A. Bacut 
251 
the real period of fertihty exceeded that calculated, it being improbable 
that there would be no failures among the early laid eggs. As soon 
as the ceased to lay fertile eggs some of them were given the oppor¬ 
tunity of a second pairing. Particulars of the egg laying subsequent 
to the introduction of a second S are set out in Table II, and of the 
hatching of the eggs in Table V. It will be seen that a number of the 
5:? regained fertility after a second pairing, while two were afforded the 
option of a third. One of these (No. 16) apparently missed the second 
opportunity, but availed herself of the third and laid a few fertile eggs; 
the other (No. 18) survived long enough to lay a few more eggs, but none 
of them hatched. Details are given in Tables III and VI’^. 
Although the $$ are stated to have no receptaculum seminis it will 
be seen that they are able to lay fertile eggs as long as 20 days after 
the removal of the d. This does not compare so unfavourably with 
fleas, in which the receptaculum is well developed. The $ of Pulex 
irritans requires to pair several times if all the eggs she is capable of 
laying are to be fertilized. 
Table VII summarises the previous ones. 
Table VIII shows the results of an experiment in differential feeding. 
Owing to the escape of a $ from Box A, it was necessary to remove one 
from Box B, and to discard observations made prior to this reduction 
in numbers. Consequently, the experiment covers the later, instead of 
the earher, two-thirds of the insects’ lives, as was intended. It is 
probably owing to this that there is a marked falling off in the average 
number of eggs laid during the reversed order of feeding, when the 
lice in Box B were fed twice a day and those in Box A only once. 
Nevertheless, the result shows clearly how dependent egg laying is on 
food supply—the fertility of the eggs remaining unaffected. 
Hatching of eggs. Table IX shows details of the hatching of a number 
of eggs laid by 10 pairs of P. humanus from the date of their maturity 
until the 30th day. The eggs were placed in an incubator at a constant 
temperature of 31° C., the air being kept humid by a pan of water with 
slips of filter paper dipping into the water and hanging over the side 
of the pans. The high percentage hatching suggests that the conditions 
were not unfavourable. The variability shown in hatching may have 
been somewhat enhanced by the fact that the time of examination 
^ It is of course obvious that tlie percentage of eggs hatching in these Tables, as well 
as those in the parallel series for P. capitis Tables X and XII, has no real relation to the 
natural fertility of the egg.s in the correct .sense of the term ; the figures are only inserted 
as a convenience. 
Parasitology ix 
17 
