Nepidae is a family of exclusively aquatic Heteropteran insects in the order Hemiptera. They are commonly called water scorpions for their superficial resemblance to scorpions, 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. Members of the genus Ranatra, the most widespread and species-rich genus, are sometimes called needle bugs or water stick insects as they are slenderer than Nepa.
While water scorpions do not sting with their tail, they do have a painful bite (strictly speaking a sting by their pointed proboscis), but this is much less harmful to humans than a true scorpion's sting.
Range and habitat
Nepidae are found on all continents except Antarctica. They mostly inhabit stagnant or slow-moving freshwater habitats like ponds, marshes, canals and streams. Exceptionally they have also been recorded from hypersaline lakes and brackish lagoons, the Australian genus Goondnomdanepa is restricted to flowing waters, and Nepa anophthalma is adapted to life in caves in Romania.
Appearance and ecology
Nepidae are brown insects, but some species have a bright red abdomen that can be seen when the wings are open. Their body is broad and flat (subfamily Nepinae) or long and thin (subfamily Ranatrinae). They are rather poor swimmers and typically crawl about on aquatic vegetation. They can fly, but this is infrequently seen. In most species the body is between 1.5 and 4.5 cm (0.6–1.8 in) long, although the largest such as the East Asian Ranatra chinensis and South American R. magna can approach 6 cm (2.4 in). Adults of Ranatra australis, which is found in the southern parts of North America, are three to three and a half inches long. 
Respiration in the adult is achieved by means of the caudal process, which consists of a pair of half-tubes capable of being locked together to form a siphon. Air is conducted through it to the tracheae at the apex of the abdomen when the tip of the tube is thrust above the surface of the water (similar to a snorkel). In some species the siphon is longer than the body, but in others it is shorter, in a few even less than one-tenth of the body length. In immature forms the siphon is often underdeveloped and respiration takes place through six pairs of abdominal spiracles.
Their frontal legs are modified into raptorial appendages that are used to grab their prey. They feed primarily on aquatic invertebrates such as other insects, but occasionally take small fish or tadpoles. 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.
Subfamilies and genera
Among these the most diverse are the widespread Ranatra (about 100 species) and Laccotrephes (about 60), but the family also includes species-poor genera, like the Ethiopian Borborophilus, Nepella, Nepitella and Paranepa (each with one species), Indian Montonepa (one species), Philippine Borborophyes (one species), and Australian Austronepa (one species) and Goondnomdanepa (three species).
Four genera are in this subfamily: Austronepa and Goondnomdanepa are restricted to Australia. Cercotmetus is from Asia to northern Australia and resembles Ranatra (Worldwide distribution), although the former has a distinctly shorter siphon.
- Austronepa Menke & Stange, 1964
- Cercotmetus Amyot & Serville, 1843
- Goondnomdanepa Lansbury, 1974
- Ranatra Fabricius, 1790
- Nepitella (syn. Nepita)
- Eurypterid: unrelated, extinct arthropods that are commonly called sea scorpions
- Nepidae, Tree of life project
- Chisholm 1911.
- I. Lansbury (1974). "A new genus of Nepidae from Australia with a revised classification of the family (Hemiptera: Heteroptera)". Australian Journal of Entomology. 13 (3): 219–227. doi:10.1111/j.1440-6055.1974.tb02176.x.
- Ye.V. Anufriyeva; N.V. Shadrin (2016). "First Record of Ranatra linearis (Hemiptera, Nepidae) in Hypersaline Water Bodies of the Crimea". Hydrobiological Journal. 52 (2): 56–61.
- "Nepidae". The Murray-Darling Freshwater Research Centre. Retrieved 6 April 2018.
- Vasile Decu; Magdalena Gruia; S. L. Keffer; Serban Mircea Sarbu (1994). "Stygobiotic Waterscorpion, Nepa anophthalma, n. sp. (Heteroptera: Nepidae), from a Sulfurous Cave in Romania". Annals of the Entomological Society of America. 87 (6): 755–761. doi:10.1093/aesa/87.6.755.
- Randall T. Schuh; James Alexander Slater (1996). True Bugs of the World (Hemiptera:Heteroptera): Classification and Natural History (2 ed.). Cornell University Press. pp. 114–116. ISBN 978-0801420665.
- P. Chen; N. Nieser; J.Z. Ho (2004). "Review of Chinese Ranatrinae (Hemiptera: Nepidae), with descriptions of four new species of Ranatra Fabricius". Tijdschrift voor Entomologie. 147 (1): 81–102. doi:10.1163/22119434-900000142.
- Heckman, C.W. (2011). Encyclopedia of South American Aquatic Insects: Hemiptera - Heteroptera. Springer. ISBN 978-94-007-0704-7.
- Taryn B. Griffith and Jennifer L. Gillett-Kaufman, "The southern water scorpion Ranatra, fresh water and saline water." University of Florida. (2020). Online at http://entnemdept.ufl.edu/creatures/AQUATIC/Ranatra.html#:~:text=The%20southern%20water%20scorpion%2C%20Ranatra,fresh%20water%20and%20saline%20water.
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Water-scorpion". Encyclopædia Britannica. 28 (11th ed.). Cambridge University Press. p. 386.
- S.L. Keffer; J.T. Polhemus; J.E. McPherson (1989). "Notes on Critical Character States in Telmatotrephes (Heteroptera: Nepidae)". Florida Entomologist. 72 (4): 626–629.
- Polhemus, John T.; Keffer, Steven L. (Spring 1999). "Notes on the Genus Laccotrephes Stål (Heteroptera: Nepidae) in the Malay Archipelago, with the Description of Two New Species". Journal of the New York Entomological Society. 107 (1): 1–13. JSTOR 25010286.
- Lansbury, I. (1974). "Montonepa gen.n. from India with notes on the genus Borborophyes Stål (Hemiptera-Heteroptera, Nepidae)". Zoologica Scripta. 2 (2–3): 111–118. doi:10.1111/j.1463-6409.1974.tb00742.x.
- Lansbury, I. (1978). "A review of Goondnomdanepa Lansbury (Heteroptera: Nepidae)". Australian journal of marine and freshwater research. 29 (1): 117–126. doi:10.1071/MF9780117.
- Media related to Nepidae at Wikimedia Commons
- Data related to Nepidae at Wikispecies
- Water Scorpions
- Hemiptera: Nepidae
- Insects of Britain and Ireland: the families of Hemiptera