Stenus biguttatus (Linnaeus, 1758)
Widely distributed across the northern Palaearctic region from Europe to the far east of Russia, China and Japan, in Europe it is generally common from the Pyrenees to the Black Sea and north to the UK, Iceland and the Baltic countries, reaching into central Norway and northern Sweden and Finland, also present on many of the Mediterranean islands and recorded from Northern Iran but apparently absent from North Africa. In the UK it is locally common across England and Wales although less so in the west and more so in the southeast and East Anglia. The species is strongly hygrophilous and is generally confined to wetland margins, especially beside lakes, ponds and slow-moving streams, often on sandy or gravelly substrates with patchy vegetation. Adults are active in bright sun, they do not climb vegetation but run actively on damp substrate searching for prey, they often occur near the water margin and are capable of moving rapidly on the water surface when disturbed, they invariably occur in numbers and usually along with other members of the genus and so will need to be examined carefully in the field. Little is known of the biology but typical of the group they probably breed in spring and summer and the larvae predate small insects and springtails etc. on the surface or among the soil, adults occur year-round, they overwinter under debris or among matted vegetation etc., and are active over a long season from early spring until September, peaking during June. They can be sampled by searching under debris or by flooding damp substrate, and pitfall traps can produce them in numbers, usually along with numerous marginal carabids and staphs and so traps can be very destructive and should be examined frequently.
4.5-5.0 mm. A rather slender looking species, body shiny black but for a small round orange spot on each elytron, densely and strongly punctured and with fine silvery pubescence throughout, antennae and legs black, maxillary palps dark with the first segment and the base of the second segment orange. Head much broader than the pronotum; flat or with only a very weak longitudinal ridge between large eyes that occupy most of the margins. Pronotum elongate, broadest near the middle (which can be seen to be obtusely angled when viewed obliquely from above) and narrowed to obtuse angles and curved basal and apical margins, surface unevenly convex and not, or only very weakly, depressed medially towards the base. Elytra quadrate or nearly so and gently curved from sloping shoulders to recurved apical margins, surface uneven, the punctures confluent in places. Basal abdominal tergites strongly bordered, weakly impressed and strongly punctured across the base and with at most only very weak median keels, apical tergites only weakly bordered and much more finely punctured. Legs long and slender with the tarsi almost as long as the tibiae. Tarsi without bilobed segments.
Stenus biguttatus 1
From Tottenham, 1954
Very similar to S. comma LeConte, 1863 but here the head is distinctly raised between the eyes and the second maxillary palpomere is either entirely dark or only very narrowly pale across the base. Males can always be separated by the form of the aedeagus; in comma the setae at the apex of the parameres are much longer than the internal setae while in the present species the apical setae are much shorter than the internal setae.