Hygronoma dimidiata (Gravenhorst, 1806)

This generally common species occurs from lowland to mountain altitudes throughout central and northern Europe, extending south to the Pyrenees, northern Italy and Romania and north to the UK and above the Arctic Circle in Sweden and Finland, further east it is recorded from the Caucasus and extends into Siberia. Here it is locally common across Wales and England north to mid-Yorkshire and sporadic and rare further north to southwest Scotland. Typical habitats include permanently damp and well-vegetated margins of rivers, ponds and reservoirs, reed beds, wet peatland and salt marshes, adults occur year-round, peaking in abundance in the spring and autumn and teneral specimens have been found in August. Adults are diurnal and spend much of their time on wet soil or among decaying organic matter but in warm weather they also climb plant stems and may be common in swept samples, they overwinter among litter in situations not prone to flooding and may be abundant among cut or fallen reed stems but breaking open standing stems may produce them in numbers, they also occur among flood refuse and in pitfall traps. Searching among decaying organic matter sieved over a sheet is probably the best way to obtain specimens during the summer as they can move very quickly when alarmed and rapidly vanish into the soil.

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This tiny rove beetle should be immediately obvious due to its flattened form and bicoloured elytra. 2.5-3.0 mm. Body entirely black but for the apical half of the elytra which is abruptly yellow, legs and palps yellow, antennae black with the basal four segments yellow. Form flat and parallel-sided the head about as wide as the pronotum and elytra and the abdomen slightly dilated towards the apex. Head proportionally very large; smooth and finely and densely punctured, eyes coarsely faceted and moderately convex, continuous with the outline, temples curved and contracted to the base, vertex and frons flat in the female, slightly concave between the eyes in the male. Antennae 11-segmented with segments 5-10 quadrate to slightly transverse. Pronotum quadrate, broadest before the middle and narrowed to rounded posterior angles, anterior angles rounded, surface finely and densely punctured and pubescent, with a variable median longitudinal impression which is usually broader in the male. Elytra quadrate to slightly elongate and only a little longer than the pronotum, with rounded shoulders and minutely produced posterior angles, basal margin sinuate, surface densely punctured and pubescent, more finely so than the pronotum. Abdomen with strongly-raised lateral borders and slightly dilated to the apex, very finely punctured and pubescent. Legs long and slender with short 4-segmented tarsi; segments 1-3 bilobed, terminal segment broadly rounded, claws simple; long and slender and lacking a basal tooth.