a genus of hemipterons of the family Nepidae (water scorpions). The body is elongate and almost cylindrical, and the abdomen ends in a long respiratory tube. The approximately 30 species are distributed throughout the world. Three species are encountered in the USSR. R. linearis, which has a length of 30–35 mm (including the respiratory tube, up to 65 mm), resembles a twig in shape and coloration. The species is found throughout the European USSR in stagnant waters. R.linearis is a predator, feeding on small aquatic animals, mainly insects. Two other species— R. chinensis and R. unicolor —are found in the Soviet Far East.