174 
Biology of Pediculus humanus 
cavity; such, insects invariably die after a short time. Gorging may 
be observed in lice kept at 28-37° C. and fed only once or twice a day, 
or after longer periods of starvation. 
Several authors have taken the pains to weigh lice before and after 
they have gorged in the manner described. Widmann (1915, p. 1336; 
18. viii. 1915, p. 290) states that an adult corporis may increase 0-7- 
1-2 mg. in weight (=0-6-1 c.m. of blood); Heymann (18. viii. 1915, 
p. 307) found that lice imbibed 0-2-0-3 mg. of blood at a meal; Rocha- 
Lima (i. 1916, pp. 17-31) reports that Sikora and Halberkann found the 
d imbibed 0-325 and the ? 0-89 mg.; finally Galli-Valerio (1916, p. 37) 
found adult capitis capable of imbibing ca. 0-5 mg. of blood. The 
interest in these weighings may be twofold ; they may afford (a) a measure 
of the loss of blood the host may suffer when infested by a given number 
of adult lice, and ( b ) give an indication of the multiplication of a virus 
taken into the louse’s body with the blood it imbibes, say in the case of 
typhus. In the latter, for instance, the blood of a typhus patient may 
be infective by inoculation in doses of 5 c.c., whereas the bodies of 
10 crushed lice, hypothetically equal to say 10 c.mm. of blood imbibed, 
may prove equally infective, and the inference that the virus has 
multiplied may be correct. As a measure of the loss of blood these 
weighings are fallacious, for normally the louse only gorges infrequently 
upon the host and the amount of blood imbibed in many little meals 
may be greater than when large meals are taken at long intervals 
although the insect perhaps fills itself to bursting point. Moreover, 
the loss of blood is not merely due to adults, since the immature stages 
usually preponderate upon infested persons. 
The time occupied in feeding. 
When hungry lice are placed upon the skin, they instantly proceed 
to feed, be they in daylight or darkness, or in the presence or absence 
of so-called repellants, and, usually in a few moments, the blood may be 
seen entering the midgut. At times there is some delay before the 
blood flows, because the insects must first find a blood vessel. A 
number of authors have timed the process, stating that it may take: 
10-15', at times 25', in larvae, and 20-30' in adults (Fantham, 1912, 
p. 514); 5-10' (Legroux, 1915, p. 470); 7' (Musselius, 1915, p. 170, 
who fed them on himself); 3-5' (Widmann, 18. viii. 1915, p. 290); 
8-21', at most 23', when they have starved (Hase, 1915, p. 85, cited by 
Muller); larvae fed twice a day took 9-22', but fed more rapidly when 
fed thrice a day (Peacock, 1916, p. 39); all stages are satiated in 2-15' 
