A GUIDE TO THE STUDY OF BRITISH WATERBUGS. 149 



under water,* although no special modifications have apparently 

 arisen in the structure of these bugs in relation to an incipient 

 aquatic life. The next link is indicated by a curious and some- 

 what isolated family, the Ochteridae,! which is distinguished from 

 Acanthia at first sight by the greater continuity of curve from 

 head to abdomen in the lateral margins. The rostrum is very 

 similar to that of Acanthia, and the legs are simple, but the 

 antennae are very short, being concealed under the head. Never- 

 theless they are not strictly aquatic, but frequent the banks of 

 rivers, feeding upon small insects, after the manner of some of 

 the species of Acanthia. The nymph lives beneath moist sand. 

 There are unfortunately no British species of the family, but 

 Ochteriis {=^ Pelogotius) marginatas, Latr., is found so near as 

 France, Spain, and Italy. + 



With the NaucoridsB we enter upon the true aquatic forms. 

 Comparatively little adaptation to an aquatic life has been 

 effected even here, but in the higher forms the posterior legs are 

 more natatorial, and the antennae more greatly modified. The 

 exotic Belostomatidse are superficially much like the Naucoridse, 

 but may be at once separated by the peculiarly shaped antennae 

 and more natatorial posterior legs. The Corixidae have un- 

 doubtedly originated from a Naucoroid ancestor, and are all 

 strictly aquatic. The Notonectidae are also Naucoroid, and are 

 perhaps the most specialised of all aquatic insects ; they certainly 

 ought not — as in the arrangements of most authors — to be inter- 

 polated between the Naucoridae and CorixidaB, but should be 

 placed immediately after the latter. 



The Nepidae are — apart from some profound structural differ- 

 ences — readily distinguished by the fact that they respire by 

 means of long filamentary tubes at the anal end of the body,§ 

 and, while the Naucoroidea move the opposite legs together, the 

 Nepoidea move them alternately. The following table will 

 separate the three families into which the Naucoroidea are 

 divided : — 



1. Somewhat flat and rounded ; anterior legs in- 

 serted on the anterior margin of the pro- 

 sternum (fig. 29) 1. Naucoridae. 



la. Oval or elongate oval ; anterior legs inserted 

 on the posterior margin of the presternum 

 (fig. 30) 2. 



* J. E. Mason, E. M. M. xxv. p. 236. 



f Pelogonitlse, auctt. 



X The extra-British Mononychidae {Mononyx, Spin., Gelastocoris, Kirk., 

 &c.) need not be considered here, as, although they are intermediate between 

 the true landbugs and true waterbugs — being riparian and kryptokeratous — 

 they do not indicate any of the intermediate stages of evolution. 



§ The strap-like processes in the Belostomatidae are sexual. 



