INSECTS AND DISEASE 



221 



The eggs are deposited in 

 horse manure if it is to be 

 found, or in other similar 

 matter, about two hundred 

 being laid by each female. 

 They hatch in one day into 

 the larval form which we call 

 maggots, and in this stage 

 do some good as scavengers. 

 After eating and growing for 

 about five or six days, the 

 larvae pass into the pupal 

 condition, inside the last lar- 

 val skin, which thus takes 

 the place of a cocoon. From 

 this the adults emerge in 

 about a week. The whole 

 process occupies about two 

 weeks, begins early in spring, 



American Museum of National History 

 FIG. 73. Eggs of the housefly. 



Courtesy of the American Museum of Natural History 

 FIG. 72. Common house (typhoid) fly. 



and continues till cold 

 weather. Supposing that 

 half the eggs produced fe- 

 males and these reproduce 

 at the same rate, calculate 

 the number of flies that 

 might be produced by one 

 adult which had survived the 

 winter, and the enormous 

 number of flies in existence 

 will be accounted for. 



Danger from Flies. Flies 

 have always been regarded as 

 more or less of a nuisance, as 

 they crawl over our food and 

 our bodies, fall into milk and 

 other liquids, and annoy man- 



