EXTERNAL ANATOMY OF INSECTS. 397 
many important purposes, that at first sight do not strike 
the observer, may be served: such as giving firmness 
to the crust in those places where it is most wanted; di- 
minishing its powers of resistance in others, so that it 
may yield somewhat to the action of the muscles; in- 
creasing or deducting from the weight of the body, so as 
to produce a proper equipoise during its motions, whe- 
ther on the earth, in the air, or in the water. The de- 
pressions of the outer surface of the crust, in many in- 
stances, produce an elevation of it in the interior, and 
so afford a useful point of attachment to certain muscles. 
This observation seems more especially applicable to 
those excavations that are common to particular tribes 
or genera: thus the dorsal longitudinal channel to be 
met with on the prothorax of most of the ground-beetles 
(Eutrechina) on the inside of the crust have a corre- 
sponding ridge. In Locusta Dux, also, (a Brazil locust,) 
the same part has four transverse channels, correspond- 
ing with which on the inside are as many septa, or ridges, 
to which muscles are attached; and those larger im- 
pressed puncta denominated puncta ordinaria, which 
distinguish the same part in Geotrupes and many of the 
Scarabeide, within are elevated, so as to form a kind of 
ginglymous articulation with the base of the anterior 
coxe. ‘The other impressed puncta so often to be seen 
on the different parts of various insects, which sometimes 
so entirely cover the surface that scarcely any interval is 
discoverable between them, though in many cases they 
appear to be mere impressions that attenuate but do not 
perforate the crust—yet in others, perhaps equally or 
more numerous, they are real pores, which pass through 
the integument. If, for instance, you take the thoracic 
