Moulting — Metamorphosis 1 1 5 



organs. Consequently as the newly-hatched nymph or 

 larva develops into the perfect insect, it has to under- 

 go a series of skin-castings or moultings (ecdysis). 

 The process begins with a splitting of the skin on the 

 upper surface of the thorax, this is continued forward 

 to the head, which opens along the sutures. The 

 head and thorax of the new stage (itistar) are then 

 worked out by an energetic wriggling motion of the 

 insect, and the old skin is gradually slipped off from 

 before backwards like the finger of a glove (fig. 73). 

 In caterpillars it is known that a fluid, secreted by 

 glands in the hypodermis, is present at moulting-times 

 between the new and the old skin, which it helps to 

 separate. The head of the new instar in a caterpillar 

 can be seen, before the moult takes place, through the 

 first thoracic segment. It is, of course, larger than 

 the head of the previous instar, and, after moulting, 

 looks disproportionately large as compared with the 

 body. During the succeeding stage, the body grows 

 larger, while the head retains its original size, appear- 

 ing, when the time for the next moult draws near, 

 disproportionately small (2, 3). 



Incomplete and Complete Metamorphosis. — 

 Through such a succession of moults, then, the in- 

 sect arrives at its adult condition — becomes an hnago. 

 But the details of the changes undergone vary greatly 

 in different insects. In some cases the insect, through- 

 out its development, is able to move and feed. The 

 grub of a May-fly, for example, after the first few 

 moultings, shows the rudiments of wings in outgrowths 

 from the terga of the second and third thoracic 

 segments. At each successive moult these rudiments 

 become larger, and the nymph, in its final stage 

 (fig. 71 a), quits the water ready for the emergence 

 of the fly. This gradual change from the larva to 

 the perfect insect is known as incomplete metamorphosis ; 



