Stenus guynemeri Jacquelin du Val, 1850

This very local Western Palaearctic species occurs mostly in upland and mountain areas of Western and Central Europe and North Africa; in the south it extends from Portugal to Austria and Hungary and to the north it reaches Germany, Poland and the UK which seems to be the northernmost limit of the distribution. A further subspecies, S. g. hercules Puthz, 1967 is known from Spain and North Africa.  Many European records are from mountain altitudes between 1000 and 1500 m where it is often common but it seems to be very scarce outside this range. In the UK it is widespread and occasionally common in Wales and Northern England and scattered and more local in Northern Scotland and Northern and Western Ireland, beyond this there are scattered records from the South eastern England, the midlands and North and South Devon. A hygrophilous and cold-loving species, adults typically occur in fissures on rock walls or on gravel slopes or under stones and debris on sparsely vegetated banks of upland rivers and streams, they also occur among sand, gravel and mosses near springs, cascades and waterfalls, in which cases they often occur alongside Dianous coerulescens (Gyllenhal, 1810). Little is known of the biology but adults occur year-round, they are active in all but the coldest winter periods and generally active from March until October, peaking in abundance during May and June and again in late summer.

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5.6 mm. A rather broad and robust species, body strongly sculptured and shiny black, antennae black, palps brown, usually with the segments darker apically, legs bicoloured; black with the femoral bases and median parts of the tibiae yellow. Head strongly and densely punctured and with fine pale pubescence, with three longitudinal ridges one along the centre and one beside each eye. Pronotum quadrate, broadest in front of the middle and rather strongly constricted before rounded posterior angles, anterior angles obtuse or rounded, basal and apical margins weakly curved, surface strongly punctured and very uneven. Elytra quadrate, much longer than the pronotum and smoothly curved from rounded shoulders to recurved apical margins, surface roughly and in placed confluently punctured. Abdomen strongly bordered and roughly punctured throughout, with yellowish pubescence that tends to lie obliquely and to be longer and more striking than that on the pronotum and elytra, basal tergites depressed across the base but without keels. Legs slender and very long, with tarsi as long as the tibiae. All tarsal segments simple, claws long, slender and not, or only very weakly toothed at the base. Aedeagus produced at the apex.