DISTRIBUTION OF DISEASE GERMS 



105 



119. Typhoid bacteria carried by flies. Disease bacteria 

 may be transmitted by many other agencies. For example, 

 typhoid bacteria may live in waste animal matter and may 

 then be carried by flies and deposited upon solid food or in 

 milk or water, and later gain entrance to the human body. 

 The flies are not causes of disease ; they are merely the agents 

 of transmission. If no typhoid bacteria had been allowed to 

 get to the organic matter 



upon which the flies fed, 

 they could not have carried 

 them. Since many people 

 are so careless as to allow 

 distribution of disease bac- 

 teria to places from which 

 flies may carry them, it is 

 important that we should 

 study the habits of the fly, 

 with the hope of prevent- 

 ing further distribution of 

 bacteria (fig. 52). 



120. The life history of 

 the house fly. An individ- 

 ual house fly lays about 

 120 eggs. They are usually 

 laid in horse manure, but 



they may be placed in any animal or plant refuse. The eggs 

 hatch in from six to eight hours, forming larvae. The larvae 

 (fig. 53) are small, white, wormlike creatures called maggots. 

 For about five days they feed upon the refuse in which the 

 eggs were laid, after which they become quiet and change 

 into pupae. After five or six days the pupae transform into 

 adult flies. In a few days these flies lay eggs, and another 

 generation is begun. In ten days 120 flies may develop from 

 one pair of house flies. Suppose that half the flies of the 

 second generation are females and that they lay eggs within 



FIG. 52. The house fly 



The rough and hairy body of the fly is well 

 adapted to carrying bacteria. After Howard 



