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, 



