Stenus boops Ljungh, 1810
This species is generally common throughout the Palaearctic region from Portugal and Asia Minor to the far east of Russia, China and Japan, it occurs from lowlands to lower mountain altitudes from the Mediterranean to the UK and the far north of Fennoscandia and Russia, increasing in abundance with latitude. In the UK it is common and often abundant throughout England and Wales, including all the islands, and sporadic and very local further north to Orkney and across much of Ireland. Typical habitats are wetland marshes and permanently wet areas of floodplains; they are often abundant on sandy or silty substrates beside rivers and streams and among patchy vegetation beside standing water and may occur in numbers beside ponds on dune slacks and at cliff seepages, in general they occur in open habitats exposed to the sun but in Northern Europe they are also common beside rivers and ponds in deciduous and mixed woodland. Adults are present year-round, peaking in abundance in early spring and late summer; they overwinter among litter or in tussocks away from marginal areas and return in early spring although we have found them active in numbers on damp soil during December, January and February, they are diurnal and spend much of their time among vegetation on wet soil but during the warmest days will be found in abundance running in the open. Adults are wing dimorphic, they do not climb vegetation and, typical of the genus, both adults and larvae are terrestrial predators of springtails and other small invertebrates. Little is known of the biology but the phenology would suggest breeding occurs in the spring and new-generation adults appear in late summer, in most years adults become scarce during the warmest parts of the summer suggesting an aestivation or, more likely, a general dying-off of the current generation. