CH.xili CONTRIVANCES OF AQUATIC INSECTS 383 



(Gyrinus) darts to and fro upon the surface, changing 

 its course every moment by slight adjustments of its 

 peculiar paddles, but when alarmed it dives into the 

 depths beneath. The larvae of the Gnat, Dixa, Peri- 

 coma, Anopheles, Stratiomys, and Hydrobius suspend 

 themselves from the surface-film by an unwettable 

 basin at the tail-end of their bodies, which admits 

 air to the spiracles, and at the same time allows the 

 head and jaws to sweep through the water in search 

 of food. Some aquatic larvse (Eristalis, for in- 

 stance) may be seen at times to creep upon the 

 underside of the surface-film. The same manoeuvre 

 is practised by other aquatic animals, such as the 

 leech-like Triclads, the Pond-snails, and Cyclas. 

 These have been erroneously said to creep upon the 

 air. A very simple experiment will show that the 

 body is at such times completely immersed. If a 

 little lycopodium powder is sprinkled over the water, 

 the light grains are not parted when the animal creeps 

 beneath them. In some cases, for instance in the 

 Triclads, the body is heavier than water, and sinks in 

 a moment if it is detached from the surface-film. A 

 drop of water let fall upon the creeping animal will 

 send it to the bottom. The Water-boatman (Noto- 

 necta) is buoyant, and comes to rest in an inverted 

 position, with its tail and the extremities of its oar-like 

 legs resting upon the surface. It is then well placed 

 for intercepting drowned or drowning Insects. When- 

 ever it chooses to leave the surface, its oars propel it 

 swiftly through the water, and the air is so distributed 

 throughout its body that it swims most easily in its 

 resting position, that is with its back downwards. 



