2 INSECT PHYSIOLOGY 



will become more pliable and extensible after feeding; and a hor- 

 mone from the central nervous system may control its hardening and 

 darkening at the time of moulting. 



Composition of the Cuticle 



The most familiar constituent of the cuticle is the nitrogenous poly- 

 saccharide chitin; but this rarely forms more than 50 per cent, of the 

 substance of the cuticle. It is most abundant in the flexible and elastic 

 endocuticle ; it is not responsible for the hardness of the exocuticle, 

 where it is reduced in amount, and it is entirely absent from the epi- 

 cuticle. Chitin is closely related to cellulose, and like cellulose it 

 exists in the form of sub-microscopic crystallites or micellae. In the 

 endocuticle these tiny rodlets are arranged at random in horizontal 

 sheets which are able to slide over one another to some extent in the 

 form of lamellae. The crystallites of chitin are bound together by a 

 protein matrix; indeed, the two may well be chemically united to 

 form a mucopolysaccharide. 



The protein of the endocuticle is readily demonstrated by the pro- 

 tein colour tests: the biuret, Millon's and xanthoproteic reactions, 

 all of which are strongly positive. The best test for chitin is that of 

 van Wisselingh, which consists in its conversion into chitosan by 

 saturated caustic potash at a high temperature, and the recognition 

 of chitosan by its solubility in acids and by the violet colour it gives 

 with iodine. 



In the exocuticle the protein component has become converted to 

 a horny substance termed 'sclerotin'. Sclerotin bears some resem- 

 blance to vertebrate horn or keratin; but whereas keratin is des- 

 cribed as 'vulcanized protein', a substance in which adjacent protein 

 chains are chemically bound together by means of sulphur linkages, 

 sclerotin is described as 'tanned protein'. It is produced by the action 

 of quinones which are formed in the cuticle from the oxidation of 

 various diphenols. The quinones react with the free amino groups in 

 adjacent protein chains and bind these firmly together, converting a 

 soft, white, extensible material into a hard and horny substance that 

 may vary in colour from amber to deep brown. Gelatin tanned with 

 benzoquinone gives rise to somewhat similar material. It was the 

 invention of sclerotin and keratin which made possible a truly 



