BODY LOUSE 



389 



tesque in appearance in some species, which closes' back like a 

 finger against a thumblike projection of the next segment of the 

 leg (Fig. 173). There are not even rudiments of wings. 



The body of a louse is clearly divided into head, thorax and 

 abdomen (Fig. 174). The thorax is always broader than the 

 head, a characteristic 

 which distinguishes at a 

 glance a true louse from 

 the broad-headed bird 

 louse (Fig. 172). The 

 abdomen is divided into 

 segments, normally eight 

 of them in the human spe- 

 cies; the terminal one 

 is indented in the female, 

 but is rounded in Ihe 

 male with the large 

 spikelike copulatory or- 

 gan often projecting at FIG. 173. Front leg of body louse, Pediculus 



o 4-\ n ^TT;. 17/1 ,- QOfVi humanus. Note huge claw and thumb-like op- 

 b Ig. 1 /4, p. 6VV). poging procegg Qf next segment> x loa 



The digestive tract, as in 



most other blood-sucking insects, is furnished with capacious 

 pouches branching from the stomach, 'which serve as food reser- 

 voirs. The tracheal system is well developed and opens by prom- 

 inent spiracles on the sides of the abdomen. 



Most species of lice are quite closely limited to a single host, 

 and sometimes even genera are thus limited. Kellogg has sug- 

 gested that the evolutionary affinities of different birds and mam- 

 mals may be demonstrated by the kinds of lice which infest them. 

 The lice infesting man have generally been regarded as belonging 

 to three species, each selecting a different portion of the body as a 

 habitat; these are the head louse, Pediculus humanus (capitis), 

 the body louse, P. corporis (vestimenti) , and the crab louse, Phthi- 

 rius pubis. The head louse and body louse are now thought 

 to be mere races or varieties of a single species, P. humanus. 

 Both Pediculus and Phthirius are probably normally confined to 

 human beings. 



Body Louse. The body louse (Fig. 174) is by far the most 

 common, as it is the most important, louse infesting man. It 

 closely resembles the head louse, but is often larger, more ro- 

 bust and less active. Fertile offspring result from hybridization 



