BODY LOUSE 



389 



species, which closes back like a finger against a thumbhke pro- 

 jection 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, six to eight of 

 them in the human spe- 

 cies; the terminal one 

 is indented in the female, 

 but is rounded in the 

 male with the large 

 spikelike copulatory or- 

 gan often projecting at 

 its tip (Fig. 174, p. 390). 

 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 

 prominent spiracles on the sides of the abdominal segments. 



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

 and sometimes even genera are thus limited. Kellogg has 

 suggested that the evolutionary affinities of different birds and 

 mammals may be demonstrated by the kinds of lice which in- 

 fest them. There are only three species which infest man, each 

 selecting a different portion of his body as a habitat; these are 

 the head louse, Pediculus capitis, the body louse, Pediculus 

 humanus (vestmenti) and the crab louse, Phthirius pubis. The 

 genus Pediculus is peculiar to man and the apes, with the ex- 

 ception of one species which infests the monkey, Ateles. The 

 genus Phthirius is represented only by the human species. 



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

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

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

 bust and less active. Fertile offspring result from hybridization 



Fig. 173. Front leg of body louse, Pediculus 

 humanus. Note huge claw and thumb-like op- 

 posing process of next segment. X 100. 



