1972] 
Hlavac — Prothorax of Coleoptera 
131 
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- 
phaga is also found in other suborders, but its function, in these 
groups, given rigid external pleural walls, is problematical (figs. 2, 
9 M12). 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. 
Prothoracic Structure of the Ancestral Group 
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: 1, i960: 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 portion of pleuron enclosed by notum 
but the pleural rim only narrowly folded over; there is no deep, hori- 
zontal, endopleural invagination 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 
