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 
