Hydraena nigrita Germar, 1824
This widespread European species occurs very locally from Spain to northern Italy and Greece in the south and to the UK and southern provinces of Fennoscandia in the north, it extends east into Poland and Ukraine but is absent from most of the Baltic countries, in the UK it occurs throughout England, Wales and the south of Scotland, including Anglesey, Man and the Isle of Weight, and there are scattered records from the Hebrides and the north and west of Ireland. Adults occur throughout the year, peaking in abundance between April and June and again in late summer, they occur from lowlands to low mountain altitudes in a wide range of wetland habitats; in Northern Europe often by streams and backwaters in mountain areas, but more generally about the margins of streams and rivers, often in wooded areas and on heavy clay substrates although they also occur in ditches and at temporary ponds in the summer. Often described as aquatic, it is only the adults that regularly enter the water, they crawl among submerged moss, plants and substrate but will also be found among marginal stones and moss etc, they are phytophagous, consuming algae and microscopic plant life, and the life-cycle is probably typical of the family with breeding occurring in the spring and larvae developing through the summer to produce adults that will overwinter. Oviposition occurs out of the water in marginal substrate and larvae are terrestrial, living among marginal moss and stones and feeding on algae, they pupate in the soil or under stones and adults usually enter the water soon after emerging from the soil. Adults may be sampled by sweeping among vegetation in shallow water, they generally float to the surface after being disturbed and can be scooped up with a sieve, splashing marginal moss can also be productive or marginal moss can be carefully removed and submerged to reveal the beetles. They will need to be taken for critical examination as members of the genus are superficially very similar.
1.7-2.1 mm. Mature specimens are darker than most other UK Hydraena species; H. gracilis Germar, 1824 is also very dark but is otherwise distinctive, see below. Body entirely black or very dark grey, appendages and mouthparts dark reddish-brown except the tips of the maxillary palps which are narrowly black. Head transverse with protruding convex eyes, vertex weakly convex and strongly punctured, frons less strongly punctured and without impressions, clypeus finely punctured laterally and smooth and shiny medially, labrum deeply emarginate. Antennae 9-segmented with a 5-segmented pubescent club, maxillary palpi very long, the curved basal segment longer than the antennae and about as long as the head, and the second segment shorter than the long terminal segment. Pronotum broadest about the middle and narrowed to obtuse angles, the lateral margin slightly sinuate towards the base, surface longitudinally and broadly depressed either side of the disc and usually with distinct transverse depressions in the basal and apical thirds, strongly punctured towards the margins, a little less strongly on the disc. Elytra broadly-oval, about 1.5X longer than wide, and widest a little behind the middle, with rounded shoulders and evenly curved to separately rounded apical margins, striae punctured throughout, more strongly so in the basal half and evanescent towards the apex, with at least seven striae between the suture and the shoulder – there may be up to ten but they are sometimes difficult to count – interstices narrow and mostly flat. Femora and tibiae long and slender, tarsi with five unmodified segments.