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. n.—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 
