have evolved from a general- ized stock of lower Holometabola, closest to Neuroptera and Mecop- tera (Crowson 1956: I, 1960: 111).
There are enormous differences in prothoracic structure between the Coleoptera and other Holometabola; and there is only moderate variation within the Lower Holometabola. Based on study of mem- bers of all major groups, it is possible to diagnose a presumed prim- itive configuration as follows:
GENERALIZED HOLOMETABOLUS PROTHORAX: Noto-pleural joint, loose, membranous. Dorsal prtion of pleuron enclosed by notum but the pleural rim only narrowly folded over; there is no deep, hori- zontal, endopleural imagination as in Coleoptera. The pleuron does bear a vetrical invagination, or apophysis, which divides it into an episternum, anteriorly and an epimeron, posteriorly. Pleural apophy- sis fused internally to sternal apophysis. Trochantin motile, closely



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132 Psyche [September
attached to pleuron and as heavily sclerotized as the other structures. Sternum joined only internally to pleuron, does not extend in front of the coxa. Anterior and posterior collars absent, coxal a.rticulation external so that considerable cervical, intersegmental, and pericoxal membrane is exposed (fig. 23).
This configuration is most similar to the extant Australian genus Ithone (Neuroptera) (fig. 20) but with a large, articulatory tro- chantin, as in the Trichoptera (fig. 22). Reducing the trochantin while maintaining associated musculature may be a modification for increasing the angle of coxal flexation. Division of the pleuron into Figs. 20-22. Prothoracic structure of the Lower Holometabola. Fig. 20. External lateral view of Ithone sp. (Neuroptera). Fig. 21. Internal lateral view of Panorpa virginica (Mecoptera). Fig. 22. External lateral view of coxal articulation of Ptilostomis ocel- lifera (Trichoptera) .




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19721
'Hiavac - Prothorax of Coleoptera
Fig. 23. Generalized Prothorax of the Lower Holometabola; Fig. 24, Hypothetical stem prothorax of Coleoptera. Membranous regions stippled. two parts by an apophysis, or pleural suture is a common feature of pterothoracic segments. Among Holometabola this arrangement is preserved in the prothorax only in Ithone and a few related forms (fig. 20, Eps, Epm, Invg). In the Trichoptera, a detached epimeron is present (fig. 22). In all other Holometabola the posterior rim of the apophysis is membranous. Ventral enlargement, and fusion of sternum, pleuron and even the cervical sclerites of the Corydalidae and Raphidioidea are doubtless specialized features readily derived from a generalized configuration
(Kelsey 1954, figs. I, 7 ; Ferris
and Fennebaker I 939, fig. 61 ) .
Comparison of the generalized holometabolous prothorax (GH) with that of the hypothetical stem coleopteran (SC) yields differ- ences in five major categories (figs. 23, 24). The abbreviations (GH and SC) are employed below for simplicity and to emphasize the fact that two abstract assemblages are being considered rather than elements of actual organisms.
A). Head-Prothoracic Joint. - In SC an anterior articulation- collar composed of notal, pleural and sternal flanges encloses part of the head and all cervical membrane as well as the cervical sclerites. The sternum is developed anteriorly and joined to the pleuron. An articulation-collar is absent in GH, the head may be slightly en-



