396 EXTERNAL ANATOMY OF INSECTS. 



minated : it is not properly analogous either to real horn, 

 shell, skin, or leather, &c. This seems to result from the 

 following circumstance : — Most of the excretions of ver- 

 tebrate animals, as horn, skin (at least when tanned), 

 feathers, wool, hair, &c. when exposed to the action of 

 fire liquify, more or less, before they incinerate; emitting 

 at the same time a peculiar and disagreeable scent : but 

 upon applying this test to the parts of insects of the dif- 

 ferent Orders, I found, in every instance, that incinera- 

 tion took place without liquefaction, and was unaccom- 

 panied by that peculiar scent which distinguishes the 

 others. Even the claws, which to the eye appear, as to 

 their substance, exactly like those of Mammalia, birds, 

 &c. burn without melting, and retain their form after 

 red heat. That the insect integument is not calcareous 

 like that of the Crustacea, and the shells of Mollusca?, 

 you may easily satisfy yourself, by immersing them in an 

 acid test. I made this experiment upon portions of in- 

 sects of several of the Orders, in an equal mixture of mu- 

 riatic acid and water, and the result was, not only that 

 all hexapods, but octopods, Arachnida, and even Scolo- 

 pcndridce, upon immersion only emitted a few air-bub- 

 bles ; while, when the other myriapods, Polydesmus, lu- 

 lus, Glomeris, &c. and the Oniscidce, were immersed, a 

 violent effervescence took place; proving the different 

 nature of their substance. It is remarkable that the two 

 great branches of the Myriapods, the Scolopendridce and 

 Iulidcc (Chilopoda and Chilognatha Latr.), should in 

 this respect be so differently circumstanced — the latter 

 having a calcareous integument, and the former not. — 

 A further difference distinguishes these two tribes : old 

 specimens of the Iididce usually lose their colour and turn 



