24 THE HOUSE FLY DISEASE CARRIER 



tecting nymphal membrane. A fully formed pupa 

 taken from the pupal sheath, or puparium as it is called, 

 is shown in figure n. 



In this stage in Washington in midsummer the 

 writer has shown the normal duration to be about five 

 days. Mr. Newstead gives the period as from -five to 



Fig. ii. House fly puparium (at left) and pupa (at right) ; 

 greatly enlarged. (Original.) 



seven days in cases where there is heat produced by 

 fermentation, but where there is no such heat the stage 

 may last from fourteen to twenty-eight days, or even 

 considerably longer. Doctor Hewitt states that with 

 a constant temperature the adult flies may emerge be- 

 tween the third and fourth day after pupation, but that 

 the period is more usually four or five days, since the 

 larvae when ready to pupate as a rule leave the hotter 



