Stenus nitens Stephens, 1833
Generally distributed throughout much of the Palaearctic region from Portugal to parts of eastern Siberia, this species is locally common in Europe from the northern Mediterranean borders north to the UK and far above the Arctic Circle in Scandinavia and Russia, although in the southern Baltic countries it is very local and rare. In the UK is locally common in south-east England, East Anglia, South Wales, south-west Scotland and across Ireland but otherwise sporadic and rare, reaching north the northern Scottish Highlands. Adults are present year-round; they are most frequently recorded from November until May and tend to become scarce in late summer. Typical habitats are wetland margins, reed beds, marshes and flood plains, and in northern Europe also in damp woodland. They are diurnal and usually occur on the ground among dense vegetation or among sphagnum or tussocks of various sedges etc, and through the winter are frequent in samples of leaf and reed litter, often among numbers of other beetles. Little is known of the biology but both adults and larvae are predatory and it is likely larvae develop through the summer in much the same habitats as the adults. Sampling is easiest by sieving litter etc or by brushing aside litter among dense vegetation and searching the soil surface; they usually occur in numbers but will need to be taken for critical examination as several very similar species are also common in these habitats.
3-4 mm. A very slender species with the head and elytra distinctly wider than the pronotum, entirely black or with the tarsi a little paler, forebody and elytra densely punctured but very shiny due to the lack of microsculpture. Pubescence pale grey, recumbent and rather sparse. Head with large and strongly convex eyes that occupy almost the entire margin, surface between eyes flat and only weakly raised along the centre so not distinctly depressed beside the eyes, labrum smoothly rounded and wider than the shortest distance between the eyes. Palps and antennae entirely dark. Pronotum elongate, broadest about the middle and narrowed to distinct angles, basal and apical margins narrow and slightly curved, punctures moderately large and dense but for the most part discrete, and with a narrow but deep median longitudinal impression which reaches, or very nearly reaches, the apical and basal margins. Elytra elongate, very slightly wider than the head and only weakly dilated towards the apex, with rounded shoulders and obliquely-converging apical margins. Abdomen narrow, almost parallel-sided in the basal third then smoothly narrowed to the apex, basal tergites strongly bordered, all tergites finely and evenly punctured; first tergite with four strong longitudinal keels in the basal half, the following two or three tergites are also keeled but not so strongly so. All tarsomeres simple or only very weakly lobed, hind tarsi much shorter than the hind tibiae.
Stenus nitens 1
Stenus nitens 2
From Tottenham, 1954
Several other UK species have the characteristic keeled tergites and median impression on the pronotum, but these either have distinct microsculpture between the punctures or, in S. melanopus (Marsham, 1820), the impression is abbreviated and does not extend onto the apical third. Aedeagus distinctive; parameres narrow and at most only slightly extending beyond the tip of the median lobe which is smoothly narrowed to a pointed apex – this will always separate males from males of melanopus where the median lobe is strongly narrowed towards the apex and slightly produced medially.