388 
Biology of Phthirus pubis 
Why does Phthirus infest the head so rarely? 
In view of the frequency with which man harbours crab-lice it is of 
interest to find an explanation of why it so uncommonly infests the 
head. According to Waldeyer (1900, p. 496), this cannot be due to the 
influence of light, for the lice may occur on the eyelashes and eyebrows. 
It cannot be due to the odour of the secretions about the genitalia and 
axillae being more attractive (Fischer, 1910, p. 118), because the insects 
occur elsewhere on the trunk and limbs. Waldeyer advances the reason¬ 
able hypothesis that the site of parasitism is determined by the distri¬ 
bution of the hair on the skin, although he believes that the insect 
perhaps finds it easier to cling to the flat crinkled hair of the pubis and 
axilla than to the hair of the head which is finer, straighter, and round 
or oval in cross section. As Wihofe found, the number of hairs upon a 
square J inch of scalp averages 223, whereas there are but 34 hairs upon 
a similar surface of pubic skin. Waldeyer’s enumerations show that 
the number of hairs on the head is 6-10 times greater than upon the 
rest of the body. Phthirus does not run about like Pediculus humanus, 
its legs, when stretched apart, span a distance of 2 mm. and this indicates 
that it will best cling to hairs that are further apart than on the head. 
The distance between the hairs of the head is about 1 mm., whereas the 
body hairs occur 2 or more mm. apart. Waldeyer believes that the dis¬ 
tance between the hairs on the shin is the factor which governs the distribution 
of Phthirus upon yuan’s body. This also explains why the insect, when 
present on the head, occurs in situations where the hairs are sparsest. 
From what I have observed of the behaviour of crab-lice on the 
body, I believe that Waldeyer’s explanation is correct. I would note, 
however, that whereas the adults and larger larvae are usually found 
clinging to two hairs, each hair being grasped by the legs of one side 
only, the first stage larvae often cling to a single hair with the legs of 
both sides (see p. 401). Under such circumstances the young larva feeds 
best when grasping a thick hair, because it can hold it more firmly and 
it affords better support. I have seen a larva try in vain to feed whilst 
anchored to a thin hair, whereas it succeeded at once in doing so when 
transferred to a thick hair. 
Mode of Dissemination and Infestation. 
Although the crab-louse is usually spread by coitus, there appear 
to be many exceptions to the rule. As the evidence of the occurrence 
of the parasite on infants indicates (see cases cited on p. 386), the insects 
