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 larvae 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 
