156 



INSECTS 



and pass their entire life upon it. They are incapable 

 of existence away from this host and have no power 

 of gaining food from any other source. The young 

 are not very different except in size, from the adults, 

 and there is no obvious metamorphosis. 



Three species live on man and their distribution 

 is coincident with that of humanity. The head louse, 

 Pediculus capitis, lives among the longer hair of the 

 body, usually confined to the head. Its legs are not 

 scansorial, it has the anchoring process at the base of 

 the head well-developed, and the 

 eggs or "nits" are attached to the 

 hair. The crab louse, Phthirius in- 

 guinal is, lives among the coarser, less 

 abundant hair of the pubic and ax- 

 illary regions and in hairy individ- 

 uals also on other parts of the body. 

 The legs here are scansorial, as of 

 necessity they must be to enable the 

 insect to maintain itself, and it also 

 glues its eggs to the hair among 

 which it lives. The body louse, Pediculus vestimenti, 

 lives among the thinner body hair and almost alto- 

 gether on parts normally covered by clothing. Its legs 

 are not well fitted for grasping, but the anchor proc- 

 esses of the mouth are well developed. The peculiarity 

 of this species is that it remains on the body of the host 

 only while it is feeding and at other times hides in his 

 clothing, where also it deposits its eggs. Man, there- 

 fore, has worn clothing for a period long enough to 

 enable this parasite to adapt its mode of life to this 

 habit, and to depend upon his garments for protection 

 and as a nidus for its ova. 



These parasites sometimes become exceedingly 

 abundant when men are herded together in camps, 



Fig. 65. — An egg 01 

 nit, attached to a hair. 