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1 34 Psyche [September
closed by the notum, cervical membrane, and sclerites are exposed, and the sternum is not externally joined to the pleuron. B). Pro-Mesothoracic Joint. - In SC a partial posterior articu- lation-collar is formed from notal and pleural flanges, which rest on the mesothorax and enclose dorsal and lateral intersegmental mem- brane. Such flanges are absent and intersegrnental is exposed in GH. C). Notlo-Pleural and Pleuro-Sternal Joints. - In SC these con- nections are of the rim-fold type and result in a rigid frame pro- thorax. The horizontal invagination that forms the endopleuron is located close to the notal rim. In GH, the noto-pleural joint is loose and there is neither an external ~leuro-sternal connection nor D) . Trochantinal and Coxal Articulations. - In SC the coxa and trochantin are connected to one another and to the sternum and pleuron via loose rim-fold joints, which enclose peritrochantinal and pericoxal membrane (fig. 56). Loose rim-fold joints are absent in GH and these membranous regions are exposed. E.)
Attachment of Sternal and Pleural Imaginations (or 40- physes). -There is no vertical pleural apo~hyseal invagination in any extant coleopteron; the sternal apolphyses are always present. In all other Holometabola, both sternal and pleural apophyses are pres- ent, and these invaginations are solidly fused together (fig. 2 I ) . These differences are of two major types, those that result in the enclosure of membrane between moving parts (A, B, D) and one that results in a rigid frame prothorax (C). Major features of the hypothetical stem prothorax of beetles re- sult in great improvements in structural integrity over the ancestral condition due to development of rigidly attached segmental walls, and widespread enclosure of membrane. Two differences seem to be side effects of an increase in structural stability. An anterior, exter- nal sternal attachment provides potentially rigid attachment for this sclerite to the pleuron or notum, and permits the sternum to form the ventral body wall, and part of the anterior collar. An internal attachment can give only rigidity.
To have both is redundant. Loss
of both the internal sterno-pleural attachment and the pleural apo- physis itself may be a structural simplification occurring after devel- opment of a multi-purpose anterior sternal attachment. Parenthet-
ically, the line of fusion between the posterior rim fold and the body



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19721 Hlavac - Prothorax of Coleoptera I35 wall, is sometimes incorrectly called a pleural suture (= apophysis) in the taxonomic literature especially in the Adephaga. The endopleuron ~rovides increased surface area for muscle at- tachment. It is argued below that the key event leading to the unique coleopterous locomotory system was entrance into a substrate adaptive zone. Strength and power are both important in substrate locomotion. The features described above provide increased strength. The endopleuron is part of a mechanism for improving power gen- eration.
All features of the stem prothorax are either direct improvements in structural integrity or ancillary modifications. The unique structure of the pterot'horax and abdomen of Coleop- tera also represents a great increase in structural integrity over the ancestral condition.
In Lower holometabolous groups, the pterothoracic segments and wings are quite similar. The two pairs of wings are membranous. There are generally considerable patches of exposed membrane be- tween abdominal segments. In beetles the pterothoracic segments and wings are highly differentiated in structure and function. The mesothoracic wings of beetles are modified into rigid, heavily sdero- tized elytra whose rims can be fitted together, via a tongue-groove device, and which also lie on the pleural margins olf the pterothoracic and abdominal segments, thereby forming a structurally stable unit protecting abdominal tergites and folded wings. The abdomen is reduced in relative length, does not often extend behind the elytra, and the sternites are connected by rim-fold joints or are solidly fused together.
The membranous metathoracic flight wings are folded lengthwise, as well as widthwise, and are generally completely enclosed at rest, within the cavity formed by union of the body and elytra. The pterothoracic segments themselves are also highly differentiated. The metathorax, which houses all flight muscles, is much larger and high- ly modified as compared to the mesothorax. The characters discussed above as improvements in structural in- tegrity or side effects thereof encompass all major adult diagnostic features of the order Coleoptera.
Improvements in structural integrity can be responses to two po- tentially quite different but blendable selection pressures involving locomotion or defense. In a surface zone (i.e., crawling on a leaf) where environmental geometry does not oppose forward motion, high structural integrity can be the mechanical portion of an anti-predator system. For example, many arthropod predators, even relatively large



