II FLIES WITH AQUATIC LARV^ 179 



larged by muscular pull, so that the pressure within 

 the cup becomes less than the external pressure. 

 The edge of the cup is roughened, as in Simulium. 

 by numerous minute teeth, which prevent slipping. 



The larva, when feeding, holds on by its tail-sucker, 

 and sticks its body straight out into the current. But 

 if the current is unusually strong, the larva, when not 

 feeding, often doubles up its body, and holds on to 

 the surface of the leaf by both suckers. 



The life of a submerged Insect in a rapid current 

 has of course its own special difficulties. The Simu- 

 lium larva has to creep from leaf to leaf, to change 

 its position as the stream rises and falls, and to avoid 

 dangerous enemies. Of these enemies aquatic larvae, 

 and especially Caddis-worms, are the commonest and 

 most formidable. In moving about there is always 

 risk of being accidentally dislodged, and the conse- 

 quences of this might be serious. For if a larva 

 should let go or miss its hold in a rapid stream, what 

 is likely to happen ? It seems inevitable that it will 

 be swept away, and who knows where it will come to 

 rest ? The little rivulet which I am accustomed to 

 visit for the purpose of observing this larva is a 

 bright clear stream, flowing over water-cress and 

 brook-lime and forget-me-not. A few feet lower 

 down it ends in the wide and stony Wharfe, a stream 

 of quite different character, in which I have never 

 been able to discover a single specimen of the same 

 species. Other brooks in which the larvae are plen- 

 tiful, empty themselves into rivers unsuited to an 

 Insect of habits so peculiar muddy, sluggish, or 

 brackish. But this difficulty has been provided 



N 2 



