Nepidae is a family of insects in the order Hemiptera, suborder Heteroptera. They are commonly called water scorpions for their superficial resemblance to a scorpion, due to the modification of the legs, of the anterior pair for predation, and to the presence of a long slender process, simulating a tail, at the posterior end of the abdomen. There are eight genera in the family, in two subfamilies, Nepinae and Ranatrinae. Members of the genus Ranatra are sometimes called needle bugs or water stick insects as they are more slender than Nepa and its relatives. The common British species (Nepa cinerea) lives in ponds and stagnant water, and feeds upon aquatic animal organisms principally of the insect kind. Respiration in the adult is effected by means of the caudal process, which consists of a pair of half-tubes capable of being locked together to form a siphon by means of which air is conducted to the tracheae at the apex of the abdomen when the tip of the tube is thrust above the surface of the water. In immature forms the siphon is undeveloped and breathing takes place through six pairs of abdominal spiracles. The eggs, laid in the stems of plants, are supplied with seven filamentous processes which float freely in the water.
In Nepa the body is broad and flat; but in an allied water-bug, Ranatra, which contains a single British species (R. linearis), it is long and narrow, while the legs are very slender and elongate.