PREVENTIVE MEASURES 191 



ment is there made that they will breed in almost any 

 fermenting organic material. They prefer horse ma- 

 nure, but will breed in human excreta, in cow dung, 

 and the dung of pigs, fowls, and other animals, in fer- 

 menting spent hops, bran, in ash barrels containing 

 more or less organic matter, and in everything of the 

 sort. Search must therefore be made for every ac- 

 cumulation of refuse of this kind within a large radius. 

 To gain the requisite conditions for fermentation, it is 

 necessary as a rule for the substances in which flies 

 will breed to accumulate until a considerable quantity 

 is reached, at least such an amount as will be readily 

 noticeable; so that the search for the breeding places 

 of the bulk of the flies of a given neighborhood need 

 not be a very close one. 



The question arises, however : In how small an 

 amount of breeding material will fly larvae be found ? 

 Certain breeding materials will remain moist in small 

 quantity longer than others; a single dropping from 

 a cow is very liquid, but it hardens so rapidly on top 

 and its exterior becomes so tough that the house fly 

 seems to find difficulty in issuing from it, and perhaps 

 that is one of the reasons why this substance is not a 

 more prolific breeding place for this species than it is; 

 though certain other flies, such as the horn fly of cattle, 

 breed in cow dung in great numbers. 



Horse dung is so mixed with the materials which 

 have been eaten that it dries very quickly indeed all 

 through the mass ; so that a single dropping of a horse 

 in a pasture under ordinary summer conditions will 



