Aquatic Insects 119 



which do not exceed half a millimetre in 

 length. They possess wings fringed with 

 hair and are able to descend into the 

 water in search of the eggs of the ' ' boat- 

 men" and dragon-flies in which to lay 

 their own eggs. They are, in fact, egg 

 parasites. The very minute larva, on 

 hatching, proceeds to devour the con- 

 tents of its host egg and then pupates 

 within it. 



There is another and somewhat larger 

 parasite that selects the caddis-fly larva 

 as its host. The adult insect crawls down 

 some plant stem into the water and 

 having found an unsuspecting caddis- 

 worm lays an egg upon it. The parasitic 

 larva feeds on its host, but is careful not 

 to injure the vital organs until it itself 

 is ready to pupate, which it does within 

 the caddis's case. 



The exploiters of the surface are not 

 nearly as numerous in species as the 

 under-water dwellers ; but they are none 



