Stenus clavicornis (Scopoli, 1763)
A very widespread and generally common species occurring from lowland to mid-mountain altitudes throughout the Palaearctic region from Europe east through Asia Minor and Russia to Mongolia, India, China and Korea, it has also become established in the northern United States and southeast Canada, having been first recorded there in 1968. Here it is common and often abundant throughout the UK north to the Outer Hebrides and Orkney but apparently absent from Shetland although it does occur on Faroe. Adults appear in March or April and are present into the October or November although may also occur during mild winter spells, on the continent they are generally associated with damp and humid habitats from grassland to forests where they occur under stones, among moss and leaf-litter etc, while in the UK they may be found in both damp and dry habitats; we have recorded them from wetland and arable margins, dune slacks, dry peat cuttings, reedbeds and calcareous grassland on hillsides exposed to the sun. Sweeping long grass or herbage in almost any open habitat in bright sunny weather is likely to produce adults, especially during late spring and early summer, otherwise they may be found by grubbing among tussocks or among plant debris samples, they usually appear in numbers and are among the commonest of our larger species of the genus.
Adults are distinctive; large with strongly bordered abdominal tergites, bicoloured legs and long tarsi but S. providus Erichson, 1839 is closely similar and occurs in the same habitats. 5.0-6.5mm. Entire body dark grey with sparse and very fine pale pubescence. Head wider than the pronotum, densely punctured and with large convex eyes that occupy the lateral margins, palps entirely pale yellow, antennae entirely dark, long and slender with the last three segments only a little wider than the others. Pronotum slightly elongate, broadest in front of the middle and more strongly narrowed to the base than the apex, punctation strong and dense but discrete, confluent, if at all, only towards the basal margin. Elytra quadrate or nearly so, with broadly rounded shoulders and slightly widened towards the apical margin, surface dull matt with strong and dense punctures which are mostly separated by less than a puncture width and may be confluent in places. Abdomen rather parallel-sided and with strongly raised borders, basal tergites with a short but distinct median ridge and all tergites evenly and regularly punctured throughout. Legs substantially pale; the distal quarter of the femora, at most, and the tarsi darker. Tarsi long, slender and without bilobed segments; the middle and hind tarsi almost as long as the corresponding tibia, the basal metatarsomere distinctly longer than the apical segment. The form of the aedeagus is very distinctive with a deeply cleft apex to the median lobe and this feature, coupled with the entirely dark elytra, will identify the species with certainty.