A Dangerous Neighbor 213 



has come, as is most likely the case, from a 

 revel in simple filth, he is just a nuisance, if 

 from infective filth, he is also a menace. 



Flies are fond of milk, and they usually fall 

 in. After this beauty bath in which a few odd 

 thousands of living bacteria are transferred 

 to the milk, the fly may scramble out. But 

 most bacteria, among them the typhoid 

 bacilli, grow excellently in milk. Thus again 

 and again have typhoid epidemics started 

 through the intervention of the domestic fly. 



A very simple experiment will illustrate 

 what happens when a house-fly with dirty 

 feet walks over food stuff on which bacteria 

 can grow. One of the Petri plates, which was 

 described in an early chapter, was partly 

 filled with warm nutrient gelatin in which 

 bacteria flourish. When it cooled, a smooth, 

 solid surface was formed. An unwary house- 

 fly was caught and his legs and body were 

 dipped into a dish of very dirty sewage water. 

 He was pinched a little, to quell his flying 

 ambitions, and set down upon the surface 

 of the gelatin. Here he was permitted to 

 wander about for a moment and then met his 



