174 



DIPTEEA. 



the air. In about a fortnight they change into pnpaB, 

 equally active, but very diiferent in their shape, for in- 

 stead of the respiratory tube near the tail possessed by 

 the larva, the pupa breathes by means of two trumpet-shaped 

 pipes af&xed to the back of the thorax. The time at 

 length arrives when the aquatic pupa has to give birth 

 to an insect, whose filmy wings would be spoiled by the 

 slightest wetting. The process by which this is accom- 

 plished affords a very interesting spectacle. The pupa 

 having risen to the surface, elevates its thorax above the 

 water, the skin soon splits down the back and exj)oses the 

 foreparts of the gnat, which are quickly protruded, and 

 the gnat floating on its old skin as in a boat, extricates its 

 T\dngs from their cases and its legs from their boots. The 

 wings unfold themselves, and the little creature flies away 

 to enjoy its new existence in another element. 



The Crane Flies (Tipula) constitute a very numerous 

 race, some of which are nearly related to the gnats, and 

 pass the first period of their existence in the water. In 



Fig. 134. — METAMORPHOSES OF BLOOD WOKMS. 



their larva state they resemble little red worms, having 

 their tails furnished with long curling filaments, some- 



