92 THE HOUSE FLY—DISEASE CARRIER 
puparium. Observations were carried on through suc¬ 
ceeding months and the duration of the life cycle was 
carefully studied ( Psyche, February, 1910). The life 
cycle is longer in the spring and shorter in summer, 
the average life cycle being twenty-two and one- 
half days with usual temperature. They found that 
one female was able to parasitize twenty-two puparia 
and another one seventeen. The authors suspect that 
the phenomenon of polyembryony, that is to say, the 
development of a number of adult individuals from a 
single egg, takes place with this species. Counts of 
several thousand reared specimens of the parasite in¬ 
dicated that fifty-eight and nineteen-hundredths per 
cent, of them were female and forty-one and eighty-one- 
hundredths per cent, were males. They found that the 
adult parasites issue from the host puparium through 
from one to three circular holes of various positions, 
several issuing from each hole. As to the abundance 
of the parasite, the authors indicate that during Sep¬ 
tember and October, 1908, they reared 8,000 or more 
specimens, and these were reared quite accidentally, that 
is to say, without conscious effort on their part to in¬ 
crease the number. The local abundance of the para¬ 
site was indicated by the fact that in a portion of a 
given experiment the percentage of parasitism was as 
high as ninety per cent. This percentage of mortality 
on the part of the house fly, however, was by no means 
general, and the parasite had apparently concentrated 
its attack on certain spots. The authors made an un¬ 
successful attempt to propagate the species artificially 
