vi] LARVAE AND THEIR ADAPTATIONS 77 



a stellate guard to the spiracles. These processes 

 can pierce the surface-film of the water, and place 

 the tracheal system of the maggot in touch with the 

 pure upper air; while its mouth may be far down, 

 feeding among the foul refuse of the ditch, it can still 

 reach out to the medium in which the end of its life- 

 story must be wrought out. 



Reverting to the first great division of the Diptera, 

 we find varied adaptations to aquatic life among many 

 grubs that possess a definite head. The larva of a 

 Gnat (Culex 1 ) has projecting from the hind region 

 of the abdomen a long tubular outgrowth, at the 

 end of which are the spiracles, guarded by three 

 pointed flaps forming a valve. When closed these 

 pierce the surface-film of the water in which the 

 larva lives ; when opened a little cup-like depression 

 is formed in the surface-film, from which the larva 

 hangs. Or having accumulated a supply of air, it 

 can disengage itself from the surface-film and dive 

 through the water, its tracheal system safely closed. 

 Another mode of breathing is found in the 'Blood- 

 worms' and allied larvae of the Harlequin-midges 

 (Chironomidae) whose transformations are described 

 in detail by Miall and Hammond (1900). These larvae 

 have two pairs of cylindrical, spine-bearing pro-legs 

 one on the prothorax and the other on the hind- 

 most abdominal segment ; the latter structures serve 



1 See Frontispiece, A. 