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136 Psyche [September
ones, are unable to crush, crack, or pierce cuticle but must attack through vulnerable membranous zones. Enclosing membrane hinders such forms. Of course, this argument applies to substrate dwelling forms as well. In addition, a substrate form not dependent on in- terstitial spaces must generate force against obstacles to make prog- ress. Consequently, features increasing structural integrity can be adaptations which prevent structural deformation as those locomotory forces are generated and also prevent membrane from coming in con- tact with abrasive materials. Increasing structural integrity can be a defense adaptation in either zone or a locomotory adaptation in a substrate zone.
Extant members of the groups closest to the presumed ancestral stock of Coleoptera are surface dwellers, except possibly Merope ( Mecoptera) . The gross structure of fossil Lower Holometabola is similar to that of extant forms and suggests that these groups have been surface grade forms throughout their long history. Beetles are the dominant adult insects in substrate environments. Specialized coleopterous faunas occur in: leaf litter, soil, living, dy- ing and decomposing woody plants, dung, carrion, etc. In each hab- itat many members of at least several families are present. However, a diverse assortment of beetles is also found in the surface zone, par- ticularly on vegetative surfaces.
The major diagnostic features of Coleoptera are adaptations for improved structural integrity. Beetles have entered and radiated in the substrate locomotory zone; and many higher taxa contain sur- face dwellers clearly derived from substrate grade forms. An increase in structural integrity can either be an adaptation for substrate loco- motion or for mechanical defense against predators. These facts and assumptions indicate that penetration of the substrate zone and de- velopment of these adaptations are related, and that both occurred rather early in the history of Coleoptera. Within the framework provided by Bock ( I 965)) then, the following historical diagnosis is suggested.
The suite of unique diagnostic characters of Coleoptera originated and/or became coordinated as a response to selection pressure for increased structural integrity encountered during initial entrance and radiation in a substrate locomotory zone. Individual characters, which may have originated on a surface zone as a mechanical defense adaptation are preadapted for a change in zone and shift in function. The adaptive transformation of the coleopterous prothorax is but a part of the large scale reorganization of the locomotory system stim- ulated by a major change in ecology.




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19721 'Hiavac - Prothorax of Coltoptera I37 The central position of the stem configuration here is an artifact of analysis. Due to the great structural discontinuity and lack of intermediate forms between Coleoptera and Lower Holometabola, several important matters can not be considered. In the preceding analysis, then, no statement is made or implied on: monophyly of the stem form and extant suborders, parallel or convergent develop- ment and origination sequences of structural adaptations. Of the four suborders of Coleoptera, only the Adephaga and Poly- phaga are large and biologically diverse. Polyphaga. The great size (90% of known beetle species) and taxonomic confusion/complexity (20 superf amilies, ca. I 55 f am- ilies) of the Polyphaga makes even a limited discussion of structural variation quite difficult. A sampling technique can be used to facili- tate matters. Much of prothoracic diversity consists of interwoven variations on a pair of adaptive themes- power and structural in- tegrity. And, strong relationships exist between biology, structure and prothoracic volume. These variations can be demonstrated by comparing forms from several higher taxa which differ widely in relative prothoracic size (figs. 25-48). A stem polyphagous prothorax may be diagnosed as follows : Primitive/Generalized Polyphagous Prothorax: Notal volume low. Noto-sternal joint membranous. Notal projection, if present, may not extend below trochantinal apex. Pleuron motile. Endo- pleuron with short, broad unconstricted base and moderately flared apex. Coxal articular region and trochantin large, not completely enclosed. Sternum does not reach coxal apex. Sternal projection does not extend below coxa and is not attached to the notal projec- tion forming complete collar, but may be flattened and extend behind the coxa (figs. 6, 7, 23). Vestiges of the anterior flange and zone of fusion between trochantin (figs. 16-18 AF) may be present. This sort of configuration is likely to be primitive for Polyphaga, representing a minimal divergence from the hypothetical ancestral condition and it is frequently associated with the two vestigial pleural structures noted above. The adaptations and functional specializa- tions, discussed below, are absent or poorly developed in this pro- thorax. This could be additional evidence for primitiveness or could represent a generalized, though advanced, configuration at or near the midpoint of an adaptive spectrum. In either case, this assemblage provides a useful reference standard in discussing adaptive extremes.



