HYDROPORINAE Aubé, 1836
With more than 2200 described species this is the largest subfamily of the Dytiscidae, it is usually divided into 10 tribes although some of these are tenuous and some have variously been ranked as subfamilies and others have been proposed and will be found in the literature, it is cosmopolitan in distribution and northern temperate regions have large and diverse faunas. Laccornellini Miller, K.B. & Bergsten, 2014 includes 2 species of a single Neotropical genus and about 40 species of a single genus from South Africa and Madagascar. Laccornini Wolfe & Roughly, 1990 includes 10 species of the single genus Laccornis Gozis, 1914, it is Holarctic in distribution and most diverse in North America; 2 species occur in Europe of which the widespread L. oblongus (Stephens, 1835) extends to the UK. Methlini Branden, 1885 includes 2 genera, one is widespread in the New World but most diverse in the Neotropics, and the African Methles Sharp, 1882 which includes 8 species, 2 of which extend north into southern Europe but have not been recorded from the UK. Pachydrini Biström, Nilsson & Wewalka, 1997 includes 2 genera, one from the New World and primarily Neotropical and one from Africa. Vatellini Sharp, 1880 includes 2 genera, one primarily Neotropical and one primarily African, none have been recorded from Europe. Hyphydrini Sharp, 1882 includes 14 genera and is almost cosmopolitan in distribution although this is due mostly to Hyphydrus Illiger, 1802, a large genus of about 140 species which occur throughout the Old World; it is the only genus to occur in Europe and of the 4 European species the very widespread H. ovatus (Linnaeus, 1761) extends to the UK. Other genera tend to be either small and restricted in distribution e.g. Hovahydrus Biström, 1982 with 4 species from Madagascar, or Hydropeplus Sharp, 1882 with 2 species from South Africa, or larger and confined to larger regions e.g. Microdytes J. Balfour-Browne, 1946 with about 50 species from the eastern Palaearctic and south east Asia, or Desmopachria Babington, 1842 with about 120 species spread throughout Central and South America. The group also includes 5 monotypic genera, 4 from South Africa and one from Japan. Hygrotini Portevin, 1929 includes 2 genera although some subgenera of Hygrotus Stephens, 1828 are often quoted as full genera. Clemnius Villastrigo, Ribera, Manuel, Millán & Fery, 2017, with 8 species, is primarily Nearctic but the European (and British) species Hygrotus decoratus (Gyllenhal, 1810) is now included. The Holarctic genus Hygrotus includes about 130 species; 24 species occur in Europe
and 8 of these extend to the UK. Hydrovatini Sharp, 1880 includes 2 genera; Queda Sharp, 1882 with 3 Neotropical species, and Hydrovatus Motschulsky, 1853 with more than 200 species which occur throughout the world but are most diverse in tropical and subtropical regions; 8 species occur in North America and only 2 species in Europe, both of which are recorded from the UK. Bidessini Sharp, 1880 is a very speciose group of 48 genera, it is cosmopolitan but mostly tropical and subtropical in distribution and temperate regions are only poorly represented; 8 genera occur in North America and only 19 species of 3 genera occur in Europe. Bidessus Sharp, 1882 is a widespread Eurasian and African genus of 50 species, 14 of which occur in Europe, Hydroglyphus Motschulsky, 1853 includes 90 species and occurs throughout the Old World but only one species occurs in Europe, and Yola Gozis, 1886 is a mainly African genus of 50 species but one, Y. bicarinata (Latreille, 1804), is widespread in western Europe although not recorded from the UK. The UK fauna includes 3 very widespread European species, Bidessus minutissimus (Germar, 1823), B. unistriatus (Goeze, 1777) and Hydroglyphus geminus (Fabricius, 1792). Hydroporini Aubé, 1836 is also very speciose, it includes 38 genera in 4 subtribes and is cosmopolitan with the greatest diversity is in northern temperate regions; the European fauna includes about 180 species of 15 genera, and New World faunas tend to be small by comparison. Sternopriscina Branden, 1885 is a speciose group of 11 genera confined mostly to Australia but with a few species also from New Zealand. The other subtribes are more generally northern in distributed and all are represented in Europe. Siettitiina Smrẑ, 1982 includes 11 genera and is mostly Western Palaearctic in distribution, the UK fauna includes 6 species of 3 genera; Graptodytes Seidlitz, 1887 (4 spp., 22 total and 14 from Europe), Porhydrus Guignot, 1945 (1 sp., 4 total, all in Europe) and Stictonectes Brinck, 1943 (1 sp., 12 total and 8 from Europe.) Hydroporina Aubé, 1836 includes 7 genera and is Holarctic in distribution. The group is well represented in Europe