THE A DA P TA TIONS OF A Q UA TIC INSECTS. 735 



decidedly heavier than the water. How is it then able to remain suspended at 

 the surface ? To explain this it will be necessary to say something about the 

 surface film. By means of this surface film other aquatic insects are enabled 

 to run along the surface of the water (as certain kinds of Gerris). The surface 

 of water is covered with an invisible film and it requires a certain amount of 

 mechanical force to break through this. A simple experiment may be made 

 to demonstrate this surface film. Gently lower a needle on the surface of still 

 water and it will float, but, by applying just sufficient pressure to force one 

 end of the needle through the film it at once sinks. Hence the larva of a gnat 

 is enabled to suspend itself tail upwards to the surface film by means of its 

 respiratory syphon which has fine flaps open or closed at will by means of 

 certain muscles. When these flaps are open they form a hollow which does not 

 allow the surface film to enter. As the larva rises to the surface the ends 

 of the flaps come in contact with the surface film and adhere to it. 

 The muscles then, draw apart the flaps and the hollow formed is expanded 

 and filled with air. The surface film now is pulling the edges of the hollow 

 and this is sufficient to counterbalance the greater density of the body of the 

 larva, which hangs without effort from the surface (Miall). 



When the larva wishes to descend the valves are closed their tips being 

 brought to a point and the. resisting pull of the surface film is reduced to an un- 

 important amount. I once watched the movements of a number of whirligig 

 beetles (Gyrhius^) in their whirling dance which was kept up at intervals 

 of five minutes They then rested in a conglomerated mass frequently head 

 to head, any stray member outside the mass always sooner or later being 

 attracted to it without any movement on the part of the insect whatever. 

 This was no doubt due to the contraction of the surface film. The respira- 

 tion effected by the pupa of a gnat is extremely interesting and exactly the 

 reverse to that of the larva. The pupa of a gnat unlike many other species is 

 active and not quiescent and always remains suspended to the surface film 

 head uppermost. The head end is composed of the wings, legs and antennse 

 which can all be seen folded up in their place by aid of a microscope, at the 

 tail end are a pair of swimming flaps. Like the larva the pupa is supplied 

 with external air and obtains this by means of a pair of tubes behind the head, 

 the tops of which always remain flushed with the water. When the pupa 

 descends below the surface water cannot enter the tubes on account of certain 

 hairs found on their inner walls. When the pupa rises from below the tubes 

 are covered by a film of water which bursts when the surface is reached. The 

 inversion of the organs of respiration in larva to pupa is explained when the 

 time of emergence of the perfect insect takes place. Gnats m their perfect 

 state avoid getting wet as much as possible and the slightest immersion would 

 be fatal to them. The fly escapes from the pupa skin through a slit in the upper 

 portion of the thorax and emerges high and dry without touching the water on 

 the back of the cast pupa skin. The larva of a dragon fly always creeps along 



• Gyrinidse are abundant in India, the commonest being Dineutes iiulicus (Lefroy iu lit). 



