176 HYDROMETKIDJE. 



Division GEBIUNARIA. 

 Subfam. Gerridae, Bianchi, Annuaire Mus. St. Petersb. 1896, p. 70. 



Body comparatively long and narrow ; length to breadth being 

 at least 4 : 1. These are all freshwater insects and are widely 

 distributed. 



Synopsis of Genera. 



A. Aotei-ior tarsi short, not nearly so long as 

 tibiie. 



a. E^es large, prominently exserted. 



a. Claws inserted before apex of tarsus . Gerris, p. 17r>. 



b. Claws inserted at apex of tarsus .... Eotrechus, p. 182. 



b. Eyes not exserted, ad pressed to head. 

 a'. Intermediate and posterior legs each 



more than twice as long as body; 



rostrum reaching middle of nieso- 



sternum Onychotrechus, p. 182. 



b'. Body longer than half the length 



of either intermediate or posterior 



legs ; rostrum scarcely or about [p. 184. 



reaching anterior coxse Cylindrostethus, 



15. Anterior tarsi nearly or about as long as 

 the tibias ; intermediate femora fringed 



with long hairs Ptilomera, p. 185. 



'-^^ 



Genus GERRIS. 



Gerris, Fabr. Ent. Syst. iv, p. 187 (1794) ; Ami/. Sr Serv. Hem. p. 414 



(184;5) ; Saund. Hem. Het. Brit. Islds. p. 151 (1892). 

 Ily drome tra, Fabr. Syst. BJujny. p. 256 (1803); Fieber, Fur. Hem. 



pp. 33 & 106 (1861). 

 Aquarius, Schellenh. Land- u. Wasserw. p. 25 (1800). 

 Subg. Tenagogonus, StSl, Ofv. Vet.-Ak. Fiirh. x, p. 263 (1853). 



,, Limnometra, Mayr, Reise Novara, Hein.Tp. 174 (1866). 



,, Lininogonus, Stal, Hem. Fabr. i, p. 132 (1868). 



., Limnotrechus, Limnoporus, and Hvgrotrechus, Stal, Ofv. Vet.- 

 Ak. Fiirh. 1868, pp. .•.96, 397. 



„ Lamprotrechus, Rent. Ofv. Finska Vet. Soc. Fork, xxv, p. 40 

 (1882). 



Type, 6r. lacusfris, Linn., a Palajarctic species. 



Distribution. Universally distributed. 



Head short, eyes large and prominent, ocelli absent ; antennae 

 with the first joint long, sometimes very long ; rostrum short ; 

 pronotum elongate, posteriorly concavely produced, constricted 

 anteriorly ; hemelytra (sometimes absent) \vith the veins very 

 prominent ; anterior coxaj consideralily removed from the inter- 

 mediate pair, intermediate and posterior coxte situate close 

 together ; intermediate and posterior femora and tibiae long, their 

 proportions variable in different species ; tarsi t\\ o-jointed ; claws 

 emerging before apex. 



