10 THE LIFE-STORY OF INSECTS [CH. 



to each other, the motions being due to the con- 

 traction of muscles which are attached within the 

 exoskeleton. 



Now this jointed exoskeleton an admirably 

 formed suit of armour though it often is has one 

 drawback : it is not part of the insect's living tissues. 

 It is a cuticle formed by the solidifying of a fluid 

 secreted by the epidermal cells, therefore without 

 life, without the power of growth, and with only 

 a limited capacity for stretching. It follows, there- 

 fore, that at least during the period through which 

 the insect continues to grow, the cuticle must be 

 periodically shed. Thus in the life-story of an insect 

 or other arthropod, such as a lobster, a spider, or a 

 centipede, there must be a succession of cuticle-cast- 

 ings ' moults' or ecdyses as they are often called. 



When such a moult is about to take place the 

 cuticle separates from the underlying epidermis, and 

 a fluid collects beneath. A delicate new cuticle 

 (see fig. 10 cu) is then formed in contact with the 

 epidermis, and the old cuticle opens, usually with 

 a slit lengthwise along the back, to allow the insect 

 in its new coat to emerge. At first this new coat is 

 thin and flabby, but after a period of exposure to the 

 air it hardens and darkens, becoming a worthy and 

 larger successor to that which has been cast. The 

 cuticle moreover is by no means wholly external. 

 The greater part of the digestive canal and the whole 



