394 EXTERNAL ANATOMY OF INSECTS. 
in hardness resembling horn or shell, it passes through 
the intermediate degrees of that of leather and parch- 
ment, almost toa thin membrane. Yet in all cases there 
is enough of rigidity and hardness to answer the princi- 
pal uses of a skeleton—to afford, namely, a sufficient 
point of attachment for the muscles, and to support and 
defend the interior organization; so that the play and 
action of the vital and secretory systems may not be in- 
terrupted or impeded. 
With respect to the principles which enter into the 
composition of this integument, very little seems to be 
known at present; but few insects having been submitted 
to a chemical analysis. The blister-beetle (Cantharis 
vesicatoria), from its importance in medicine, has, how- 
ever, been more than once analysed; and though the 
products have not been very precisely stated, yet we find 
amongst them phosphate of lime, albumen, and some 
other usual components of the substance of vertebrate 
animals*. But which of these products belong to the 
integument, and which to its contents, cannot be ascer- 
tained, without a separate process for each; which would 
not, I conceive, be very feasible. ‘The substance, how- 
ever, of the integument of insects, though we know not 
its precise contents, which probably vary in different ge- 
nera, &c. appears not to be exactly of the nature of any 
of those substances after which it has usually been deno- 
minated: it is not properly analogous either to real horn, 
* Thenard Traité de Chimie Elémentaire, iii. 637. n. 2005. The 
other products he mentions are—a green oil, a yellow substance, a 
black ditto, acetic acid, uric acid, phosphate of magnesia. The vesi- 
cant matter consists of little micaceous laminz soluble in boiling al- 
cohol and oil, but insoluble in water. But see Mr. Children in 
Zoolog. Journ. Nov I. 
