HYDRAENIDAE Mulsant, 1844
These small beetles occur around any salt- or freshwater margins, and can be found at any time of year. Many species are common and often abundant.
Around the World
This large and cosmopolitan family of mostly wetland beetles includes more than 1600 described species in about 40 genera and 4 subfamilies although many hundreds of species remain to be described, especially from tropical and southern temperate regions and especially from the genus Hydraena Kugelann, 1794. About 85% of all species belong to 3 very speciose genera; Hydraena, Limnebius and Ochthebius, all of which are cosmopolitan. Most regions have a diverse fauna and there are many island endemics; the Palaearctic fauna includes about 950 species, and of the 500 or so European species 50 occur in Northern Europe and about 30 extend to the UK, and even mostly arid areas are well-represented, 15 species of 3 genera occur in Egypt. The Australasian fauna includes more than 350 species in while that of the New World marginally exceeds this total, and about 100 species occur in the Nearctic region. The Afrotropical fauna includes about 450 species. Prosthetopinae Perkins, 1994 includes 10 genera from Tropical Africa while the 3 genera of Orchymontiinae Perkins, 1997 occur in Australia and New Zealand. The cosmopolitan Hydraeninae Mulsant, 1844 includes 5 tribes and the majority of the species with about 1000 in Hydraena. Hydraeniini Perkins, 1908 includes 3 Neotropical genera. Hydraenini Mulsant includes Hydraena and the small Neotropical genus Adelphydraena. Liminebiini Mulsant, 1844 includes the cosmopolitan genus Limnebius Leach, 1815 with about 150 species, and 3 species of the Palaearctic genus Laeliaena Sahlberg, 1900. Madagastrini Perkins, 1997 includes 3 genera from southern Asia and Africa. Parhydraenini Perkins, 1997 includes 6 genera from Africa and the Indian Ocean islands. Ochthebiinae Thomson, C.G., 1859 is divided into 2 tribes. The small tribe Ochtheosini Perkins, 1997 includes the monotypic Edaphobates Jäch & Díaz, 2003 and Ginkgoscia Jäch & Díaz, 2003, both from southern Asia, and 2 species of the Neotropical genus Ochtheosus Perkins, 1997. Ochthebiini Thomson, C.G., 1860 includes 12 genera in 5 subtribes and is cosmopolitan; the largest genus (and only large genus) is the worldwide Ochthebius Leach, 1815 (in the broad sense, including Eniocerus Stephens, 1829) with about 350 species.
POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
The vast majority of species are small, 1-3mm, the largest is Prosthetops wolfbergensis Bilton, 2013 at 4.2mm (Prosthetopinae) from South Africa, and superficially resemble small hydrophilids with which they often occur in marginal wetland habitats. They are distinct in having 6 or 7 visible abdominal sternites and a small sternite between the metacoxae, a densely pubescent ventral surface and 9-segmented antennae with a long 5-segmented club, in addition many species have very long maxillary palps and/or a sculptured dorsal surface which make them distinctive with a little experience. Most species are semi-aquatic with at least the early instar larvae living out of, but near to, the water. Adults of most are aquatic and most occur at the margins of streams, ponds and slow-moving water; many live in, or are restricted to, brackish water and perhaps the best example are species of the New World genus Neochthebius Orchymont, 1932 which live in rock crevices in the intertidal zone, but more generally many occur in salt marshes and brackish estuarine habitats. Many species, species groups or genera are restricted to certain habitats and some are very specialized e.g. species of the Australian genus Tympanogaster Janssens, 1967 (Ochthebiinae, Meropathina Perkins, 1997) are the only beetles recorded from hygropectric habitats (where water flows over rock surfaces). In temperate regions they may be particularly common on sparsely vegetated marginal habitats where the substrate is frequently washed and so becomes partitioned with respect to particle size and organic content and often develops areas of permanent algal matting, here several species of the three large genera may occur together and may display some microhabitat preference. In such situations they are readily sampled by flooding the substrate or by working a net through any marginal debris and vegetation at a very low angle. Both adults and larvae of most species are omnivores and generalist grazers on damp surfaces, especially stones, sand grains and among decaying leaves etc. where they feed upon protozoans, algae, bacteria and fragmenting decaying vegetation. Some species live among wet moss in marginal situations and are rarely found elsewhere, brackish water species tend to have the same lifestyles as freshwater species but many specimens may be found by searching under debris and among roots in damp situations away from areas that become flooded, in such situations they may also be seen active on the substrate during warm weather. Adults are truly aquatic as they can carry a plastron below the abdomen which allows them to remain submerged for extended periods and they may sometimes be seen crawling among aquatic vegetation or upside down under the surface of the water. In general the larvae are semi-aquatic and occur in the same situations as the adults; many species oviposit on plant stems, stones or debris near the margin and just below the water surface and freshly emerged larvae leave the water and live among damp marginal soil and roots.
