Gr. H. F. Nuttall 
343 
Activity. That typical capitis are more active in their movements at a 
lower temperature than are corporis is well established, this depending, I 
believe, upon the temperature conditions prevailing in their usual habitat, 
that in the hair being usually lower than that adjacent to the clothed human 
body to which corporis has, within limits, adapted itself. This difference may 
be regarded as merely due to adaptation. 
Climbing on hair. Owing to typical capitis being stouter limbed and usually 
smaller than corporis, they climb more rapidly along hair (Nuttall, XI. 1917, 
pp. 101-3), their muscles being relatively more powerful and their weight less. 
In this respect, corporis may be regarded as somewhat modified, owing to 
the altered surroundings in which they live, i.e. mainly in clothing to which 
they can cling with greater ease than hair. This is compensated for, however, 
by the habit corporis has acquired of gorging, which allows it to tide over 
fasting periods brought about by the host periodically discarding his clothing. 
Feeding. There is practically no difference in the feeding habits of capitis 
and corporis when they have unlimited opportunities of sucking blood, for 
under such conditions they imbibe small meals at frequent intervals (ibid. 
p. 173). The opportunities of snatching small meals upon the head are much 
greater than upon the body, because the hair to which head-lice cling is rooted 
in the skin, whereas the clothing to which corporis chiefly cling (owing to 
insufficiency of hair upon the human body), shifts about when the body is 
in movement. A consequence of this is that corporis becomes ravenous and 
imbibes large meals, i.e. gorges at intervals determined by the resting periods 
of the host or the periods when he removes and replaces his clothing. This 
habit of gorging is one which capitis may, however, likewise acquire if trained 
to feed at longer intervals of time. It is due in my opinion to altered feeding 
conditions that capitis appears more difficult to raise experimentally than 
corporis, it must first acquire the ability to gorge. 
Bearing of feeding habits on size and form. The smaller average size of 
capitis is attributed by Sikora (ix. 1917, p. 282) to the effect of its feeding 
ground on the insects’ nutrition and possibly to a difference in the temperature 
of the habitat. 
On the contrary, it appears to me that capitis should be regarded as the 
normal standard for comparison in seeking to explain the usually larger size 
of corporis, there being no reason to suppose that capitis suffers from mal¬ 
nutrition. By raising capitis under similar experimental conditions to corporis , 
that is, by altering its usual feeding habits, it becomes habituated to feeding 
at longer intervals and acquires with time the size and form of corporis. It 
appears to me that the periodic gorging, which necessarily leads to increased 
internal pressure, explains the larger size of corporis, in that the chitinous 
exoskeleton, prior to ecdysis and whilst still soft, is more stretched than is 
normally the case with capitis. It is only exceptional with corporis to gorge 
when provided with unlimited opportunities of feeding, they behave in other 
words like typical capitis. It is without doubt due to this that corporis raised 