and the UK. A single species of Hydrocolus Roughly & Larson, 2000 occurs in the far north of Europe but is absent from the UK, the genus otherwise includes 11 Nearctic species. The Holarctic genus Hydroporus Clairville, 1806 includes 80 European species (about 190 species in total and the only member of the subtribe to occur in Asia) of which 30 extend to the UK. The species often listed as Suphrodytes dorsalis (Fabricius, 1787) and S. figuratus (Gyllenhal, 1826) are now included in Hydroporus. The remaining genera are confined to the New World with only a few species occurring in Central and South America. Deronectina Galewski, 1994 includes 8 genera and is Holarctic and African with the greatest diversity in the Palaearctic region. Stictotarsus Zimmermann, 1919 is Holarctic with a few species extending into Central and South America; it includes 18 species of which 7 occur in Europe and one extends into the UK. Scarodytes Gozis, 1914 is a Western Palaearctic genus of 11 species of which the widespread S. halensis (Fabricius, 1787) occurs in the UK. Of the 30 species of the Holarctic genus Oreodytes Seidlitz, 1887, 5 occur in Europe and 4 of these extend to the UK. Nebrioporus Régimbart, 1906 is a large Genus of about 60 species, it is primarily Palaearctic with a few species occurring in North America and tropical Africa; 22 species are known from Europe and 4 of these extend north into the UK. Deronectes Sharp, 1882 is a Palaearctic genus of about 60 species, 22 occur in Europe but only one, D. latus (Stephens, 1829) is known from the UK.
UK species are of a characteristic small size (<6mm) and most are flattened and elongate-oval in shape, they are mostly drab brown or black, often with definite or vague paler markings, but a few e.g. among Oreodytes and Graptodytes are strikingly coloured. They are distinguished among our diving beetle fauna by the scutellum being hidden beneath the posterior margin of the pronotum and the pseudotetramerous front and middle tarsi, the fourth segment being tiny and hidden within the third segment, the hind tarsi slender and lacking lobes (c.f. Laccophilinae) and the prosternal process extending back to the mesocoxal cavity. The dorsal and ventral margins of the hind tibiae and tarsi bear swimming hairs in both sexes. Most are very typical diving beetles; long-oval and continuous or discontinuous in outline, flattened or (e.g. in Hydrovatus or Hyphydrus) convex or even globose, with the head variously retracted into the thorax, weakly convex eyes that follow the outline and slender filiform antennae. The pronotum is transverse, usually has distinct angles and, apart from various weak impressions or series of punctures, usually lacks sculpture, the prosternum, and more particularly the form of prosternal process, is often a valuable aid to identification. The elytra are usually finely and extensively punctured and lack distinct striae although longitudinal puncture series or ridges are sometimes present. Many are extensively glabrous but many have fine dorsal pubescence, dorsal microsculpture is usually present and the form and distribution of this can be a valuable aid to identification. The front and middle legs vary but are usually shorter and broader overall than the hind legs, and the front and middle tarsi often have expanded segments, the extent of which may be sexually dimorphic. This is a rather simple description of the group based on the UK fauna but much more variation is seen in foreign species e.g. the group includes many cave dwelling forms that lack pigment and eyes and have long appendages that are not adapted for swimming. The 6 UK tribes are readily separated on morphological features; they fall very conveniently into two groups according to whether the epipleura have a raised oblique line just before the humeral angle. Tribes with this line include Bidessini, identified by short impressed furrows that align at the base of the pronotum and elytra, and Hydroporini, which have the prosternal process unbordered, and Laccornini, in which the apex of the process is bordered. Tribes lacking the raised epipleural line include Hyphydrini, in which the front claws are unequal in length, and Hydrovatini and Hygrotini in which they are more-or-less equal; in Hygrotini the elytral apex is rounded or acuminate while in Hydrovatini it is mucronate. This is a very simple outline but the groups will soon become familiar with a little experience; the patterned species can be picture matched and the size and shape in many cases are very distinctive. Having said this, identification at the specific level, especially in Hydroporus, can be very challenging and will sometimes require dissection but fortunately there are several very good and lucid keys to the group available.
