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 isth, there may be by September loth, 

 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 (191 1) 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 htmdred 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 

 Hkely 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 (19 13) 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 



