Stenolophus mixtus (Herbst, 1784)
This Western Palaearctic species is generally common and often abundant throughout its range; from Portugal to Asia Minor, Iran and the west of Russia and from North Africa to the UK and the southern provinces of Fennoscandia. Here it is common throughout lowland Wales and England north to North Yorkshire though mostly absent from the West Country, it occurs on all the islands north to Man and is widespread though very local across Ireland. In central Europe it also occurs in highland and low mountain areas up to 1000m. Typical habitats in the UK are various wetlands, especially on clay or muddy soils; marshes, pond and river margins and permanently damp pasture and they may be particularly abundant among damp litter in reed beds but in many areas of Europe also in damp woodland and sometimes among damp leaf-litter and in compost heaps. Adults occur year-round although in warm seasons may aestivate and so be absent for a while; they overwinter among damp litter and layered vegetation and become active from the first mild days of spring, they are nocturnal but easily alarmed and run readily during the day, are predatory, fly well and may come to suitably situated light traps in very large numbers from late spring until late summer. Breeding occurs in the spring and new-generation adults appear from late summer. Adults are easily found in suitable habitats, they usually occur in numbers and often in company with other common wetlands carabids such as Agonum fuliginosum or Paranchus albipes.
A medium sized ground beetle, 5-6mm which may be recognized in the field by the characteristically coloured pronotum. Dorsal surface glabrous, head black, pronotum black with narrow pale margins, elytra iridescent due to strong microsculpture, very variable in colour but usually dark with the suture, lateral margins and shoulder area pale, the extent of the pale areas varies and in extreme forms it is bright red with only a triangular dark mark around the scutellum. Head smooth; the vertex convex and clypeus flattened, without frontal furrows and with a single setiferous puncture beside each eye, eyes large and prominent, long and slender; dark with only the basal segment pale. Pronotum transverse and rounded laterally, with only a single long sensory seta which is situated near the middle, anterior and posterior angles rounded, basal margin unbordered. Pronotal disc convex and relatively smooth, the basal area with wide, shallow and ill-defined fovea, and larger punctures either side of a roughened median area. Elytra long and moderately curved laterally to a continuously rounded apical margin, each with eight unpunctured striae and an abbreviated scutellary striole, interstices convex; the third with a pore puncture in the apical third which usually joins the second striae. Legs pale to moderately dark brown, often with the femora and tibiae darker towards the apices. Tarsi 5-segmented, male with basal segments of pro-tarsi dilated. Claws smooth.