THE STUDY OF FRESH-WATER ANIMALS. 191 



The creatures walking or running upon the surface film 

 should be noted, and the peculiarities enabling them to do 

 so should be carefully studied. Whirligig beetles have 

 the two hind pairs of legs very broad and fringed, by 

 which means they paddle themselves both upon the 

 surface and below it. The fore legs are used more parti- 

 cularly for prehension and holding prey, etc. Pond snails 

 and flat worms creep upon the under side of the surface 

 film. Pond skaters skate or jump upon it. 



The properties of the film, enabling these animals to use 

 it as they do, should be explained to the pupils. 



Other adaptations which might be investigated are those 

 connected with feeding, e.g. note the perforated sucking 

 jaws of the water-beetle larva, the knife blade-like fore 

 limbs of the water scorpion, the water- sweeping " brush " 

 of the gnat, etc. 