Elongate-oval and continuous (Limnebius) or discontinuous in outline, most are drab; brown to black or with indistinct pale areas, and many are metallic or at least partly so, dorsally with scattered setae; ventrally with dense hydrofuge pubescence, only rarely limited to discreet areas. Head transverse to elongate and sometimes produced anteriorly e.g. Eniocerus, always prominent and visible from above, with convex and prominent eyes and in some with ocelli near the posterior margin of the eye, vertex and frons punctured and rugose or with various sculpture; fovea, transverse or longitudinal depressions etc. Antennae inserted below the clypeus near the anterior margin of the eyes, in European species 9-segmented with a hydrofuge-pubescent 5-segmented club, the fourth usually forming a cupule. At rest the antennae are held beneath the eyes with the club in a ventral depression. Clypeus well-developed and distinct, labrum very variable; small to large and curved, emarginate or excavate anteriorly, mandibles mostly hidden beneath the labrum or visible laterally. Maxillary lacinia small with robust scraping edge or process and/or rows of stiff setae, palpi 4-segmented; the basal segment reduced and usually minute, 2-4 variable, in Hydraena and Limnebius much longer than the antennae, in Ochthebius about the same length. Labial palpi 3-segmented and small, often partly or wholly concealed under the mentum. Pronotum wider than the head and very variable; laterally smoothly rounded, sinuate and narrowed anteriorly or posteriorly, or variously indented and, sometimes, with membranous borders, in some Hydraena almost hexagonal in outline. Surface variously punctured and microsculptured, from smoothly convex e.g. in many Hydraena and Limnebius, to characteristically sculptured with transverse, oblique and longitudinal channels e.g. in Ochthebius where they are often important aids to specific identification. Scutellum usually small but visible. Elytra entire; completely covering the abdomen or sometimes leaving the pygidium partly exposed, and continuously rounded apically, almost always with well-developed and punctured striae, sometimes up to 15, and often lacking a scutellary striole although the area around the scutellum is sometimes confusedly or confluently punctured, lateral margins variously explanate, interstices usually simple punctured and flat to weakly convex, in some variously raised or carinate, in some Ochthebius they may be obliterated by very strong puncture rows and transverse sculpture, in most Limnebius without striae, finely and randomly punctured and microsculptured. Abdomen with 7 visible sternites, although the apical sternite may be retracted and so not visible, and an approximately triangular inter-metacoxal sternite. Legs very variable; long to short and usually undeveloped, tibiae parallel-sided or, especially the anterior pair, broadened apically, often with rows or patches of small and stiff setae, in some species sexually dimorphic e.g. in male Limnebius truncatellus (Thunberg, 1794) the metatibiae are strongly constricted before the base, apical spurs often tiny and inconspicuous. Tarsi 5-5-5 but often appearing 4-4-4 due to a tiny basal segment, terminal segment long and expanded apically, often as long as the others combined; segments not obviously lobed. Claws usually well-developed, smooth and separate to the base.
Our fauna is comparatively diverse and typical of northern temperate faunas, a familiarity with which will allow most of the family to be recognized. Octhebiinae Thomson, C.G., 1859 are characterized by the maxillary palps being about as long as the antennae, our UK list includes 16 species in 3 genera, all formerly included in Ochthebius. Our single species of Eniocerus Stephens, 1829, E. exsculptus (Germar, 1824) is a very local insect occurring in wet moss beside shallow streams in the north of England and Scotland, it is distinct having widely explanate elytral margins and lacking transverse pronotal impressions. Conversely our single species of Aulacochthebius Kuwert, A. exaratus (Mulsant, 1844) is a very local and scarce insect occurring in reed beds etc in the south east of England. Ochthebius is represented by 14 species in 4 subgenera although several others have been listed which require confirmation. Several are locally common and widespread e.g. O. dilatatus Stephens, 1829, O. minimus (Fabricius, 1792) and O. bicolon Germar, 1824 while some are very local and rare e.g. O. poweri Rye, 1869 or O. pusillus Stephens, 1835. O. poweri Rye, 1869 is a very local species of water seepages on cliffs while several are associated with brackish conditions e.g. O. punctatus Stephens, 1829 and O. viridis Peyron, 1858, or the very rare and mostly western coastal O. lejolisii Mulsant & Rey, 1861; most species are predominantly southern but O. lenensis Poppius, 1907 occurs in rocky coastal pools around Inverness. O. aeneus Stephens, 1835 has not been recorded for more than a century and is now considered extinct in Britain. Hydraeninae is represented by 2 genera, in each case recognized by the maxillary palps being much longer than the antennae; Limnebius are smoothly convex and more or less continuous in outline whereas Hydraena have distinctly punctured elytral striae and are discontinuous in outline. Of the 5 species of Limnebius only 2 are widespread and generally common; L. truncatellus (Thunberg, 1794) occurs throughout the UK while L. nitidus Marsham, 1802 becomes scarce in the north and is absent from the north of Scotland. L. papposus Mulsant, 1844 and L. aluta Bedel, 1881 are widespread but local in the south of England and Wales while L. crinifer Rey, 1884 is a very rare species of southern Kent. Several of our 10 species of Hydraena are widespread and common including H. riparia Kugelann, 1794, H. britenni Joy, 1907 and H. gracilis Germar, 1824 while H. testacea Curtis, 1830 is common only in the south and H. flavipes Sturm, 1836 has a western and northern distribution. Several are rare or have a restricted distribution e.g. H. palustris Erichson, 1837 which is sporadic in the south, or H. pygmaeus Waterhouse, G.R., 1833 which is widespread. Members of our 3 diverse genera can be very difficult to identify from external morphology and so males should be dissected in all but the most obvious cases, fortunately there are very good and well illustrated keys, including figures of the aedeagi, in the publications given below.