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19721 Hlavac - Prothorax of Coleoptera 1 39 Since a number of forms are to be compared in some detail, it is convenient to treat variation separately in three functional units: I ) Size (volume, muscle housing)
2) Pleuro-coxal mechanism (housing, motility of coxa, trochantin, pleuron )
3 ) Intersegmental attachment ( pro-mesothoracic joint ) Size.
Variation in muscle volume and consequently in gross pro- thoracic size represents adaptive equilibria to the sharply different power requirements of substrate and surface locomotion. Obviously, compression of substrate requires more power than simply counter- ing the force of gravity on a surface. Changes in prothoracic size are most easily demonstrated through consideration of one dimension height, i.e., prothoracic height and that of component structures/ height of metathorax plus elytra ( = Thi, etc./Th3). In the high volume prothoraces of substrate dwellers, the notum is tall (Ni/Th3 ca. b%), the sternum extends to near the coxal apex and the entire assemblage is frequently ca. 85% Th3 (figs. 27, 29, 32, 34, 37, 39, 42, 44, 47, 49). At the other extreme, in the' low volume prothoraces of a surface or interstitial space inhabitants the Figs. 25-49. Prothoracic variation within five higher taxa of Polyphaga. Each set, bounded by brackets, compares two forms differing widely in prothoracic volume and consists of five figures: a pair of lateral views, drawn parallel to the coxal long axis, a smaller pair of posterior views and a set of graphs comparing the height, length and width of the pro- thorax with the dimensions of the metathorax. The set on the left, e.g. fig. 29A, contrasts the cumulative heights of prothoracic structures expressed as a percentage of height of metathorax plus elytra, measured at the level of the posterior edge of the noto-sternal joint, and depicted in the same order as the drawings. While the paired, crossed lines on the right, e.g. fig. 29B, contrast the length (vertical line) and width of the pronotum expressed as a percentage of maximum width of metathorax plus elytra; the percentage scale on the right applies here as well. Figs. 25-29; Elateriformia; figs. 27, 28 Lutrochus geniculatus (Limni- chidae) ; figs. 28, 29, Perothops muscida (Perothopidae). Figs. 30-34, Cleroidea; figs. 30, 31 Malachius aenaeus (Melyridae) ; figs. 32, 33, Ternnochila chorodia (Trogositidae). Figs. 35-39; Staphylinidae; figs. 35, 36, Philonthus cyannipenniq figs. 37, 38, Trigonurus crotchii.
Figs. 40-44; Cucujoidea ; figs. 40, 41, Epicauta pennsylwanica (Meloidae) ; figs. 42, 43 Alobates pennsylwanica (Tenebrionidae). Figs. 45-49; Scararbaeidae; figs. 45, 46, Euphoria limbalis; figs. 47, 48, Copris fricator.
Figs. 50-55; compaction and enclosure in Polyphaga. Figs. 50-52, lat- eral, posterior view of prothorax and lateral view of body of Agathidium sp. (Anisotomidae). Figs. 53-55, same of Chelonarium lecontei (Chelonari- idae). See text for explanation.




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1 40 Psyche
[September
notum is low ( N I / Th3 ca. 35 % ) , the most ventral point of the sternum is far above the coxal apex, and the coxa is quite elongate so that Th1/Th3 is ca. 70% (figs. 30, 34, 35, 39, 40, 45, 49). Large changes in gross prothoracic size are associated with great modifica- tion in relative sizes of structural units due to the geometry of pro- thoracic musculature.
Major locomotory muscles are housed within the notum (Larsen 1966, fig. 64). Sharp increase in muscle mass, then, implies increased notal volume. And notal enlargement effects other prothoracic struc- tures. There appears to be a distinct upper limit to prothoracic height in most taxa; Th1/Th3
rarely exceeds 90%. Given this
boundary to vertical growth, large notal increase is accompanied by sternal reduction and coxal modification. The complimentarity of notal and sternal heights effects coxal structure. In generalized forms, the most dorsal point of the coxa is level or nearly so with the noto-sternal joint (fig. 6). As notal height increases, the coxa is either reduced with the sternum, maintaining initial geometry (fig. 27) or the coxa is elongate and enclosed deeply within the notum (figs. 42, 45, 47). Each strategy has been adopted numerous times in the Polyphaga, while only the former occurs in the Adephaga (figs. 8,61, 65).
Variation in prothoracic length and width are correlated with height. The width of a prothorax is rarely greater than that of the metathorax (figs. 29, 34, 39, 44, 49). Prothoracic height and width are then usually limited by the corresponding dimensions of the largest segment of the body.
Structure of substrate grade configurations can be explained as an optimization of muscle volume given geometric limiting factors. However, prothoracic structure of extreme surface grade inhabitants represents not only reduced muscle volume but also a mechanism for increasing coxal flexation, see below.
Pleuro-coxal Mechanism. This complex and highly variable sys- tem divides naturally into two subunits involving: housing and function of pleuron, trochantin and coxa. Housing.
Concealment of moving parts and surrounding mem- brane produces improved structural integrity. Enclosure can occur
around the entire cozxal perimeter, dorsally by notal and sternal cowlings, anteriorly and ventrally by the sternum and sternal projec- tion, and posteriorly by the notal projection. Dorsal and ventral enclosure are particularly variable.
Enlargement of rim folds, which originally protect only membrane around the coxa and trochantin, forms cowlings that partially to



