278 FRANZ SCHRADER 
of the animal is limited to a small area at first. Crowding out 
of one line by the other would progress very fast in that case. 
SEX RATIOS 
Knowing that unfertilized eggs give rise to males in the Amer- 
ican race, it is to be expected that, in a random collection, there 
would be a preponderance of that sex. Such is not the case, 
however, and this is due, no doubt, as Williams has already 
pointed out, to a difference in the duration of life of the two sexes. 
In one of my breeding experiments the female lived twenty-five 
days, while the males were generally dead by the tenth or eleventh 
day. Hargreaves (714) mentions an adult that lived thirty- 
eight days, but does net give the sex of this specimen. He notes 
further that the duration of life is to some extent dependent on 
temperature, cold weather ‘‘making them live more slowly.” 
Judging from the cytological evidence, the production of the 
two sexes seems not to be due to two kinds of spermatozoa, but 
to be dependent merely on fertilization or its omission. Like 
the earlier observers, I found that virgin females produced only 
males, eight broods ranging from eight to fifty-two individuals 
having been raised to confirm this evidence. Mated females 
produced both sexes, and the proportion of these was very vari- 
able. This irregularity in sex ratios is open to two possible ex- 
planations: either the female has control over the spermatheca 
and, reacting to some stimulus, permits each egg to be fertilized 
or not, as the case may be, or the spermatheca acts perfectly 
mechanically and every egg is normally fertilized until the sup- 
ply of spermatozoa is exhausted. In either case, the sex-ratio 
would be liable to great variation. 
To determine this, I mated virgin females and kept a careful 
account of their offspring. I found that the eggs first laid were 
also the first to hatch and develop to the adult stage. In doing 
this, I did not keep track of every egg laid—a physical impos- 
sibility—but only of the batches deposited on successive days. 
If the female has control over the spermatheca, it is probable 
that the offspring will belong to either sex from day to day, the 
first eggs giving rise to a mixture of sexes. If, on the other 
