EXTERNAL FEATURES 227 



substance called sclerotin, which is much harder and less 

 permeable to water than arthropodin, its predecessor. It is 

 sclerotin which has made possible the rigid arthropod exo- 

 skeleton, and so, as a special development, insect flight. Outside 

 the exocuticle is a very thin layer, a few microns only, called the 

 epicuticle. It has three main layers but contains no chitin ; 

 on the inside a tanned protein called cuticulin, then wax, and on 

 the outside a protective cement which is probably also a tanned 

 protein. Almost all the waterproofing of the insect cuticle is done 

 by the waxy layer of the epicuticle, yet it is never more than a 

 fraction of a micron in thickness. The total thickness of the cuticle 

 in the cockroach is about 40 microns ; in many other insects it is 

 much less, and in some, such as the larva of the water beetle 

 Dytisciis, much more. 



The whole of the cuticle is secreted by the cells of the epidermis 

 (hypodermis) ; cuticulin first, then the exocuticle, and then the 

 endocuticle. The endo- and exocuticle, and probably the cuticulin 

 of the epicuticle, are at first traversed by vertical pore canals, 

 which contain cytoplasm. When the cuticulin and exocuticle have 

 been formed, a somewhat temporary layer, the polyphenol layer, 

 is spread over the surface, and the waxy layer is next spread over 

 this ; presumably both these come through the pore canals. 

 At this point the old skin is shed in ecdysis. The cement layer is 

 almost immediately spread over the surface from the dermal 

 glands, and the hardening of the exocuticle takes place. The endo- 

 cuticle is continuous with the exocuticle, but is laid down after 

 ecdysis. While the first part of the new cuticle is being secreted, 

 and before ecdysis, a liquid called moulting fluid appears between 

 the new epicuticle and the old endocuticle. It contains a protease 

 and probably a chitinase, and gradually dissolves the old endo- 

 cuticle, the products being absorbed, through the new cuticle, 

 into the epidermal cells. The exocuticle and epicuticle are not 

 attacked, but as there are lines where no exocuticle has been 

 laid down, the cuticle here becomes reduced to the epicuticle, 

 and so is very thin. It is along these lines that it splits in ecdysis, 

 so that the insect can crawl out of its own skin. 



In some insects the pore canals retain their cytoplasm through- 

 out life, and the cuticle is therefore living ; in others they become 

 occluded. The various stages of the formation of cuticle, 

 and so the ecdysis, are controlled by hormones secreted by 

 structures called corpora allata, situated just behind the brain. 



