Nepidae is a family of exclusively aquatic Heteropteran insects in the order Hemiptera. They are commonly called waterscorpions for their superficial resemblance to scorpions, which is due to their raptorial forelegs and the presence of a long slender process at the posterior end of the abdomen, resembling a tail. There are 14 genera in the family, in two subfamilies, Nepinae and Ranatrinae, and they can be found on all continents except Antarctica. Members of the genus Ranatra, the most widespread and speciose genus, are sometimes calledneedle bugs or water stick insects as they are more slender than Nepa and feed primarily on invertebrates, but occasionally take small fish or tadpoles. 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 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 often underdeveloped and respiration takes place through six pairs of abdominal spiracles. The eggs, which are laid above the waterline in mud, decomposing vegetation, the stems of plants or rotting wood, are supplied with air by filamentous processes which vary in number among the genera.
Species in the subfamily Nepinae tend to have a broad, flat body measuring over two inches in length, whereas those in the subfamily Ranatrinae tend to have a long and narrow body, with similarly long and slender legs.
(From Wikipedia, February 2015)