Description

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.

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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.