INSECTA : DIPTERA 



343 



face tension on them, it sinks to the bottom. In rising 

 again, it jerks its whole body, and also makes use of the 

 special swimming organ which projects laterally from the last 

 segment of the tail, ending in four lobes, and bearing at one 

 side a plate of stiff bristles which possibly serve as a rudder 

 to guide the movement through the water. 

 T , p In two or three weeks, the larva moults its 



skin for the third or fourth time, and at this last 

 moult it changes its shape com- 

 pletely and becomes a pupa. It 

 now floats head uppermost, and 

 the rudiments of the eyes, wings, 

 and appendages are clearly visible 

 through the pupal skin which 

 covers the big rounded mass at 

 the front end. The abdomen is 

 little changed, except that to the 

 eighth segment is attached only 

 a single pair of " tail-flaps," which 

 are used in swimming; for this 

 pupa, though it does not feed, 

 is not always quiescent like 

 most other insect pupae. If 

 touched, it at once darts down in 

 the water, only to rise to the 

 surface again as soon as it stops 

 struggling. Since the head-end 

 is now uppermost, we find that 



the pupa breathes through two little trumpet-shaped tubes 

 on its head, the tail tube being entirely absent. The inner 

 surface of these " trumpets " is hairy, and so water is pre- 

 vented from entering them. 



Finally, when the body of the imago has been 

 P erf ected within, the skin splits along the back 

 between the two air trumpets, and the perfect insect 

 begins to emerge. The head and thorax push up first into 

 the air, and then the legs and wings are carefully withdrawn. 

 The few moments before the tips of the legs are free are 

 the most critical in the life of the gnat, for the top-heavy 

 body is supported merely by the frail bark made of the pupal 

 skin, and the least breath of air sends it scudding across the 



FIG. 265. Pupa of the Common 

 Gnat (Culex pipiens). 



r, Respiratory tubes. 



