284 INSECTS AND MAN 



may be due to differences in the calibre of the hairs, to 

 which they are compelled to cling and to which their feet 

 are adapted. It would be quite impossible for claws fitted 

 for clinging to very coarse hair to obtain a firm hold on 

 fine hair, and vice versa. Again, thickness of skin may 

 have its bearing on this phenomenon, mouth parts adapted 

 only for piercing a thin skin would prove useless on a 

 thick hide. Difference in temperature, odour, and taste of 

 the host may have their effect. 



The three species of louse common to man are the head 

 louse, Pediculus capitis, the body louse, Pediculus vesti- 

 menti, and the crab louse, Phihirius inguinalis (fig. 81, 

 A, B, c). They are all three commonly and correctly as- 

 sociated with a dirty, unkempt condition of their host, but, 

 unfortunately, owing to the exigencies of present-day 

 civilisation, even the cleanest among us cannot claim im- 

 munity from their unwelcome attentions; though it is 

 comforting to know that while some two hundred and 

 fifty years ago these parasites were the constant com- 

 panions of even the upper classes in England, they are 

 now of comparatively rare occurrence. 



The head and body louse closely resemble one another, 

 and are often confused ; both of them appear to have been 

 known from the earliest times, and the former is the com- 

 moner. It is usually confined to the hair of the head, being 

 rarely found on other parts of the body. The small, white, 

 pear-shaped eggs (fig. 82) are laid singly, by the female, 

 and, at the same time, glued towards the end of a hair, the 

 back of the ears being a favoured position. These eggs, 

 popularly known as "nits," give the hair somewhat the 

 appearance of having been singed, and are more obvious 

 than the adults to the casual observer. The young, which 

 appear in about a week after oviposition, are very similar 

 to the adults except in size, and, till they have had their 

 fill of blood, their abdomens are proportionately smaller. 

 The adults of both sexes are almost white, with indistinct 



