Platystethus nitens (Sahlberg, 1832)
Locally common throughout the Palaearctic region from Europe to eastern Russia and northern China, this is among the most widespread members of the genus; it occurs throughout Asia Minor and North Africa and is recorded from the Canaries, Azores and most of the Mediterranean islands. The species is present from lowlands to subalpine mountain altitudes from the Mediterranean borders north to the UK, Southern Finland and Sweden; it is generally common in warmer southern regions but more sporadic further north. In the UK it is locally common across Southern England, the midlands and South Wales, more local and scarce further north to Yorkshire and it seems to be absent from Scotland and Ireland. Typical habitats in northern Europe, including the UK, are pond and river margins, marshes, reedbeds and peat bogs, the species also occurs by brackish water environments such as saltmarshes and tidal estuaries and it is sometimes abundant by pools on dune slacks. In southern Europe the species is more eurytopic, occurring also on arable (among corn crops etc.) as well as uncultivated land, dry meadows, hillside and mountain forests, reforested areas of pine and spruce, and among subalpine scrub (Rhododendron, Dryas and Dwarf Mountain Pine, Pinus mugo etc) In central and northern Europe adults have been recorded from dung, compost and, once, from a mole’s nest, and in northern China a specimen was found on carrion. Adults are present year-round, they overwinter among tussocks or in the soil but they have also been found far from water at this time, overwintering in groups in rural situations, they are active over a long season from April until November and peak in abundance during April and May. Little is known of the life-cycle but larvae have been found in spring and summer burrowing in moist riparian soil and mud. Adults are easily sampled by searching the soil among marginal vegetation although they are also active on bare soil in warm weather; they fly well and sometimes swarm on wetland margins during the evening. They are known to overwinter away from water but are often active through all but the coldest winter spells in riparian situations; they often occur in sedge tussocks etc. at this time and may be common in winter flood refuse.
Platystethus nitens 1
2.5-3.0 mm. Forebody and abdomen shiny black, elytra at least partly brown, usually with the basal and lateral margins black, antennae black, mandibles and legs brown, usually with the femora darker. Forebody and elytra without microsculpture, abdomen with faint and mostly transverse microsculpture. Head large and transverse, broadest across small and slightly prominent eyes and with long sinuate temples, surface sparsely punctured and pubescent, laterally with a strong curved impression from beside each eye towards the base. Clypeus expanded laterally over the antennal insertions and more or less truncate apically. Mandibles with an internal tooth extending almost to the apex, so appearing bifid. Penultimate maxillary palpomere expanded internally from the base, terminal segment diminutive. Antennae long and slender; only weakly broadened from the fifth segment. Pronotum transverse and only slightly broader than the head, distinctly bordered and more or less evenly rounded from slightly-projecting anterior angles and with a widely sinuate apical margin, surface sparsely punctured and pubescent; sometimes with the disc almost impunctate, central longitudinal furrow often fine. Elytra transverse, dilated from rounded shoulders to separately-rounded apical margins, sutural and apical margins rather strongly bordered, surface without striae; sparsely punctured and pubescent, the punctures separated by much more than their diameter and the hairs short and not overlapping. Abdomen short and usually rather straight-sided, tergites sparsely punctured and pubescent, the basal five strongly bordered, and the sixth widely incurved apically. Legs short and robust. Femora unarmed. Front and middle tibiae narrowed subapically and with strong lateral spines. Tarsi 3-segmented, the basal segments short and unmodified and the terminal segment long and curved. Claws long and slender, smooth and lacking a distinct basal tooth. Males have a pair of spines on the anterior clypeal margin, although these may be reduced and hardly noticeable, and a short tooth either side on the apical margin of the eighth abdominal sternite.