THE HYDROCORES. 405 



If the Gerrides, like the Whirligig Beetles {Gyrini), 

 select the surface of the water for the scene of their 

 existence, there are not wanting Bugs which, like the 

 true Water-beetles, plunge freely into the depths of 

 their aquatic home, and although none of the British 

 species of these Water-bugs can rival our large Water- 

 beetles in size or power, many of the exotic species 

 must rank as the largest insect inhabitants of the 

 water. Most of these aquatic Bugs, forming the tribe 

 of the Hydrocores"^, are easily distinguished from 

 those members of the present order which we have 

 hitherto had under consideration, by the structure of 

 their hinder and intermediate legs, which are usually 

 elongated, compressed, and fringed with hairs, to 

 render them efficient natatory organs ; but when this 

 is not the case, the antennae furnish another character, 

 which is universal throughout the tribe. These organs, 

 instead of being elongated and exserted, are composed 

 of three or four very short joints, and inserted beneath 

 the eyes so as to be entirely concealed. 



The common Boat-fly (Notonecta glauca), which 

 may be found in every pond, closely resembles the 

 true Water-beetles in many of its habits. It may be 

 seen constantly floating near the surface of the water 

 with its tail higher than its head, and with its long 

 hind-legs, the principal agents of natation, extended 

 straight out on each side of the body, exactly in the 

 manner of a Dyticus ; and its object in this position 

 is also the same, namely, that of taking in a supply 

 of air between its wings and its body to serve for 

 respiration when it descends. There is this difl'erence 

 between the positions of the Bug and the Beetle, 

 however, that whilst the latter swims in what may be 

 * Gr. hiidor, water ; koris, a bug. 



T 5 



