74 



ENTOMOLOGY 



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 C 9 H 15 NO 6 or C 18 H 15 NO 12 by Krukenberg, and Packard 

 adopts the formula C 15 H 26 N 2 O 10 ; though no two chemists 

 agree as to the exact proportions of these elements, owing 



probably to variations in the 

 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 setae and pharyn- 



geal teeth of annelid WOriTlS, 



'he shell of Lingula and the 



FlG - 



Section through 



ntegument of a 



secondary cuticula; h, hypodermis cell; p en o f the CUttle fish (Krilkeil- 

 n, nucleus. After TOWER. 



berg). 



The chitinous integument (Fig. 88) of most insects con- 

 sists of two layers : ( i ) an outer layer, homogeneous, dense, 

 without lamellae 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 



