Hydrochus

Hydrochus is the only living genus of beetle in the family Hydrochidae, which belongs to the superfamily Hydrophiloidea, and was formerly treated as a subfamily of Hydrophilidae. Hydrochus includes about 180 species, which are found worldwide. The name "Hydrochus" has also been used for a fly genus in the family Dolichopodidae, but this is a junior subjective synonym of the genus Rhaphium.
They are found in slow moving streams or stagnant water bodies, where they are associated with dense vegetation. In adults air is stored in a bubble on the underside of the body, with the antennae used to transfer atmospheric air to the bubble. The larvae live at the bottom of water bodies, indicating that they can breathe underwater. The diet is only known for the larvae of one species, H. japonicus, which feed on annelid worms belonging the family Naididae. The larvae also burrow into crevices, such as hollow dead grass, including to pupate. The adults are sluggish crawlers along surfaces, and are thought to consume algae and plant detritus. Eggs are laid in silk cases, often attached to substrates.
The club of the seven segmented antennae consists of three segments with a cup-like basal segment. The number of tarsi on the legs are usually 5-5-5 or 4-4-4. The pronotum narrows towards the rear but is narrower than the base of the elytra. An African species was formerly described under the genus Kiransus, but along with several other African species, it is now placed in Hydrochus. Hydrochids are considered to be an older branch of the Hydrophiloidea. Hydrochids have been suggested as a sister group of the Spercheidae and Hydrophilidae, but the relationships are unclear. Molecular phylogenetic comparisons suggest that they are related as a sister group to Helophorus and Georissus.
The oldest fossil of the modern genus is Hydrochus relictus from the Green River Formation of North America, dating to the Eocene. An extinct genus of the family, Ponohydrochus, is known from the Early Cretaceous Khasurty locality in Russia.

Selected species

More than 180 species have been described in the genus. The following list includes some of the species in the genus:Hydrochus aljibensis Castro & Delgado, 1999^gHydrochus angusi Valladares, 1988^gHydrochus angustatus Germar, 1824^gHydrochus argutissimus Hydrochus aschnaae Hydrochus basilaris Hydrochus bicarinatus Hydrochus bituberculatus Hydrochus brevis ^gHydrochus brevitarsis Knisch, 1922^{i c g}Hydrochus brianbrowni Makhan, 2005^{i c g}Hydrochus callosus LeConte, 1855^{i c g}Hydrochus collaris Hydrochus conjunctus Hydrochus corruscans Hydrochus crenatus ^gHydrochus currani Brown, 1929^{i c g b}Hydrochus debilis Sharp, 1882^{i c g}Hydrochus denarius Hydrochus drakei Knisch, 1921^gHydrochus elongatus ^gHydrochus ensifer Hydrochus excavatus LeConte, 1855^{i c g b}Hydrochus falsus Hellman in Worthington, Hellman, and Lago 2016Hydrochus farsicus^gHydrochus flavipennis Küster, 1852^gHydrochus formosus Hydrochus foveatus Haldeman, 1852^{i c g}Hydrochus grandicollis Kiesenwetter, 1870^gHydrochus granulatus Blatchley, 1910^{i c g b}Hydrochus hellmani Hydrochus ibericus Valladares, Diaz & Delgado, 1999^gHydrochus ignicollis Motschulsky, 1860^gHydrochus inaequalis LeConte, 1855Hydrochus jaechi Makhan, 1995^{i c g}Hydrochus japonicus Sharp, 1873^gHydrochus jiawanae Makhan, 1996^{i c g}Hydrochus kellymilleri Hydrochus kirgisicus Motschulsky, 1860^gHydrochus lachmoni Hydrochus leei Hydrochus lobatus Hydrochus mauriciogarciai Hydrochus megaphallus van Berge Henegouwen, 1988^gHydrochus minimus Blatchley, 1919^{i c g}Hydrochus neosquamifer Smetana, 1988^{i c g}Hydrochus nigeriensis Hydrochus niloticus Hydrochus nitidicollis Mulsant, 1844^gHydrochus nodulifer Hydrochus nooreinus Berge Henegouwen & Sainz-Cantero, 1992^gHydrochus obscurus Sharp, 1882^{i c g}Hydrochus octocarinatus Hochhuth, 1871^gHydrochus pajnii Makhan, 2000^{i c g}Hydrochus pallipes Chevrolat, 1863^{i c g}Hydrochus pictus Hydrochus pseudosecretus Hydrochus pseudosquamifer Miller, 1965^{i c g b}Hydrochus pupillus Orchymont, 1939^gHydrochus ramcharani Hydrochus roomylae Makhan, 2001^{i c g}Hydrochus rishwani Hydrochus rufipes Melsheimer, 1844^{i c g b}Hydrochus rugosus Mulsant, 1844^{i c g b}Hydrochus sagittarius Hydrochus scabratus Mulsant, 1844^{i c g b}Hydrochus schereri Makhan, 1995^{i c g}Hydrochus setosus Leech, 1948^{i c g}Hydrochus shorti Hydrochus simplex Leconte, 1851^{i c g b}Hydrochus soekhnandanae Hydrochus smaragdineus Fairmaire, 1879^gHydrochus spangleri Hellman in Steiner, Staines, McCann and Hellman, 2003^{i c g}Hydrochus squamifer LeConte, 1855^{i c g b}Hydrochus subcupreus Randall, 1838^{i c g b}Hydrochus tariqi Ribera, Hernando & Aguilera, 1999^gHydrochus tarsalis Chevrolat, 1863^{i c g}Hydrochus vagus LeConte, 1852^{i c g}Hydrochus tuberculatus Hydrochus variabiloides Hydrochus variolatus LeConte, 1851^{i c g}Hydrochus yadavi Makhan, 2000^{i c g}
Data sources: i = ITIS, c = Catalogue of Life, g = GBIF, b = Bugguide.net