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, 

 wc see that there is time for the development of seven 

 generations between April 15th and September loth. 

 Flies, it is true, continue to emerge from manure piles 

 and other breeding places much later than September 

 loth, 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. 



