G-. H. F. Nuttall 
117 
larger. The proportion of the sexes is seen to vary considerably; in 
some broods the sexes are nearly equal in numbers, in others especially 
the males predominate. 
Table showing the Relative Numbers of the Sexes. 
(a) In the offspring of capitis d + corporis ? (from Bacot). 
Parents 
Progeny raised 
<? ? 
Pair 1 
71 
25 
2 
130 
22 
„ 3 
51 
49 
„ 4 
76 
35 
2nd generation. 
Progeny of pair 1 
211 
181 
3rd generation. 
Descendants of pair 1 ... 
93 
110 
( b) In the offspring of corporis <J + capitis $ (from Bacot). 
Parents 
Progeny raised 
<J ? 
Pair 1 
27 
22 
„ 2 
17 7 
2nd generation. 
Progeny of pairs 1 and 2 
143 
109 
3rd generation. 
Progeny of pairs 1 and 2 
63 
64 
The cause of the variation in the proportion of the sexes in the off¬ 
spring of different pairs remains to be determined. Further experi¬ 
ments under varying conditions may perhaps serve to explain the 
interesting phenomenon. The occurrence of male and female broods 
doubtless prevents excessive interbreeding among the descendants of a 
given pair, and, by insuring cross fertilization, promotes the maintenance 
of the species. 
OVIPOSITION. 
Oviposition in P. humanus, under favourable conditions, commences 
24—36 hours after the emergence of the female from the third larval 
skin; it is delayed or inhibited by a low temperature. The number 
of eggs laid by a female is dependent upon the food supply and the 
temperature at which she is maintained. A few eggs may occasionally 
be laid by females some days after they have been fed, thus Hase 
(1915, p. 75, cited by Muller, 1915) saw female corporis lay after fasting 
2—3 days, in one case even after fasting for 5 days. I have moreover 
seen 3 corporis (Obs. v. 16), after one feed, lay no eggs until 48 and 
