150 
Biology of Pediculus humanus 
The young louse usually frees itself from the egg-shell in 2-5 minutes 
after the springing of the operculum. If, however, the temperature is 
too high, as I have frequently seen in eggs hatching in the thermostat 
or on the body, a considerable proportion of the larvae die in the act 
of hatching; death occurring at any stage of the process from the 
inability of the insect to free itself quickly, before the delicate and soft 
vitelline membrane hardens and the insect’s moist body dries and sticks 
to the membrane. On the one hand, the larva may die without bursting 
the membrane which encloses it within the shell, in which case only the 
back of the head and thorax protrude from the orifice of the shell; or 
the larva may be unable to extricate the first, second, or third pair of 
legs, remaining stuck to the hardened and shrivelled intima, in the 
position depicted in Fig. 10. 
We may add that when the pumping process begins, the larva 
appears pale, but as the moment of hatching approaches, it becomes 
markedly darker, especially in the region of the head, claws, etc. In 
other words the chitin hardens before the larva issues, and this explains 
why the insect is ready to feed at once on emergence, as War burton 
first observed. I have, by means of a needle, helped a larva to free 
itself and transferred it to the back of my hand upon the needle point; 
it then instantly proceeded to feed. The chitinous exoskeleton becomes 
still darker in the course of a few hours after the emergence of the 
larva, this being specially noticeable in more pigmented forms of capitis 
where the larva shows well-marked dark lines at the sides of the thorax. 
The Empty Shell. 
The empty shell, when viewed by reflected light, presents a white 
pearly appearance to the naked eye. Under the microscope the shell 
is iridescent like a soap-bubble; and by transmitted light it is hyaline. 
The whitish appearance of empty shells is most striking to the naked 
eye when egg clusters are viewed in a good light upon a dark background 
of cloth or hair. The very thin and structureless vitelline membrane 
frequently protrudes out of the orifice of the shell like a funnel or cuff, 
this being the portion which has been pushed out and ruptured by the 
larva that has issued (Fig. 9, 11-13), and crumpled traces of it may be dis¬ 
covered within the shell or hanging like a sack in the aperture to whose 
margins it remains stuck. This membrane is purposely omitted from 
Fig. 10. 
In describing the orientation of the egg in the section on oviposition 
(see p. 123) reference was made to the manner in which the operculum 
