558 HYDROMETRINA. 



the centre of the disk mostly with a longitudinal keel, which ceases 

 before the apex of the scutellar process. Elytra (sometimes entirely 

 wanting) flat, nearly or quite as long as the abdomen, lying within 

 the connexivum ; nerves strong, touching in places, and thus forming 

 5 long, narrow, closed cells, which do not reach the margins ; the 

 2nd cell projecting beyond the others. Sternum with a shallow 

 longitudinal channel, or convex, the sides next the pronotum sepa- 

 rated from it by a deep longitudinal channel, so that the sides of the 

 pronotum, although it is narrower than the sternum, are visible from 

 beneath ; Prosternum short, with a short rostral channel ; Meso- 

 sternum very long, so that the 1st and 2nd pairs of legs are very far 

 apart ; Metasternum shorter. Legs ; 1st pair shortest and stoutest, 

 the ihiglis somewhat flattened, the tibfa thickened to the apex, the 

 sides channeled ; 2nd and 3rd pairs long, sub-cylindrical, flattened 

 beneath; 2nd mostly longest ; coxa; 1st pair inserted beneath the 

 prosternum ; 2nd and 3rd pairs cylindrical, inserted in a horizontal, 

 semi-conical projection on the sides of the meso- and metasternum j 

 tarsi 2-jointed ; 1st pair stout, short ; 2nd and 3rd pairs very long, 

 filiform ; claws very short, apex of the 2nd tarsal joint covering their 

 insertion. 



Abdomen above flat, beneath very convex ; on the sides of each 

 segment, within the connexivum, a fine, impressed, divisional line ; 

 Connexivum wide, reflexed ; on the 6th segment produced posteriorly 

 into a long and sharp, or triangular point, whereby the posterior 

 margin of the segment becomes concave. Genital segments ; in the 

 $ 3 ; the 1st oblique, annular ; the 2nd visible only beneath ; the 

 3rd forms a conical or rounded apex ; in the $ generally only 2 

 (2nd and 3rd) are visible above and beneath ; the 1st of them quad- 

 rangular or reversed trapeziform, cleft in the middle but not parted ; 

 the 2nd like the last segment in the $ . 



The Hydrometrce live on the surface of running or stagnant water, 

 where they propel themselves rapidly by the rowing motion of their 

 2nd and 3rd pairs of legs, feeding upon any insects that may come 

 in their way, catching them by springing upon them. They can 

 also dive when alarmed. 



De Geer has given their natural history in the 3rd volume of hi 

 ' Memoires.' 



In apterous examples the scutellar process is shorter and less 

 angulated. 



