894 



FRESH-WATER BIOLOGY 



here again is given evidence of independent adaptation to aquatic 

 Kfe, and nowhere could be found more complete intergradation of 

 habits between terrestrial and shore-loving forms and those that 

 dwell in the water. 



The shore bugs (Acanthiidae) and toad bugs (Pelogonidae) are 

 essentially terrestrial; the marsh treaders (Hydrometridae), water 



Fig. 1364. A giant water bug, Benacus, clinging to a vertical surface under water. 



striders, skaters, etc. (Veliidae and Gerridae), have passed out upon 

 the surface, a few of them having acquired the ability to dive and 

 swim. The Nepidae and Belostomatidae are fairly adapted forms 

 that do not depart far or long from the surface of the water, and 

 only the Corixidae and Notonectidae have acquired very highly 

 specialized apparatus for swimming and for carrying down a copious 

 air supply. 



There are no tracheal gills developed in this order. Nymphs 

 and adults alike must come to the surface for air. They are easily 

 collected by sweeping aquatic vegetation with a dip net. The 

 Corixidae stick more closely to the bottom than do other forms. 



