36 THE HOUSE FLY—DISEASE CARRIER 
time in any of his sets of rearings, which he tabulates 
in Public Health, May, 1908, four and one-half to six 
days from egg to pupa, and three and one-half days 
from pupa to adult fly, a minimum for the life round 
of eight days. He found great variations in this 
period, according to the prevailing temperature. 
Number of Generations 
Taking the minimum duration of a generation in 
Washington so far as observed (and this must not be 
taken as the scientific minimum, since it depends upon 
observations taken only during midsummer of a single 
year), or we will say perhaps a midsummer average 
under Washington conditions, and accepting Doctor 
Hewitt’s observations as to the time elapsing between 
the issuing of the adult flies and their sexual maturity 
as being, perhaps under American conditions, ten days, 
we see that there is time for the development of seven 
generations between April 15th and September 10th. 
Flies, it is true, continue to emerge from manure piles 
and other breeding places much later than September 
1 oth, and in fact during the season of 1910 active lar¬ 
vae were found as late as the 30th of November, while 
on the occasional warm days of that period adult flies 
were still active and laid eggs. The generations of 
springtime and of autumn, however, are of much slower 
development than those of midsummer, so that it is 
probably safe to say that there are seldom more than 
nine generations a year under outdoor conditions in 
places comparable in climate to Washington. 
