House-Flies or Typhoid-Flies 63 



fourth million bacteria to each fly. Now where 

 do all these bacteria come from? Necessarily 

 from the place where the fly breeds or where 

 it feeds. 



LIFE-HISTORY AND HABITS 



The eggs of the house-fly may be laid on almost 

 any kind of decaying or fermenting material. If 

 this is kept moist and a proper temperature main- 

 tained the larvae or maggots (Fig. 47) that hatch 

 from the eggs may develop. As a rule, however, 

 these requirements are found only under certain 

 conditions and are ordinarily found only in ma- 

 nure heaps or in privy vaults or latrines. All ob- 

 servers agree that the female fly prefers to deposit 

 her eggs in horse manure when this can be found 

 and when this is piled in heaps in the barn-yard 

 (Fig. 48) or in the field the heat caused by the 

 decay and fermentation makes ideal conditions 

 for the development of the larvae. Cow manure 

 may serve as a breeding-place to a limited extent. 

 The flies are immediately attracted to human ex- 

 crement and breed freely in it when opportunity 

 offers. Decaying vegetables or fruit, fermenting 

 kitchen refuse and other materials sometimes also 

 serve as breeding-places. 



In suitable places in warm weather the eggs will 