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142 Psyche [September
wholly enclose these structures as well as surrounding membrane (figs. 56-60). Enclosure is accomplished through ventral develop- ment (below the noto-sternal joint), overlap, or fusion of notal and sternal cowlings (figs. 27, 57, 58, 60) ; through dorsal development of the notal cowlings (figs. 5, 45, 47, 59) ; or through a combination of these two methods (figs. 40, 44, 59). Ventrally, the sternal ~rojection may be developed below the coxa, thereby protecting the most ventral section of pericoxal mem- brane. A sterno-coxal articulation is frequently present in high vol- ume forms and prevents the coxa and trochantin from being deflected (figs. 27, 32, 42).
Structures which increase structural integrity by enclosing the coxa, trochantin and surrounding membrane are common in the Polyphaga and are most prevalent in but not restricted to substrate dwelling forms.
Function.
Coxal movement, the end product of the pleuro-coxal mechanism, consists, in the Polypha.ga, of rotation and sometimes flexation as well. Rotation is simply circular movement about the coxal long axis. Flexation results in antero-posterior motion of the coxal apex and is generated only through movement of the pleuron against its notal attachment
(fig. 35). The coxa rotates but does
not flex against the pleuro-trochantinal joint. Each type of coxal movement is most suited for locomotion in one adaptive zone. Flexation is greatly emphasized in many low notal volume, ex- treme surface grade forms and in a few cursorial interstitial space inhabitants (figs. 30, 35, 40). In both sites, locomotory require- ments for power are minimal, so that the quantity of forward mo- tion generated per stroke is a valid measure of coxal performance, which is maximized by employing a combination of rotation and flexation. The effectiveness of flexation is a function of the radius and swing angle. The radius is increased by lengthening the coxa. Anterior and posterior clearances are necessary for a long coxa to traverse a broad arc. Anterior clearance is achieved by sternal re- duction so that in extreme cases, its ventral plane lies just below the trochantinal apex permitting the coxa to slide under the sternal rim (fig. 35). Posterior clearance is obtained by reducing and/or flat- tening the notal projection thereby decreasing the amount of inter- segmental overlap and exposing membrane. Modifications permitting extensive flexation also allow a large coxa to rotate extensively. A flexing coxa then imposes strict design limitations on surrounding structures.
Rotation is the sole coxal movement in almost all substrate forms



