THE HOUSE FLY AND DISEASE 77 



rasping it with the lobes of the proboscis. If the fly chances to 

 feed on substances containing bacteria, these are taken into the 

 alimentary canal and later may be extruded through the mouth, 

 or deposited in the excreta which forms " fly specks." The con- 

 clusion is evident : a fly that has access to substances containing 

 bacteria cannot help but become soiled by them and later 

 transfer them to other objects, such as articles of food (Fig. 510). 



FIG. 51 a. Plate of gelatine, showing colonies of bacteria in footprints of a 



house fly. (After Underwood.) 



Germ Diseases Transmitted by House Flies. After having 

 shown that the house fly carries bacteria, we must inquire as to 

 what sorts of diseases may be caused by these bacteria. Lack 

 of space makes it impossible to give the evidence upon which our 

 statement is made, but there is convincing proof that the germs 

 of typhoid fever, Asiatic cholera, summer diarrhoea, tubercu- 

 losis, yaws, ophthalmia, smallpox, and tropical sore are carried 

 bv flies. The relation between flies and typhoid fever is well 



