75 
CHAPTER X. 
THE HYDRADEPHAGA, OR AQUATIC CARNIVOROUS 
BEETLES. 
Sub-section 2. Hydradephaga, M'Leay. 
The members of this sub-section, the predaceous 
Water-beetles, are the aquatic representatives of the 
Geodephaga, exhibiting, 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 conspicuously separate, but for the 
most part fitting into each other closely. The eyes 
do not project ; the antennas are mostly slender, and, 
with 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 ” movement 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 sur- 
face for the attachment of the propelling muscles 
(Fig. 10, dd ; p. 35). 
The mentum is emarginate, and, in nearly all the 
species, strongly toothed in the middle; the ligula 
