Classification

The subfamily is divided into tribes as follows.

• ERETINI Crotch, 1873 includes 4 species of the single genus Eretes Laporte, 1833. Only E. griseus (Fabricius, 1781) occurs in Europe, it is the most widespread member of the genus and occurs throughout the Old World with the exception of most of Australia and the northern Palaearctic region, extending into southern parts of Europe. Another Old World species, E. sticticus (Linnaeus, 1767) occurs throughout Africa and the Middle East north to the Mediterranean but does not extend into Europe; it also occurs across Central America and extends into the Nearctic and Neotropical regions. Two other species are more restricted; E. explicitus Miller, K.B., 2002 to North America, and E. australis (Erichson, 1842) to Australia. Adults are medium-sized beetles, 11-18mm, mostly pale with darker markings to the head and pronotum and with very variable elytra which have several rows of larger punctures surrounded by dark spots, they are otherwise distinguished from the remaining tribes by the pointed prosternal process, rows of small stiff spines along the apico-lateral elytral margins and a row of setae on the basal meta-tarsomeres. They are strong fliers and often occur in temporary habitats; E. sticticus has been recorded developing from egg to adult in only 15 days, a shortest cycle of any dytiscid. For a good insight to the group as well as a delightful read see: Revision of the genus Eretes Laporte, 1833 (Coleoptera: Dytiscidae) Miller, Kelly B. 2002. Aquatic Insects 24(4):247-272.

• AUDEHYDRINI Guignot, 1942 includes 2 Neotropical species of the single genus Notaticus Zimmermann, 1928. They are small, 6.5-10.0mm and proportionally more elongate than other members of the subfamily and have a characteristic prosternal process broad across the apex, the scutellum concealed and a fringe of golden setae across the apex of the meta-tarsomeres, as in the Eretini. They are dark beetles with a variously pale pronotum and an extensive pale macula across elytral base. Both occur in tropical wetlands.

• DYTISCINI Leach, 1815. This tribe now includes only the single genus Dytiscus Linnaeus, 1758 although it may often be found to include a further genus, Hyderodes Hope, 1838 which is now generally included in its own tribe HYDERODINI Miller, K. B., 2000. Members of these genera are morphologically very similar but differ, among other things, in the form of the frontoclypeal suture, in Hyderodes it is interrupted medially whereas in Dytiscus it is complete. Hyderodes includes 2 species and is restricted to Australia. Dytiscus includes 27 species; it has a Holarctic distribution and extends south into northern Africa and Central America, 11 species occur in the Nearctic region and 13 are Palaearctic while 2 species have a Holarctic distribution, of the 7 northern European species, 6 occur in the UK. Dytiscus generally occur in richly-vegetated still-water habitats, ponds and lakes etc. but a few may occur in slow-moving rivers. They are large beetles which are known to possess chemical defences against predators which may explain why they often appear in habitats rich in fish. Several species are widespread and generally common in Northern Europe and among these both D. marginalis and D. semistriatus should soon be found by UK beetlers. Species are readily recognized by their large size, 22-44mm and bicoloured dorsal surface; dull brown to green with various pale margins to the pronotum and elytra, the only confusion might be with Cybister but here the metatibiae are short and have unequal spurs and the metatarsi have a single claw, in Dytiscus the metatibiae are longer and have equal spurs and the metatarsi have 2 claws. Species tend to look rather similar, differing in size and detail although the splendid D. latissimus Linnaeus, 1758 has broad explanate elytral margins which extend beyond the normal ‘dytiscid’ outline. Sexual dimorphism is strongly developed in Dytiscus with males having broadly dilated pro-tarsal segments furnished ventrally with adhesive setae arranged in various patterns, females of some species have longitudinally-sulcate elytra although in all cases females with smooth elytra also occur and the proportion varies between species as well as between populations, the function of the sulci was once thought to be to assist with mating but this is uncertain.

• HYDATICINI Sharp, 1880 includes almost 140 species of the single genus Hydaticus Leach, 1817 and has an almost cosmopolitan distribution. The species are divided into 2 subgenera with the majority, about 130 species, included in Prodaticus Sharp, 1882; these are mostly tropical, with a few extending into the Palaearctic and Nearctic regions while Hydaticus s.str. includes 7 species and is restricted to the Holarctic region. Of the subgenus Hydaticus 2 species are Nearctic and 4 are Palaearctic while H. aruspex Clark, 1864 is Holarctic; both our UK species, H. seminiger (DeGeer, 1774) and H. transversalis (Pontoppidan, 1763) are included. They are typical of the subfamily, occurring in well-vegetated ponds and lake margins; they breed in the spring, larvae develop through the summer to produce new-generation adults in late summer and autumn that will disperse and overwinter away from aquatic habitats. During spring and summer they often occur in small water bodies such as garden ponds, and towards the end of summer and into the autumn they may appear in large numbers. Among our UK fauna they are distinguished by the size, 12.0-14.5mm, and the dark elytra with pale margins; in H. transversalis it is there is a narrow undulating pale transverse line across the base of each elytron whereas in H. seminiger the base is entirely dark.

