Platystethus nodifrons Mannerheim, 1830
This is a mostly northern Palaearctic species extending across the region from Europe to the far east of Russia and Japan although it is absent from China, it is the most northerly member of the genus and in northern regions it is generally the commonest, it is generally scarce in the south where the limit of the distribution seems to be Turkey. In Europe it is sporadic and generally scarce in the south, reaching mountain areas in south eastern France and northern Italy, it is absent from the Iberian Peninsula and the western limit of the distribution is the UK, it remains scarce in the southern Baltic countries but is generally common further north extending far beyond the Arctic Circle in Scandinavia. The species is widespread in the UK but seems to be frequent only across the English midlands and East Anglia; it is otherwise very sporadic with records across the south and north east England, Wales, Scotland, including the Outer Hebrides, and Ireland. Adults are present year-round and are often active through the winter in all but the coldest spells; they are active from early spring until the autumn and peak in abundance during spring and late summer. Typical habitats are undisturbed margins of stagnant still and slow-moving water, marshes and fens, although in northern Europe they often occur among matted vegetation in forested foothills and valleys. Little is known of the biology but, typical of the genus, breeding probably occurs in the spring and larvae develop through the summer. Adults may be found by searching among marginal soil and tussocks etc. although they are sometimes active on open soil during warm spells and they fly well and swarm on warm spring evenings. During the winter they appear in extraction samples or among flood litter, but they are often active and may be sieved from reasonably dry marginal leaf litter.
© Lech Borowiec
2.5-4.0 mm. Forebody and abdomen shiny black, elytra at least to some extent brown, usually with the basal and lateral margins black, antennae black, legs pale yellowish-brown, usually with the femora darker. Forebody and elytra without microsculpture, abdomen with faint, mostly transverse microsculpture, Head large, only slightly transverse, broadest across small convex eyes and with long curved and converging temples, surface moderately strongly and rather closely punctured from the base to the antennal insertions and longitudinally impressed from beside the eyes towards the base. Mandibles long and toothed internally towards the apex, terminal maxillary palpomere diminutive. Antennae inserted laterally in front of the eyes, the insertions hidden by a raised lateral part of the clypeus, long and slender, the basal segment especially so, only weakly thickened from the fifth segment. Pronotum transverse and almost semi-circular from slightly protruding anterior angles, surface moderately strongly and discretely punctured, about the same as the head, with a distinct median longitudinal impression and often with indistinct impressions towards the base, basal and lateral margins bordered. Elytra slightly dilated from rounded shoulders to separately-rounded apical margins, surface rather strongly punctured, these in places form close groups where punctures are separated by at most their diameter, pubescence sparse and short, nowhere overlapping. Abdomen long and usually only weakly curved laterally, tergites strongly bordered and narrowly impressed across the base, otherwise smooth and very finely punctured and pubescent. Legs long and rather slender. Femora unmodified. Front tibiae emarginate externally before the apex, front and middle tibiae spinose externally, hind tibiae with only fine setae along the external margin. Tarsi 3-segmented, basal segments short and unmodified, terminal segment long and curved. Males are usually more robust, especially in head structure, but this may only be obvious by comparison, the sexes are otherwise similar as males lack clypeal projections and the eighth sternite is unmodified.