302 INSECTS AFFECTING MAN AND HOUSEHOLD 



excrement, spittoons, decaying vegetable matter, and garbage of 

 all sorts. Eggs are deposited in groups of 100 or more, and the 

 female may lay several groups of eggs before dying. These eggs 

 hatch in twenty-four or more hours, depending upon temperature. 

 The maggot stage lasts a week, and the pupal stage also about a 

 weeks. Adults (Fig. 311) may differ in size, due possibly to a 

 difference in kind and quality of food supply. It is estimated that 

 one pound of horse manure will produce 1200 flies. The house-fly 

 cannot fly a long distance, but it is carried passively on street cars, 



FIG. 310. Petri dish containing agar over which a fly was allowed to walk. White spots are 

 colonies of bacteria coming from germs left by the fly when crawling over the jelly. 



horse vehicles, on trains, ships, delivery wagons, and by other 

 means, and, once started in a community previously free, it 

 soon is present in enormous quantities. In fact, so rapid is its 

 increase that it is estimated that during a season enough flies 

 arise from the batches of eggs laid by one mother, supposing all 

 to live, to bury the entire earth four to seven feet deep. Putting 

 it in different words, enough descendants would come from the 

 laying of one mother during the course of a season to go around 

 the earth at the equator, if placed end to end, many times. The 

 length of life of the adult fly cannot be stated with absolute 

 accuracy. It may live three weeks or longer. 



