Cell Structure— Chitin 



Fig. 2. — Section through Insect's skin 

 cc. Chitinous layer; c. its generat- 

 ing cell ; g. nerve-cell ; h. hair ; 

 hy. hypodermis. From Miall & 

 Denny's "Cockroach." 



nuclear activity is still a disputed question among 

 naturalists (56). 



Chitin. — The outer skin of the cockroach is hard and 

 firm, owing to the presence 

 of a chemical substance 

 known as chit'm} which is 

 specially characteristic of 

 arthropods. The chitinous 

 skin, or exoskeleton, is com- 

 posed of a number of thin 

 layers which are secreted by 

 the layer of active cells form- 

 ing the under-skin {hypoder- 

 mis) of the insect (fig. 2). 

 The hard coat of an insect is 

 built up of a number of distinct chitinous plates, each of 

 which is known as a sclerite. Sometimes a number of 



sclerites are firmly joined 

 together so as to form a 

 rigid part of the skeleton 

 — the head-capsule for 

 example ; the lines of 

 junction between such 

 sclerites are known as 

 sutures. In other cases 

 sclerites maybe separated 

 from each other by inter- 

 vening tracts of soft mem- 

 branous skin ; this is 

 necessarily the case where 

 motion between the parts 

 is required, as in the 

 segments of the hind- 

 body or of the legs, 

 head is rounded above 



Fig. 3. — Head of Cockroach (Blaifa 

 oricntalis), front view. Magnified 

 10 times. From Miall & Denny's 

 "Cockroach." 



Head, — The Cockroach's 



1 The chemical formula of chitin in C15H26N2O10. 