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19721 ~Hlavac - Prothorax of Coleoptera I43 and is compatible with high muscle volume and structural integrity particularly of the pro-mesothoracic joint (see below). In many substrate dwellers, the coxal rotation axis is distinctly inclined away from the vertical, so that the force generated by coxal movement has a vertical component and can, for example, be used in substrate compression (figs. 33, 38, 48).
Because the locomotory needs of the two zones are met not only by varying muscle volume but frequently by differences in coxal motility as well, there is great variation in internal mechanism in the prothorax of Poly~haga. In flexing forms, the endopleuron is rather small and does not extend far above the coxal apex (figs. 30, 35). This arrangement increases the distance between the dorsal surface of the endo~leuron and notal wall and reflects an increase in the major generator of coxal flexation -the noto-pleuralis mus- cle. And, of course the noto-pleural joint is membranous and highly motile.
Some of the important coxal rotator muscles and part of the femo- ral depressor, a major vertical force generator, originate on the undersurface of the endopleuron (Larsen 1966: 143, fig. 64). In- crease in volume of these muscles is correlated with gross notal de- velopment and is diagnosed by a lengthened, f requentl~ stal k-like, endopleural base with an expanded apex. Such modifications increase both attachment surface area and the distance between origin and insertion (figs. 23, 34, 37, 44). In many forms where coxal move- ment is mechanically restricted to rotation by the notal projection, the pleuron is motile and the noto-pleuralis functions as an indirect coxal rotator. But in many substrate dwellers, and others as well, noto-pleural joint is solidly sclerotized so that pleural motility is lost. In some of these cases, the endopleuron lies against and may be solidly fused to the dorsal notal wall, thereby maximizing the length of several important muscles. Loss of a moving part - the pleuron - could also be an improvement in structural integrity in these "heavy duty" systems.
Pro-Mesotheracic Joint. Based on criteria of locomotory func- tion, as well as those of static and dynamic structural integrity, three heterogenous classes of intersegmental attachment can be dis- tinguished : motile, rigid, defensive. In many substrate inhabitants, prothoracic motility is employed to push and compress substrate; the prothorax, as a whole, then, is part of the locomotory system. In such forms, a complete collar formed from the union of notal and sternal projections, plus a mesothoracic clearance, permits the pro- thorax to move through a wide arc with all intersegmental mem-



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144 Psyche
[September
brane enclosed
(figs. 32, 33, 41, 43, 45-48). A complete collar (sometimes called closed coxal cavities in the taxonomic literature) then increases the structural integrity of a major body joint during movement and can be our adaptation for substrate locomotion. The posterior collar is only partially developed in many Poly- phaga. Commonly, the notal projection extends to the level of the trochantinal apex, and mechanically restricts the coxa to rotation and the sternal projection extends behind the coxa. When the pro- thorax is in its most ventral position, latero-ventral intersegmental membrane is enclosed but will be exposed if the prothorax moves dorsally (figs. 8, 26). The collar is least devebped, providing pos- terior clearance in forms capable of extensive flexation (figs. 30, 35, 40). In these extreme surface grade beetles, prothoracic movement does not contribute directly to locomotion but may aid in leg place- ment in a multi-planar environment.
In a few substrate dwellers (e.g., Histeridae), the prothorax is rigidly held against the mesothorax during locomotion and the en- tire dorsal surface of the squat body is employed to compress mate- rial. In some streamlined aquatic groups, the pro- and mesothorax are tightly joined, sometimes by complex interlocking mechanisms, thereby avoiding potential turbulence during swimming. In these two cases, a non-motile joint plays a role in locomotion. Frequently, a potentially rigid joint is an important part of an anti-predator de- fense system.
As pointed out above, structures increasing structural integrity can be adaptations for substrate locomotion, defense against preda- tors, or both. However, several specializations of the pro-mesotho- racic joint and peripheral structures function exclusively as part of an anti-predator system.
Given a thick uncrackable cuticle and widespread enclosed mem- brane, the remaining vulnerable sites are the appendages and the major body joints, i.e., connections between he,ad-prothorax, pro- thorax-mesothorax, metathorax-abdomen, elytra-body. The strength of the pro-mesothoracic joint is increased in some beetles (esp. Elateriformia) by a complex series of interlocking mechanisms involving ball-socket and groove-ridge devices (figs. 25- 28, 53-55). Interlocking occurs apparently only after .attack or dis- turbance; during walking the segments are widely separated expos- ing much intersegmental membrane.
Appendages can be protected via two strategies: compaction and enclosure. In each case, ,a smooth continuous surface is formed that offers neither purchase for crushing mandibles nor a pathway to



