22 THE HOUSE FLY— DISEASE CARRIER 



might last only twenty-four hours, but at a lower tem- 

 perature or with a deficiency of moisture the period 

 was prolonged and might take several days. The third 

 stage occupied as a rule between three and four days. 



At all times the larvcC are very active. When their 

 breeding place is disturbed they wriggle actively about 

 in the endeavor to conceal themselves, and so rapidly 

 do they accomplish this purpose that it is difficult to 

 take a satisfactory moving picture of them, or indeed 

 a photograph of any kind. When full-grown and 

 ready to transform, the yellowish color becomes more 

 pronounced, owing to the proliferation of fat cells in 

 great numbers in anticipation of the resting, non-feed- 

 ing pupal condition. The transformation to pupa may 

 take place almost anywhere, but as a rule there is an 

 effort on the part of the larvae to descend deeper into 

 the manure pile or other substance in which they may 

 be living, and sometimes, when the substance upon 

 which they have fed is moist and the earth below it is 

 also moist and easy of entrance, they may descend two 

 or three inches below the surface of the ground. 



The only good word that can be said for this fly is 

 the fact that its larvae destroy enormous quantities of 

 excrementitious and waste material, greatly assisting 

 the bacteria of putrefaction. E. Guyenot (1907) 

 shows, first, that the liquefaction of albuminoid sub- 

 stances is the result of a true process of digestion un- 

 der the influence of certain germs of putrefaction; 

 second, that fly larvae, absorbing exclusively liquid 

 substances, easily assimilable, have the digestive tract 