• ACILIINI Thomson, C.G., 1867 includes about 65 species in 7 genera and is worldwide in distribution although two genera are restricted to certain regions e.g. the 8 species of Aethionectes Sharp, 1882 occur across Africa south of the Sahara and one is endemic to Madagascar, while the monotypic genus Tikoloshanes Omer-Cooper, 1956 is restricted to a few small areas in southern Africa. Thermonectus Dejean, 1833 is a New World genus of 20 species; most occur in warmer regions from the southern United States to tropical South America including the Caribbean islands. Sandracottus Sharp, 1882 includes 16 species and occurs throughout southern and south-east Asia extending south to northern and eastern Australia. Rhantaticus Sharp, 1880 includes the single species R. congestus (Klug, 1833) although recent research suggests this may comprise several species; it is relatively small for the family at 7.5-11.0mm and very variable in colour, it occurs throughout warmer parts of the Old World including most of Africa, the Middle East, southern and south-east Asia and Australasia and is among the most widespread members of the subfamily. Graphoderus Dejean, 1833 includes 12 species and is Holarctic in distribution; 5 occur in the Nearctic region and the rest are Palaearctic. Of the 4 central and northern European species 3 occur, or are known to have occurred, in the UK. Among our fauna they are identified by the size, 12.0-14.0mm, the dark pronotum; pale with the anterior and basal margins broadly dark, and the mottled elytra. Acilius Leach, 1817 includes 13 species in 2 subgenera and has a Holarctic distribution; 7 species occur in the Nearctic, 6 in the Palaearctic and none are Holarctic. The only member of subgenus Homoeolytrus Gobert, 1874, A. duvergeri Gobert, 1874, is a very local insect of southwest Europe, northwest Africa and some Mediterranean islands. Two species occur in central and northern Europe, both of which are widespread in the UK although only A. sulcatus (Linnaeus, 1758) is generally common. Members of this genus are medium sized, in UK species 14.0-18.0mm, broadly oval and have various distinctive dark markings to the dorsal or ventral surfaces, the dorsal surface of the pronotum and elytra are densely and strongly punctured and in most, including the north European species, the female elytra are longitudinally sulcate and densely pubescent. Males have very widely dilated pro-tarsi bearing several large round discs of adhesive setae as well as smaller discs or fields of setae on the ventral surface. Species of this tribe are typical of the subfamily, they occur in well-vegetated still-water bodies in a wide variety of habitats; often in small or temporary ponds or among reed bed margins, and some can be locally abundant. They are recognized among the subfamily by the form of the metatibial spurs, both of which have the apex bifid.

• More generally classified as a distinct subfamily the CYBISTRINI Sharp, 1880 includes about 130 species in 7 genera and is distributed in warmer areas worldwide; the greatest diversity is tropical regions with only a few species extending into northern and southern temperate areas, and with one exception the genera are restricted to certain regions. The monotypic genus Regimbartina Chatanay, 1911 occurs in a relatively small area of tropical West Africa while the 21 species of Megadytes Sharp, 1882 are restricted to The New World, mostly tropical South America but a few extending north throughout the Caribbean and into southern Florida. With the exception of Cybister Curtis, 1827 the other genera have restricted distributions within Australia although one species of Sternhydrus Brinck, 1945 (4 spp. in total) extends north as far as New Guinea. Cybister is the largest genus with almost 100 species, they are most diverse in tropical regions although rather poorly-represented in South America where they are replaced by Megadytes, and similarly they are only poorly represented in the Holarctic region where Dytiscus is the predominant genus of large dytiscids. The group is generally absent from the UK but one species, Cybister lateralimarginalis (DeGeer, 1774) was recorded several times from Essex during the early nineteenth century.  Members of this tribe superficially resemble Dytiscus, with or without the pale margins, and most are medium sized although the two large genera vary widely in size; Cybister from 13-43mm and Megadytes from 16.5-47.0mm, which makes them the largest extant members of the family. They differ from other members of the subfamily in having a small patch of setae under the elytra towards the base, and unequal spurs and a group of bifid setae on the metatibiae.