1972] 
Illavac — Prothorax of Coleoptera 
137 
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. 
Variation 
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 superfamilies, ca. 155 fam- 
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. 
