402 
Biology of Phtliims pubis 
swells. After this it may extend its legs so that it comes to grasp two 
hairs with the legs of each side as the later stages almost invariably do. 
The larva moults in situ and then shifts its position a short distance, 
and immediately resumes feeding. The following stages behave similarly. 
It is relatively uncommon to find the insects wandering excepting 
immediately after a moult, or after they attain sexual maturity, and 
even then they are usually found feeding. Gorging, as observed in 
Pediculus, is never seen in the crab-louse. Two tusk-like, retrograde 
processes, situate antero-laterally and ventrally upon the head, serve 
as an additional means of fixation of the mouth-parts to the skin, such 
processes being absent in Pediculus which feeds rapidly. 
As an example of the manner of feeding, the following record may be 
cited. It relates to a hungry adult which I placed on my arm and 
observed microscopically. Its pharyngeal pump was seen to act and 
blood appeared in the gut diverticula 8' after the mouth-parts had 
penetrated; after 11' a blood droplet issued from the anus. The record 
proceeds: 18|' stopped pumping, 21' pumped, 23' stopped pumping, 
24' inspissated blood issued from the anus and pumping recommenced, 
25' pumping stopped and five beads of blood were voided, 27' started 
pumping, 29' stopped, 29J' started again and voided a drop of blood, 
30|' stopped, 37' started, 38' stopped pumping, and so on. The next 
day the same kind of observation was repeated, and the examination of 
other insects subsequently showed that this is the ordinary mode of 
feeding, pumping and resting periods alternating and blood-red faecal 
matter being voided at short intervals. This frequent defaecation 
renders their surroundings very filthy in a short time. Their filthiness 
in this respect is much more observable than in Pedicidm because 
defaecation is so localized. Merely because the faeces are more scattered, 
P. liumanus capitis, relatively speaking, appears to produce less evident 
filth, whilst corporis, favoured by its retreats in the clothing, ranks 
third in this respect. 
The habit of what practically constitutes continuous feeding which 
characterizes Phtlivrus, sufficiently explains why it dies so quickly from 
hunger and drought when removed from the host. 
Reactions to Light and Heat. 
On short exposure to light, when feeding, Phthirus frequently shows 
signs of restlessness, but it does not release its hold and wander away. 
As Hewlett (1917, p. 188) showed, the insects are little affected when a 