This subfamily includes some of our most abundant and widespread water beetles, at least some can be sampled throughout the year and most wetland habitats will produce a range of species; many are most common in spring and autumn when they may occur in very large numbers e.g. during an overcast day in May 2013 we swept a small water-filled hollow produced by a fallen tree in dense woodland in South Hertfordshire and found hundreds of Hydroporus memnonius Nicolai, 1822 along with a few specimens of several other Hydroporus species, and this with a small aquarium net. Sweeping marginal vegetation or among reed beds will almost certainly produce some specimens, garden ponds and cattle troughs are also very good as are flooded tyre ruts in woodland during the spring although these tend to be dominated by larger water beetles. The presence of vegetation is not essential and even heavily shaded slow-moving woodland ditches will usually produce a few beetles. As many different habitats as possible should be sampled in order to produce a decent list of species; most obviously still and running water but also different substrates such as chalk streams, silty ponds and rivers, peat bogs and exposed moorland pools and brackish habitats near the coast. The best technique for sampling is sweeping, and the best way is to sweep the same spot a few times over five minutes or so as the first few sweeps tend to disturb more specimens and make them active, the contents of the net should always be emptied onto a white tray as many specimens may be missed among sediment and tangled vegetation in the net. Identification in the field is impossible for the beginner but with a little experience the common species become familiar and anything different will usually stand out, on the other hand it pays to simply take a few specimens at random and identify them under the microscope as this often produces surprises and so is a good way to become critical about identification. Very quickly the same few very common species will turn up everywhere, species like Hydroporus planus (Fabricius, 1782), H. pubescens (Gyllenhal, 1808), Nebrioporus elegans (Panzer, 1794) and Hygrotus impressopunctatus (Schaller, 1783) etc. but most sampling sessions will produce something interesting as there are so many locally common or supposedly rare species. Our only member of the Hyphydrini, Hyphydrus ovatus (Linnaeus, 1760) is common everywhere in well-vegetated water, Laccornis oblongus (Stephens, 1835) is very local and generally scarce, our 3 species of Bidessini and 2 species of Hydrovatini are very local and rare and Hydroporini, which contains the majority of our species, includes many common species that will soon produce a long list of local records. Adults and larvae of all species are thought to be predatory, feeding on tiny aquatic arthropods and their early stages. Adults of all species can swim freely among aquatic vegetation and all need to surface regularly to replenish air within a ventral plastron, most are capable of flight and many come to light or alight on plane-polarized surfaces. Several breeding strategies are known; adults may breed in the summer and produce eggs or larvae that overwinter to produce spring adults, or they may breed in the spring and produce adults late in the year that will overwinter, and a few are known to have flexible life-cycles and so larvae occur over a very long season and eggs and adults occur throughout the year. Eggs are laid underwater, either among the substrate or on or within plant stems etc, larval development passes through 3 instars and is generally rapid except when they overwinter, and pupation occurs in a cell constructed by the larva out of water, either among marginal substrate or under debris. Life-cycles and the distribution of many species are well-understood and there is an abundance of information and pictures both in print and online.