74 EN TOMOLOGY 
possibly a putrefaction, which accounts in a measure for the 
rapid disappearance of insect skeletons in the soil (Miall and 
Denny). By boiling the skin of an insect in potassic hydrate 
it is possible to dissolve away the cuticular framework, leav- 
ing fairly pure chitin, without destroying the organized form 
of the integument, though less than half the weight of the 
integument is due to chitin. The formula of chitin is given 
as CotlsNO, or C.ei,.NOj. by Keukenbers, and. Packard 
adopts the formula C,;H,,N.O, 9; though no two chemists 
agree as to the exact proportions of these elements, owing 
probably to variations in the 
Fic. 88. 
substance itself in different in- 
sects or even in the same species 
of insect. Iron, manganese 
and certain pigments also enter 
into the composition of the 
integument. 
Chitin is not peculiar to ar- 
thropods, for it has been de- 
tected in the setze and pharyn- 
Section through integument of a geal teeth of annelid worms, 
ee ee at fee the shell of Lingula and the 
secondary cuticula; h, hypodermis cell; pen of the cuttle fish (Kruken- 
n, nucleus.—After Tower. 
berg’). 
The chitinous integument (Fig. 88) of most insects con- 
sists of two layers: (1) an outer layer, homogeneous, dense, 
without lamellze or pore canals, and being the seat of the cutic- 
ular colors; (2) an inner layer, “ thickly pierced with pore 
canals, and always in layers of different refractive indices and 
different stainability.”” (Tower.) These two layers, respec- 
tively primary and secondary cuticula, are radically different 
in chemical and physical properties. The chitinous cuticula 
is secreted, as a fluid, from the hypodermis cells. Each layer 
arises as a fluid secretion from the hypodermis cells, the pri- 
mary cuticula being the first to form and harden. 
The fluid that separates the old from the new cuticula at 
