EXTERNAL ANATOMY OF INSECTS, 395 
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 liquefy, 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- 
pendridé, upon immersion only emitted a few air-bub- 
bles; while, when the other myriapods, Polydesmus, Iu- 
lus, Glomeris, &c. and the Oniscide, 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 Scolopendride and 
Iulide (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 Julid@ usually lose their colour and turn 
