The House-fly as a Carrier of Disease 149 
Washington. The number may be considerably increased in warmer 
climates. 
The rate at which flies may increase under favorable conditions is 
astounding. Various writers have given estimates of the numbers of 
flies which may develop as the progeny of a single individual, provid¬ 
ing all the eggs and all the individual flies survived. Thus, Howard 
estimates that from a single female, depositing one hundred and 
twenty eggs on April 15th, there may be by September 10th, 
5,598,720,000,000 adults. Fortunately, living forms do not produce 
in any such mathematical manner and the chief value of the figures 
is to illustrate the enormous struggle for existence which is con¬ 
stantly taking place in nature. 
Flies may travel for a considerable distance to reach food and 
shelter, though normally they pass to dwellings and other sources 
of food supply in the immediate neighborhood of their breeding 
places. Copeman, Howlett and Merriman (1911) marked flies by 
shaking them in a bag containing colored chalk. Such flies were 
repeatedly recovered at distances of eight to one thousand yards 
and even at a distance of seventeen hundred yards, nearly a mile. 
Hindle and Merriman (1914) continued these experiments on a 
large scale at Cambridge, England. They “do not think it likely 
that, as a rule, flies travel more than a quarter of a mile in thickly- 
housed areas.” In one case a single fly was recovered at a distance 
of 770 yards but a part of this distance was across open fen-land. 
The surprising fact was brought out that flies tend to travel either 
against or across the wind. The actual direction followed may be 
determined either directly by the action of the wind (positive anemo- 
tropism), or indirectly owing to the flies being attracted by any odor 
that it may convey from a source of food. They conclude that it is 
likely that the chief conditions favoring the disposal of flies are fine 
weather and a warm temperature. The nature of the locality is 
another considerable factor. Hodge (1913) has shown that when 
aided by the wind they may fly to much greater distances over the 
water. He reports that at Cleveland, Ohio, the cribs of the water 
works, situated a mile and a quarter, five miles, and six miles out in 
Lake Erie are invaded by a regular plague of flies when the wind 
blows from the city. Investigation showed that there was absolutely 
nothing of any kind in which flies could breed on the crib. 
The omnivorous habits of the house-fly are matters of everyday 
observation. From our view point, it is sufficient to emphasize 
