THE HOUSE FLY AND DISEASE 



75 



sterilization. We base our methods of control upon our knowl- 

 edge of the agents harmful to bacteria. Sunlight is their great- 

 est destroyer, whereas they grow best in the dark. Heat stops 

 their multiplication at 109.4 F., and cold at 60.8 F. Many of 

 them are killed at 32 F., but unfortunately the bacteria causing 

 typhoid fever and Asiatic cholera are not, and hence may be dis- 



FIG. 49. Foot of the house fly. 



A, end of foot showing claws and bristles ; B, opposite side of foot showing 

 fleshy lobes, the pulvillus; C, part of pulvillus much enlarged showing 

 hooked hairs. (After Smith.) 



tributed in ice. It is evident that the cooling of foods, such as 

 milk, will prevent the multiplication of germs and hence keep 

 them from spoiling, and that surgeons' instruments may be 

 rendered aseptic, that is, free from bacteria, by heating or boiling. 

 Gases, such as formaldehyde, and liquids, such as bichloride of 

 mercury and carbolic acid, are commonly used as disinfectants. 



