1972] 
Hlavac — Prothorax of Coleoptera 
43 
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 Polyphaga. 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 endopleuron 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, frequently stalk-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-Mesotkoracic 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- 
