Acupalpus dubius Schilsky, 1888
This is the most common member of the genus in the UK and, at least across much of the southeast, among the most common of our carabids; it is generally abundant across south and central England, including the islands, abundant around the coast of Wales though more scattered and local inland, very local in the north of England and almost absent from Scotland, it is very local and mostly coastal in Ireland though probably under-recorded. On the continent it is restricted to western and central areas; from France east to Estonia and the Caucasus and extending north to southern Sweden where it is coastal and very local, but in most areas it is rare or very rare e.g. in Denmark, Belgium, Switzerland and Germany, and it was first discovered in Norway in 2007 when a single specimen was found in a light trap. Beyond this it occurs on several islands, the Azores, Madeira and Sicily. In the UK it is a lowland species likely to occur in just about any wetland situation; water margins, sphagnum bogs, marshland, dunes and fens etc. but they may be especially common on coastal dunes and among well-vegetated areas beside reed beds and ponds on moors and heaths, we find them occasionally by sweeping in such situations but searching through leaf or reed litter often produces them in abundance. Adults occur year-round; they are active from early spring until late in the year and overwinter among vegetation, moss or litter close to water, they breed in the spring but little is known of their biology. They are fully-winged and known to fly but it seems they rarely do so.
This tiny carabid will soon become familiar even in the field but there are several similar species occurring in the same habitats and so specimens should always be examined carefully. 2.5-3.3mm. A dull brown beetle with the pronotum paler than the head and elytra. Head dark brown, smooth and without impressions, with a single setiferous puncture beside each eye, and narrower than the pronotum. Mandibles sharp apically and without a setiferous puncture on the outer margin, palps pale or weakly infuscated; terminal maxillary palpomere long and pointed, antennae pubescent from the third segment; dark with the basal and terminal segments pale. Pronotum transverse, widest in front of the
middle and smoothly narrowed to rounded posterior angles, basal border incomplete, surface glabrous and shiny; punctures confined to the basal fovea. Although the posterior angles are rounded there is usually a distinct but weakly-defined obtuse angle present, most specimens have the pronotum entirely pale but it is often gradually and diffusely darkened towards the centre. Elytra dark shiny brown, appearing iridescent in certain lights, and glabrous, they may be paler towards the lateral margins or along the suture but this is gradual and does not form a distinct pattern, with rounded shoulders and gradually widened so that they are distinctly broadest behind the middle, lateral margin sinuate just before the rounded apex. Striae distinct to the apex, scutellary striole present, interstices weakly convex; the third with a tiny puncture in the apical third which usually joins the second stria (this should be looked for very carefully as it is easy to miss and an important aspect of the identification). Legs pale but usually darkened towards the femoral and tibial apices. Males have the basal pro-tarsal segments dilated.