22 INSECTS AND HUMAN WELFARE 



Several other species of flies appear regularly in houses, 

 but in far lesser numbers, and none exhibit to such a marked 

 degree the peculiar tastes of the housefly, which wanders 

 back and forth from filth to food, feeding on each in turn. 

 In this method of feeding lies the danger of infection by 

 houseflies; they are equally fond of clean and filthj' materials, 

 and their frequent migrations from one to the other multiply 



Fig. 8. Housefly, showing method of feeding. 



their opportunities to pick up pathogenic organisms that may 

 be later deposited upon foods. The frequency with which 

 this actually happens is of course the vital point, and it is 

 upon this that it is verj"^ difficult to obtain concise data. 



It has been shown rather conclusively that adult flies do 

 not retain in the alimentary tract bacteria which they may 

 have ingested as larvae that have developed and fed in ma- 

 terial containing, for example, the bacillus of typhoid fever. 

 On the other hand, adult flies readily obtain this bacillus from 

 contaminated substances and may retain and later deposit it 

 in a living condition on food designed for immediate human 

 consumption. There can be no question but that this occurs 

 commonly under many circumstances, particularly in com- 

 munities where there is no adequate system of sewage dis- 



