XI THE WATER SPRING-TAIL 369 



and after a considerable time were seen to sink. But 

 where water-plants with floating leaves grow, the 

 Podura^ can descend at pleasure. They will not 

 attempt to do so if they are well off at the surface ; 

 but when food is scarce, or the temperature low, they 

 may occasionally be seen to drag themselves through 

 the surface-film by grasping the stem with their 

 strong and hooked legs, and so descending. When 

 the Podura climbs downwards into the water it is 

 still unwetted, and enclosed by a film of air which 

 clings to its hairy body. It can remain for many 

 hours submerged without inconvenience. 



Like some aquatic Beetles the Podura probably 

 buries itself for weeks together in the mud at the 

 bottom of ponds, and thus escapes the severity of 

 winter cold. 



The small, metallic-coloured, carnivorous Flies of 

 the family Dolichopodidae commonly seek their prey 

 in the neighbourhood of streams and pools. Hence 

 the naturalist In search of aquatic Insects cannot fail 

 to find them almost daily and hourly, sometimes in 

 swarms, sometimes singly. They come to rest on the 

 grasses, herbs, or bushes near to water, on stones in 

 the bed of a stream, or even on the surface of the 

 water itself. Some rival the Pond-skaters in the 

 agility with which they dart to and fro upon the 

 surface of rapid streams ; others hover incessantly in 

 the spray of waterfalls. At least one species of 

 Dolichopodidae preys upon Podura aquatica, and it 

 is probable that many flies fresh-hatched from a 

 variety of aquatic pupae fall victims to these swift 

 and destructive enemies. 



B B 



