26 THE HOUSE FLY—DISEASE CARRIER 
terior end of the pupal case in dorsal and ventral por¬ 
tions by means of the inflated frontal sac, which may 
be seen extruded in front of the head above the bases 
of the antennae. The splitting of the anterior end of 
the pupal case is quite regular, a circular split is formed 
in the sixth segment and two lateral splits are formed 
in a line below the remains of the anterior spiracular 
processes of the larva. The fly levers itself up out of 
the barrel-like pupa [puparium] and leaves the nymphal 
sheath. With the help of the frontal sac, which it al¬ 
ternately inflates and deflates, it makes its way to the 
exterior of the heap and crawls about while its wings 
unfold and attain their ultimate texture, the chitinous 
exoskeleton hardening at the same time; when these 
processes are complete the perfect insect sets out on its 
career.” 
The frontal sac just mentioned is the distended mem¬ 
branous portion of the front of the head. This is con¬ 
stantly distending as the fly walks rapidly about after 
issuing. When it is contracted at this early time, it 
forms a dull area, soft and fleshy-looking, and free 
from hairs. The fly possesses the power of distending 
it into a bladder-like expansion, trapezoidal in outline 
and almost as big as the rest of the head, pushing the 
antennae down out of sight. This membrane is evi¬ 
dently distended with air, and, as pointed out by Pack¬ 
ard, its connection with the tracheae and the mechanism 
of its movements would form a very interesting sub¬ 
ject of inquiry. Lowne, in his Anatomy of the Blow¬ 
fly, has described a similar structure with that insect, 
