8 INSECT PHYSIOLOGY 



surface which at this stage is still quite permeable. And all the time 

 this process is going on the epidermal cells are synthesizing and de- 

 positing the chitin and protein of the new endocuticle; indeed, the 

 deposition of these inner layers may continue for several weeks after 

 the old cuticle has been shed. 



Up to the time of moulting the new cuticle is soft and almost 

 colourless. Within an hour or two of the casting of the old skin it 

 has both hardened and darkened. The mechanism of hardening, the 

 quinone tanning of the cuticular protein to form sclerotin, has al- 

 ready been described. This process involves a considerable degree of 

 darkening, so that the two processes go forward together. In ad- 

 dition, there are probably other oxidative processes going forward in 

 the blackest parts of the cuticle leading to the production of melanin 

 from the amino-acid tyrosine. 



We are now in a position to appreciate the activities of the epi- 

 dermal cell during the formation of the new cuticle. This cell secretes 

 the cuticulin of the epicuticle, the chitin, the 'arthropodin' and 

 'resilin' and other proteins of the inner layers, the chitinase and 

 protease which dissolve the old skin, then the phenolic material 

 which will give rise to the quinones, and the waterproofing wax. 

 Finally, it produces the oxidase which completes the hardening pro- 

 cess. And all these activities are exquisitely timed (and synchronized 

 throughout the body) to follow one another within a matter of hours 

 or minutes. Indeed, in the formation and maintenance of its little 

 share of cuticle the epidermal cell has shown itself a chemical agent 

 with an amazing range. 



The mechanism of moulting and of hatching 



When the insect has digested its old cuticle and laid down the new 

 one, it has fresh problems to solve. In the first place, it has to rupture 

 the old skin and escape from it. Now any cuticle that has to be shed, 

 that is, the cuticle in every stage except the adult, has a 'line of 

 weakness' - sometimes in the head, most often in the midline of the 

 thorax. Along this line the exocuticle is wanting, and the endocuticle 

 extends right up to the very delicate epicuticle (Fig. 3, C). Conse- 

 quently, when the endocuticle is completely dissolved (and the 

 process of moulting is so timed that this happens just when the 



