vn] PUPAE AND THEIR MODIFICATIONS 87 



perforce have to dive into the water. With the 

 beautifully adapted transfer of the functional spiracles, 

 their position is appropriately arranged for the gnat's 

 emergence at the surface, and the empty pupal cuticle 

 floats serving the insect as a raft. On this it rests 

 securely and the crumpled wings have opportunity to 

 expand and harden before the insect takes to flight. 



The aquatic pupae of other Diptera, many species 

 of the midges Chironomus and Simulium for example, 

 breathe dissolved air by means of tufts of thread-like 

 gills, which arise on either side of the prothorax. The 

 pupae of Simulium rest in their curious little cup- 

 like dwellings, attached to submerged stones or plants. 

 The Chironomus pupa is usually found in an elongate 

 gelatinous case adhering to a stone. From this case 

 the pupa rises to the surface of the water, that the 

 midge may emerge into the air. Miall and Hammond 

 (1900) describe the arrangement by which, when the 

 pupal stage ends, and these gills are no longer 

 required, their connection with the air-tube system 

 is severed 'without undue violence.' The walls of 

 the fine air-tubes that pass into the gills are specially 

 strengthened, but just below the pupal cuticle these 

 walls are exceedingly thin and delicate. Thus when 

 the pupal cuticle is cast, they are readily broken 

 there, and the cuticle of the midge forming beneath 

 has a spiracular opening into the main air-trunk, 

 ready for use during the insect's aerial life. 



