Thinodromus arcuatus (Stephens, 1834)

Formerly included in subgenus Trogophloeus Mannerheim, 1830 of Carpelimus Leach, 1819. This species is widely distributed but very local and generally scarce in Europe from Spain to Italy and Bulgaria in the south and north to the UK, Denmark and a few southern provinces of Fennoscandia. Most records are from lowland sites but they occasionally occur in river valleys etc. at lower mountain altitudes. Beyond Europe the species is widespread in North Africa and Asia Minor and has been recorded from Siberia. In the UK it is widespread though very local across England, Wales and southern Scotland and there are a few widely scattered records from the north of Ireland; it is very rare in Eastern and Southern Central England but much more frequent in the West Midlands and the north. Adults occur year-round and are active from March until August or September, peaking in abundance during May and June. The species is always associated with wetland margins although the overwintering sites are unknown, usually on exposed gravel or sandy substrates with patchy vegetation beside energetic rivers or streams. Little is known of the biology but adults are predatory and known to fly. They are active in warm sun on exposed sediments but are otherwise seldom seen, they occur under boulders and stones and have been extracted from moss and matted vegetation and specimens sometimes occur at light or in flight-interception traps.

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3.0-3.5 mm. Body shiny black or sometimes with a faint blue reflection, antennae dark brown, legs dark brown with paler tarsi. Dorsal surface with conspicuous and rather dense pale pubescence which forms a distinct oblique pattern on the pronotum. Head flat and finely punctured between large convex eyes, temples produced behind the eyes and strongly converging towards a broad neck, clypeus raised laterally above antennal insertions. Maxillary palpi short, the penultimate segment elongate and expanded to a rounded apex and the terminal segment diminutive and needle-like. Antennae 11-segmented and filiform; segments 1-7 elongate, 8-10 quadrate or slightly transverse and 11 broad and asymmetric. Pronotum transverse, broadest in front of the middle, curved to rounded or obsolete anterior angles and narrowed in a more or less straight line to obtuse posterior angles and a curved basal margin, surface moderately strongly punctured throughout, with a wide oval depression either side of the disc and a horseshoe-shaped impression across the basal third (diagnostic.) Elytra weakly dilated from rounded shoulders which are slightly produced forward (view from above) to a reflexed and continuous apical margin, surface widely depressed beside the suture and sometimes raised below the shoulders but otherwise smoothly convex, moderately strongly and densely punctured throughout, about the same as the pronotum. Abdomen with strongly raised lateral margins, all tergites finely and densely punctured, these in places giving the impression of strong cellular microsculpture, tergite 8 with a concave apical margin. Femora unarmed, tibiae thin and hardly broadened from the base, pubescent throughout and without external spines or teeth. Tarsi 5-segmented, the basal segments small and often difficult to discern and the terminal segment long and curved. Claws narrow, smooth and without a basal tooth.