LIFE-HISTORY OF THE GNAT 249 



250 to 300 eggs. Each egg is cigar-shaped, with the 

 narrow end turned upwards, floating with the tip 

 projecting through the surface-film into the air. By 

 this means the float is made self-righting, for if sub- 

 merged it carries down between the pointed egg-tips 

 a bubble of air and almost instantly rises again to 

 the surface perfectly dry. The * surface-film ' is an ex- 

 pression of the elastic property of the surface particles 

 of water. The tenseness of this film is readily seen 

 in soap-bubbles, and the use of it by insects has led 

 to many biological adaptations. In the present case 

 the hold which the eggs gain on the film provides 

 them with a constant supply of air. 



The next day the larvae hatch out. They differ 

 from the early stages of the insects so far considered 

 in having no limbs. Each larva is a minute, colourless, 

 worm-like organism, consisting of a small head pro- 

 vided with biting mouth-parts, a swollen thorax of 

 three segments, and a segmented abdomen, at the 

 tip of which are some peculiar processes. The be- 

 haviour of the gnat-larva is characteristic. When at 

 rest it hangs head downwards from the surface-film. 

 From time to time it releases its hold and descends 

 passively, then wriggles again to the surface. This 

 swimming movement is performed by bending the 

 body into double and reversed curves, and progresses 

 tail foremost (fig. 52). 



The larva gains a hold on the surface-film by 

 means of a peculiar projection the siphon attached 

 to its tail. This siphon is a tube opening by fine flaps 



