140 NATURAL ARRANGKMENT OF INSKCTS. 



they will continue immovable for several minutes^ with 

 their hinder legs stretched out^ in the same attitude which 

 a boat's crew assume when ready for a sudden start. 

 The Notonectidce, like the Nepidee among the bugs, are 

 rapacious, — living upon small insects, whose juices they 

 suck. The attitude we have just described, although 

 apparently one of listlessness, is really that of extreme 

 watchfulness : the insect is lying in wait for its prey ; 

 and the moment a luckless wanderer passes within reach, 

 it darts upon it with the utmost rapidity, and is out of 

 sight in a moment. Another singular habit belongs to 

 these insects : they not only swim, but repose, upon 

 their backs ; that is, with the under side of their body 

 turned uppermost : this is a wise provision of Nature, 

 and admirably suited to the nature of their pursuits ; — 

 by resting in this position, their eyes are directed fully 

 to that quarter where they are to look for their prey ; 

 while, if they assumed the usual position of other insects, 

 they would gaze upon objects either upon the surface, 

 or above it, 



(128.) Let us now look to the analogies which these 

 insects possess to other classes of beings. The Noto- 

 nectidce are clearly an aquatic type, and offer an imme- 

 diate point of union with the typical Hemiptera; their 

 close affinity with the NepadcB is admitted by all writers ; 

 and we have frequently observed, both among annulose 

 and vertebrate animals, that one of the transitions of 

 nature is made by an aquatic group. All natatori;il 

 types, among birds, represent the shrikes and the fly- 

 catchers, who have the peculiar habit of watching for 

 their prey from a fixed station, and seizing it, not by 

 pursuit, but by surprise, just as the insects now be- 

 fore us. Natatorial types, moreover, have always the 

 largest heads ; witness the hippopotamus among the 

 pachydermatous quadrupeds, the whale among theCeta- 

 cea, and the HesperidcB in the circle of diurnal butterflits. 



(129-) In regard to the remaining family of the pre- 

 sent tribe, we can say but little. If we merely looked 

 to those genera which showed the greatest deviation 



