GNATS, MIDGES, AND MOSQUITOES 233 



being capable of enduring prolonged submergence without 

 inconvenience. The pupa is as capable of active exertions 

 as was the larva, and in fact is freely locomotive, though 

 it takes no food. This is a most exceptional circum- 

 stance amongst insects with a complete metamorphosis. 

 Nothing, moreover, could be in stronger contrast to 

 the style of life of the "short-horned" flies than that 

 of this roving gnat pupa. It will be remembered that 

 the blow-fly, which may be taken as a type of the 

 " short-horns," when about to become a pupa, does not 

 cast its skin, but becomes a barrel-shaped, absolutely 

 motionless body, by the hardening of the last larval 

 coat, whereas the gnat or mosquito does cast its skin 

 to become a pupa, and that pupa is a lively wrig- 

 gling creature, free to wander whither it chooses, though 

 110 more capable of feeding itself than the aforesaid 

 barrel. 



When the time for the emergence of the perfect 

 insect arrives, which will be about a month after the 

 hatching of the eggs, the pupa ascends to the surface, 

 and tipping up its tail, lies in a nearly horizontal position 

 with the back of the thorax just above the water. The 

 skin now splits, and the fly gradually extricates itself, 

 of course in a limp condition, and incapable of flight till 

 its wings are dried and stiffened. The empty shell of 

 the pupa gives it foothold till it is strong enough to 

 spread its wings and mount into the air for the first 

 time in its life. The occasion of the transformation 

 from pupa to fly is evidently the supreme moment in 

 the gnat's career, and the risks involved are considerable. 

 Not merely is it still exposed, as it has been hitherto, to 

 the jaws of hungry fish or predaceous water insects, but 

 there are also chances of wind and weather that may 

 prove fatal. However, vast swarms escape these perils 