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19721 Hiavac - Prothorax of Coleoptera 145



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146 Psyche [September
membrane for probing beaks. In compacted forms, elements of the proleg and sometimes the mesoleg as well are drawn tightly together and lie in thoracic cavities, so that exposed leg surfaces are flush with one another and with surrounding sclerites (figs. 53-55). Fre- quently the antennae are concealed in pronotal grooves and the mouthparts by the prosternum.
In a few groups of Polyphaga, the antennae, mouthparts, front, and sometimes middle legs are enclosed, in defense position, within a complex cavity formed from elements of the head, pro- and pterc- thorax (figs. 50-52). A "roll up in a ball" strategy requires exten- sive structural and mechanical modifications which result in great reduction of prothoracic volume.
The pro-mesothoracic joint, then, varies greatly in structural in- tegrity and may contain locomotory and defense adaptations. Adephaga.
This group consists of a broad adaptive array of sub- strate and surface inhabitants as well as several exclusively aquatic families. A uniform pleuro-coxal mechanism and high degree of structural integrity are maintained throughout (see subordinal di- agnoses above). Structural variation occurs primarily in the com- pleteness and quality of the posterior articulation collar, which is incomplete only in a few apparently primitive taxa. In all major groups of ground and arboreal carabids the collar is complete; re- versal following shift in zojne seems unlikely ( Hlavac 197 I ). Pro- thoracic size is quite variable and is, of course, strictly correlated with ecology, as in Polyphaga. In the Adephaga, prothoracic elonga- tion is particularly important and is graphically seen in the increased ventral inclination of the pleuro-sternal joint (figs. 61-65) . The pattern of prothoracic evolution in the Adephaga is similar to that found in several biologically diverse higher taxa of Polyphaga with specialized pleuro-coxal mechanisms and relatively little structural variation, e.g., Scarabaeidae (figs. 45-49). DISCUSSION
As seen above, similar prothoracic configurations and characters occur in members of unrelated taxa which share a common band on a broad ecological spectrum.
Reasons why convergence and paral-
lelism are an important aspect of prothoracic differentiation can be seen from a consideration of the relative breadth of adaptive pathway within each functional variable - structural integrity and power. The plasticity of structures varying morphological strength and en- closure of membrane is sharply limited by geometry, i.e., the adaptive pathway is narrow. For example, there are just two modes of pos-



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19721 t h a c - Prothorax of CoZeofltera 147 terior collar completion. Either a medial sternal projection or lateral edges of the cryptosternum is/are joined to notal projections, e.g., figs. 42, 43 vs. 50, 51. The presence of similar collars, differing in minor details, in numerous groups of beetles reflects a broad selec- tion pressure acting through a narrow gap-like pathway. And, of course the probability of multiple origin, and convergent improve- ment is quite high in this and other structural integrity improving adaptations.
Variability of structures increasing ~rothoracic power generation is limited by the geometry of muscle origins and insertions. Sev- eral distinct pathways are possible. For example, figs. 27, 42, 47 depict a trio of high volume ~rothoraces with three different, spe- cialized pleuro-coxal mechanisms (as compared with that of con- figurations differing slightly from the generalized reference standard, figs. 25, 32, 37). In two cases, the endopleuron is fused to the notal wall, but two different strategies of coxal reaction to notal increase have been employed (figs. 27, 44). In figs. 45, 47, deep coxal internalization is also employed but the pleuro-coxal mech- anism is completely different. The pleuron is reduced and attached to the coxa; both rotate about a noltal condyle. In each of these cases, a specialized pleuro-coxal mechanism is uniform throughout a large, biologically diverse higher taxon - i.e., superfamily. And each type of mechanism is present in, at least, several unrelated taxa. The observed diversity of internal mechanics in high volume forms is consistent with a relatively specific selection pressures having a,cted through a broad, multi-solution pathway, followed by canalization. he prothoraces of some surface dwelling forms are simply low in volume and have the same pleuro-coxal mechanism as do substrate inhabitants of the same taxon, e.g., Ad'ephaga, Scarabaeoidea (figs. 45, 47). On the other hand, the design limitations of a flexing pro- coxa have resulted in similar configurations in the several groups of surface grade beasts that have adopted this locomotory mode (figs. 30, 35, 44.
Prothoracic structure is then sensitive to changes in locomotory biology in but a limited number of ways. And, the broad adaptive radiations of Coleo~tera may be documented through the study of prothoracic morphology. But since adaptive pathways are so narrow, convergence so common, and putatively unique paradaptive features so infrequent, only limited evolutionary conclusions can be drawn solely from prothorax morphology. The obvious historical questions on ecological differentiation of individual higher taxa circumscribe a major poorly explored area of beetle systematics. Useful ideas on



