THE COCKROACH : ITS OUTER SKELETOX. 29 



moulted Cockroaches are white, but gradually darken in three 

 or four hours. Lowne* observes that in the Blow-fly the pig- 

 ment is " first to be met with in the fat-bodies of the larvae. 

 These are perfectly white, but when cut from the larva, and 

 exposed to the air, they rapidly assume an inky blackness. 

 . When the perfect insect emerges from the pupa, and 

 respiration again commences, the integument is nearly white, 

 or a faint ashy colour prevails. This soon gives place to the 

 characteristic blue or violet tint, first immediately around those 

 portions most largely supplied with air vessels." Professor 

 Moseleyf tells us that, thinking it just within the limits of 

 possibility that the brown coloration of the Cockroach might be 

 due to the presence of silver, he analysed one pound weight of 

 Blatta. He found no silver, but plenty of iron, and a remark- 

 able quantity of manganese. That light has some action upon 

 the colouring matter seems to be indicated by the fact that in a 



* 



newly-moulted Cockroach the dorsal surface darkens first. 



Chitin is not peculiar to Insects, nor even to Arthropoda. 

 The pen of cuttle-fishes and the shell of Lingula contain the 

 same substance,^ which is also proved, or suspected, to occur in 

 many other animals. 



The chemical stability of chitin is so remarkable that we 

 might well expect it to accumulate like the inorganic con- 

 stituents of animal skeletons, and form permanent deposits. 

 Schlossberger has, however, shown that it changes slowly 

 under the action of water. Chitin kept for a year under water 

 partially dissolved, turned into a slimy mass, and gave off a 

 peculiar smell. This looks as if it were liable to putrefaction. 

 The minute proportion of nitrogen in its composition may 

 explain the complete disappearance of chitin in nature. 



The Chitinous Cuticle. 



The chitinous exoskeleton is rather an exudation than a true 

 tissue. It is not made up of cells, but of many superposed 



* Anatomy of the Blow-fly, p. 11. 

 t Q. J. Micr. Sci., 1871, p. 394. 



Krukeaberg. Yergl. Physiologische Vortriige, p. 200. Halliburton, Q. J. Micr. 

 Sci., 1885, p. 173. 



Ann. d. Chem. u. Pharm., Bd. 98. 



