THE FRESH WATERS 179 



jecting from the last ring of their bodies, and 

 may be seen hanging head downwards from 

 the surface so that air may enter. There are 

 other aquatic larvae which never even get wet. 

 This is difficult to understand, because it re- 

 quires a knowledge of physical properties, but 

 it is a fact, and it is illustrated in a very varied 

 way among animals that have gone back from 

 the dry land to the water. Some water-beetles 

 can hardly become wet at all ; some keep the 

 greater part of the body dry, but not it all ; 

 some become quite wet. The little whirligig 

 beetle (Gyrinus) and the Water Boatmen 

 (Notonecta) become very slightly wetted. 

 The water-spider remains dry over a consid- 

 erable part of the hairy body. 



The time spent in the water is often very 

 long compared with the aerial life. Some of 

 the caddis-flies are said to spend three years 

 in the water, and then only to live a few days. 

 And some aerial lives are shorter still. Some 

 of the May-flies or Ephemeridae, as they are 

 called, from the shortness of their lives, live 

 only a few hours as winged insects in the air. 

 But their larval life in the water lasts for two 

 or three years, during which they feed, grow, 

 and cast their husk many times. At length 



