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CHAPTER X. 



THE HYDEADEPHAGA, OE AQUATIC CAENIVOEOUS 

 BEETLES. 



Subsection 2. Hydradephaga, M'Leay. 



The members of this subsection, the predaceous Water- 

 beetles, are the aquatic representatives of the Geodephaga, 

 exhibitini^, in many respects, a similarity of structure, 

 with modifications adapted to the change of element. 

 Their body is nearly always smooth, depressed or ovate, 

 with a continuous outline ; the parts not being conspi- 

 cuously separate, but for the most part fitting into each 

 other closely. The eyes do not project; the antennae 

 are mostly slender, and, Avith the front and middle legs 

 (which are close to each other), pack securely against 

 the body. The hinder legs are removed from the middle 

 pair, so as to allow of an extended " fore and aft'' move- 

 ment in rowing; they are, in nearly all, strong, broad, 

 and shallow — their width being in a transverse direction, 

 — and strongly fringed with hairs on the inner side. 

 The hinder coxae are much enlarged, and soldered to the 

 metasternum, thus allowing a greater internal surface 

 for the attachment of the propelling muscles (Fig. 10, 

 dd; p. 27). 



The mentum is emarginate, and, in nearly all the 



