A. Bacot 
255 
The $$ of the F. 2 generation of the cross, P. humanus S and P. 
capitis ?, laid their eggs on both cloth and hair, but they showed a 
preference for laying on hair; clustering was not so noticeable a feature 
as in the case of the $$ of the reverse cross. 
An examination of the eggs laid by these hybrids showed that on 
the whole they were clearly intermediate in size between those of the 
species. The eggs are, however, variable in size, both as regards those 
of the species and those of the hybrids, so that it was easy to match 
eggs of either capitis or humanus from the hybrid batch. 
No signs of unhealthiness, mortahty in moulting or shortening of 
life in the hybrid insects was observed; the usual life was 30 to 40 days. 
One pair of the F. 1 generation of the P. capitis d and P. humanus $ cross 
lived 45 days and their fecundity appeared to be on a par with that of 
P. humanus. They throve just as well under the artificial conditions 
of rearing as did this species. Although P. capitis can be successfully 
reared in the boxes, the colonies do not show the same rapid and vigorous 
growth. 
There was no noticeable increase of mortahty in the eggs laid by the 
hybrid insects. 
No attempts were made to carry the hybrid races beyond the F. 3 
generation; they were then in all respects healthy and fertile, probably 
they could be continued indefinitely. The variability in size referred 
to above may, however, have been the beginning of a segregating 
process which would eventually result in the hybrid races being forced 
back to the specific norm. 
Summary. 
General comparative note on the two species. 
Pediculus hwnanus {vestimenti) is a larger, more robust and less 
active insect than P. capitis ,—the ?? having a relatively greater egg¬ 
carrying capacity than those of the head louse. The eggs are larger 
and the number laid (under the conditions of these experiments) is 
greater, while the habits associated with egg laying differ, although 
placing the 9? of humanus under conditions apphcable to capitis or 
vice versa may induce a considerable degree of uniformity. Cross 
pairings between the insects are easily brought about and the offspring 
are fertile inter se. Hybrid strains were maintained until the F. 3 
generation and there seemed no reason, judging from breeding results, 
why such strains should not be continued indefinitely. Nevertheless 
