1972] 
Hlavac — Prothorax of Coleoptera 
135 
wall, is sometimes incorrectly called a pleural suture (— apophysis) 
in the taxonomic literature especially in the Adephaga. 
The endopleuron provides 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 pterothorax 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 sclero- 
tized elytra whose rims can be fitted together, via a tongue-groove 
device, and which also lie on the pleural margins of 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 
