THE INTEGUMENT 7 



means certain in all cases; but in one example that has been studied 

 in detail, that of the bug Rhodnius, it has been shown that the insect 

 is held by the adhesion or seizure of the climbing organ to the sur- 

 face. In other words, it is the surface molecular forces that are 

 directly concerned in supporting the body weight. 



The moulting, formation and pigmentation of the cuticle 



Through the possession of a more or less rigid carapace, incapable 

 of increasing in surface area once it has been laid down, the insect 

 incurs the necessity for discontinuous growth. In later pages (p. 83) 

 we shall consider some of the physiological processes which lie at 

 the back of insect growth; but one change which it always involves is 

 the casting of the old cuticle and the formation of a new and larger 

 one. This side of growth, or ecdysis, may be conveniently considered 

 here, for it throws a good deal of light upon the properties of the 

 insect cuticle. 



Chitin is a nitrogenous substance, though its nitrogen content is 

 relatively small (about 8 per cent. - only half that of protein), and 

 the suggestion has frequently been made that it is really a waste pro- 

 duct, and moulting or ecdysis an excretory phenomenon. But this 

 idea fails to take into account the fact that during moulting most of 

 the old cuticle is dissolved, and is not, in fact, lost to the insect. In the 

 bug Rhodnius only about 14 per cent, of the cuticle of the abdomen is 

 actually shed; the remaining 86 per cent, is reabsorbed by the insect 

 and is presumably used again for the formation of the new cuticle. 



During this process only the endocuticle is dissolved, the chitinous 

 and protein layer; the exocuticle and epicuticle are untouched. Solu- 

 tion is brought about by means of the 'moulting fluid'. This is a 

 thin layer of liquid which lies between the newly developing cuticle 

 and the old cuticle that will be cast off. This liquid contains enzymes, 

 a chitinase and a proteinase, which will digest the substance of the 

 endocuticle but will not touch the lipoprotein of the epicuticle or the 

 sclerotin of the exocuticle. Thus the new cuticle is protected from its 

 action by the epicuticular covering. The enzymes of the moulting 

 fluid appear to be secreted from the general surface of the new cu- 

 ticle; perhaps they are discharged at the tips of the pore canals. And 

 the products of digestion are likewise absorbed through the general 



