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 1 1 . 



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 



