British Ilemiptera-Heteroptcra. 637 



HYDR ODR OMl CA. 



I. Head very long and narrow, widening in front . . . . Hydrometra. 



II. Head short. 



A. Antennae four-jointed. 



B. Rostrum four-jointed Gerris. 



BB. Rostrum three-jointed. 



C. All the tarsi two-jointed Microvelia. 



CC. At least the two posterior pairs of tarsi three- 

 jointed. 



a, Antennre short, not nearly as long as the 



body; thighs without long projecting hairs Velia. 



b. Antennae long, nearly as long as the body; 



thighs with long projecting hairs . . . . Meitovelia. 



A A. AntenniB with more than four joints .. .. Ilebrus. 



Hydrometra. . 

 1. stagnorum, De Geer. Fig. Dougl. & Scott, Brit. Hem. 

 pi. xix. fig. 7. 

 Very elongate, black ; more or less rufescent about the 

 thorax; legs very long and thin, testaceous. Antennas 

 inserted almost at the apex of the head. 

 Length 5 — 6 lines. 



Found walking about on the surface of water ; common. 

 Its peculiar shape and habit distinguish it from all its 



allies. 



Gerris. 



I. Sixth segment of the abdomen produced at each side 



into a more or less elongate spine. 

 A, More or less feiTuginous; first joint of the an- 

 tennae shorter than the second and third 



together rvfoscutellata. 



AA. Black; first joint of the antennae longer than the 

 second and third together. 



a. Posterior tarsi about one-third as long as 



the tibiaj ; second joint not half as long 



as the first paludum. 



b. Posterior tarsi not one-fourth as long as 



the tibiae; second joint more than half 



as long as the first Najas. 



II. Sixth segment of the abdomen produced and broadly 



dentate at each side, but not spinose. 



A. Thorax more or less rufescent in parts. 



B. Larger and more robust ; very wide across the 



region of the intermediate coxae . . . • costce. 



BB, Smaller and narrower, not very mde across the 

 region of the intermediate coxa}. 



a. Longer; first joint of posterior tarsi almost 



twice the length of the second . . . . thoracica. 



b. Shorter; first joint of posterior tarsi not 



nearly twice the length of the second . . aspera. 



