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 larve 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 Ephemeridz, 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 
