188 



ANIMAL LIFE 



other parts of fish, and derive all their nutriment from the 

 body of the fish, show various degrees of degeneration. With 

 some of these fish-lice the female, 

 which looks like a puffed-out worm, 

 is attached to the fish or other aquatic 

 animal, while the male, Avhich is per- 

 haps only a tenth of the size of the 

 female, is permanently attached to 

 the female, living parasitically on her. 

 100. Parasitic insects. Among 

 the insects there are many kinds 

 that live parasitically for part of 

 their life, and not a few that live as 

 parasites for their whole life. The 

 true sucking lice (Fig. 113) and the 

 bird-lice (Fig. 114) live for their whole lives as external 

 parasites on the bodies of their host, but they are not 

 fixed that is, they retain 

 their legs and power of loco- 

 motion, although they have 

 lost their wings through de- 

 generation. The eggs of the 

 lice are deposited on the hair 

 of the mammal or bird that 



FIG. 112. Fish-louse (Ler- 

 a, adult ; b, larva. 



FIG. 113. Sucking louse (Pediculus) of 



human body. FIG. 114. Bird louse (Lipeurus densus). 



serves as host ; the young hatch and immediately begin to 

 live as parasites, either sucking the blood or feeding on the 



