EXTERNAL ANATOMY OF INSECTS. 395 



in hardness resembling horn or shell, it passes through 

 the intermediate degrees of that of leather and parch- 

 ment, almost to a 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 a . 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- 



tremely hard, while Cantharis Geoffr., Meloe ¥., and Telephones 

 Geoffr., are very soft. 



a Thenard Traile de Chimie Elementaire, iii. 637. n. 2005. The 

 other products he mentions are — a green oil, a j'ellow substance, a 

 black ditto, acetic acid, uric acid, phosphate of magnesia. The vesi- 

 cant matter consists of little micaceous laminae soluble in boiling al- 

 cohol and oil, but insoluble in water. 



