162 NATURAL HISTORY OF AQUATIC INSECTS CH. 



mobility of the head, which can be bent completely 

 back till it touches the prothorax, or screwed round 

 till the line of the mouth lies in the axis of the 

 body. If the larva should slip into deep water, the 

 respiratory cup remains free from water, and buoys 

 up the tail. If the whole body is sunk beneath 

 the surface, a bubble is carried down enclosed in 

 the fringes of the respiratory cup. The larva when 

 thus submerged, swims energetically after the manner 

 of a Chironomus larva, twitching its body this way 

 and that, and making pretty rapid, though un- 

 steady progress. It can readily regain the surface 

 of the water. 



The larva of Dixa makes use of the surface-film 

 not only to buoy up its tail, and to keep water out of 

 its air-filled basin, but also as an aid in climbing. 

 When it leaves the water it takes with it a close- 

 fitting, contractile film, which binds it to any wettable 

 surface with which it comes in contact. This use of 

 the film will become more evident if we suppose that 

 we have to climb a slippery pole. We grasp it as 

 hard as we can with arms and legs, and should find 

 the task greatly facilitated by a strong band of india- 

 rubber, which embraced both body and pole, and 

 spared us the necessity of grasping. The Dixa larva 

 is bound to the surface of a leaf by its contractile 

 water-film, and can therefore employ all its strength 

 in shoving itself along. 



The same principle is turned to account by a 

 Gyrinus when it creeps up a smooth surface, though 

 here only the ventral surface of the Insect is 

 wetted. Mr. Scourfield has shown that certain 