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148 Psyche
[September
such problems as the ancestral zone and factors responsible for shift will arise however from integration of pro-and pterothoracic varia- tion. Limited evidence suggests that the predominant direction of shift, in extant forms, has been from substrate to surface) coincident with the evolution of flowering plants and peripheral communities. Thus) characters increasing power and improving structural integ- rity are likely to be primitive, in a statistical sense) within the CoIe- optera.
In summary) adaptive responses to ecological differentiation) in- cluding initial exploitation of the substrate zone a,nd shifts back to the ancestral surface locomotory zone) account for diagnostic fea- tures of the coleopterous prothorax and also for major features of variation.
ABBREVIATIONS USED IN THE FIGURES
AF - anterior fold
AFL - anterior flange
ANFL - anterior notal flange
APLFL - anterior pleural flange
ASFL - anterior sternal flange
Cnd - condyle
CrS - cryptosternum
CS - cervicle sclerite
Cw - cowling
CxBr - coxal bridge
EndPl - endopleuron
Epm - epimeron
Eps - episternum
Is Mb - intersegmental membrane
Lb - lobe
N - notum
N pj - notal projection
PI - p1euron
PlAph - pleural apophysis
PNFL - posterior notal flange
PI-S - pleuro-sternal joint
RFM - rim fold margin
S - sternum
S Aph - sternal apophysis
SL pj - sternellar projection
S pj - sternal projection
Tn - trochantin
Zf - zone of fusion
ARNE~, R. H.) JR.
1967. Present and future systematics of the Coleoptera in North America. Ann. Ent. SOC. Amer. 60 : 162-170. BOCK, W. J.
1965. The role of adaptive mechanisms in the origin of higher levels of organization. Syst. 2001. 14: 272-287. BRUNDIN, L.
1972. Ph~logenetics and biogeography. Syst. Zool. 21 : 69-79. CROWSON, R. A.
1955, The natural classification of the families of Coleoptera. Natha- niel Lloyd & Co,, London. 187 pp.
1960. The phylogeny of Coleoptera,
Ann. Rev, Ent. 11 : 111-134.




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19721
HZavac - Prothorax of Coleoptera
FERRIS, G, F. AND P. PENNEBAKER
1939. The morphology of Agulla adnixa (Hagen) (Neuroptera: Raphi- diidae) . Microentomology 4: 121-142.
HLAVAC, T. F.
1971. Differentiation of the carabid antenna cleaner. Psyche 78: 51- 66.
KELSEY, L. P.
1954* The skeleto-motor mechanism of the dobsonfly, Corydalus cor- nutu 1: Head and prothorax. Mem Cornell Univ. Agric. Exp. Stn., No. 334, 51 pp.
LARSEN, 0.
1966. On the morphology and function of the locomotor organs of the Gyrinidae and other Coleoptera. Opusc. Ent. Supp, 30 : 1- 241.
MATSUDA, M.
1970.
Morphology and evolution of the insect thorax. Mem. Ent. SOC. Canada, No. 76, 431 pp.
PONOMARENKO, A. G.
1969. Historical development of the Archostemata-Coleoptera. Trasvs. Paleont. Inst. Acad. Sci. USSR, 125, 237 pp. [In Russian].



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