344 
Pediculus and Phthirus 
by me in a felt cell on the arm, where they could feed at will (Nuttall, xi. 1917, 
pp. 105, 161), were decidedly smaller than others raised in boxes and fed 
twice daily to repletion. These insects were also proportionately narrower 
across the abdomen as compared to those that were fed but twice daily. 
I am convinced that by modifying the method of feeding so as to lengthen 
the intervals between meals as rapidly as practicable, that capitis will be 
made to assume the form of corporis in a shorter time than four to five genera¬ 
tions as observed by Sikora. Greatly increased internal pressure, consequent 
upon the habit of gorging, although induced periodically, would appear to 
me to explain the larger average size of corporis, particularly the stretching 
of the thinner portions of the integument, especially the increase in the size 
of the abdomen coupled with the loss of angularity in the abdominal segments 
and the more widely separated hairs upon the abdominal surface. 
The effect of darkness no doubt is responsible for corporis possessing longer 
and slimmer antennae and legs than capitis. The latter is more exposed to 
light upon the head than is corporis beneath the clothing in most instances. 
It is of course well known that arthropods inhabiting dark places have longer 
antennae and legs than those living exposed to light. 
Minor biological differences have been pointed out by different authors, 
thus the fertility of capitis is somewhat less under experimental conditions (see 
Nuttall, xi. 1917, p. 141), but such experiments do not afford a just measure 
of what takes place in nature. With time, as Mr A. Bacot informs me (hi. 1919), 
capitis lay more eggs when raised in successive generations experimentally. 
Copulation. The observation of Bacot (n. 1917, pp. 253,256) that $ corporis, 
when paired with $ capitis, cause an undue mortality among the latter, is 
explained, in my opinion, by coital traumatism, due to large males copulating 
with.small females and rupturing them. The statement of Bacot ( loc. cit.) 
that oviposition differs in corporis in that these prefer to lay on cloth and lay 
awkwardly on hair, was not confirmed by me (loc. cit. p. 143), although 
capitis has been found both by Bacot and me to prefer laying on hair. This 
difference is however only one of degree. 
Interbreeding . When capitis and corporis pair, they yield a progeny that 
are fertile to the third generation, as was first demonstrated by Bacot (loc. cit.), 
and there is no reason to suppose that this fertility may not continue longer. 
The occurrence of hermaphrodites in crosses between capitis and corporis 
(see Keilin and Nuttall, this vol.) indicates that the two forms are races 
belonging to one species. 
That corporis is descended from capitis in nature has been assumed by 
systematists like Mjoberg (1910, p. 169), Fahrenholz (1910, p. 65), and Cum¬ 
mings (1916), who regarded the two forms as separate species. Without 
holding the latter view, and prior to reading these authors' papers, it also 
occurred to me, as it has to others, that capitis must represent the primitive 
form. According to this view, primitive man was hairy all over, as his body 
grew naked he adopted clothing and his very ancient pedicular ectoparasites 
