LIFE-HISTORY OF THE BODY-LOUSE 25 



the ground or in the nest of its host feeding on 

 organic debris. When this is full-grown it spins 

 a cocoon, inside which it turns into a resting stage, 

 the pupa, which is in shape somewhat like the 

 adult flea but is white and soft. Inside this pupa 

 the adult is formed, and from the cocoon emerges 

 the brown familiar flea. Such a development is 

 known as a " complete metamorphosis." The 

 development of the louse is by >; incomplete 

 metamorphosis " ; that is, the food and form of 

 adult and young are similar and there is no 

 resting stage. The growth, however, is not a 

 continuous unbroken process like that of a man, 

 but takes place in a series of three jumps, each 

 represented by the casting of the old chitinous 

 covering and the development of a new and 

 larger one. Chitin is not a very elastic substance 

 where it is thick, and so this moulting process is 

 necessary to enable growth to take place. Crabs 

 and lobsters grow in the same way by casting off 

 the old coat, which will not stretch, and develop- 

 ing a new one, and the soft dog-crab which has 

 just moulted is a familiar object on the sea- 

 shore. The young louse moults for the first 

 time when about two days old. The skin splits 

 along the back of the thorax, along the neck and 

 the top of the head. The insect then expands 

 and forces up its back, draws out its head and 

 legs and then pulls itself free of the old skin 

 (Fig. 8), which remains attached to the cloth. 

 It is a flimsy object very easily seen on a lousy 



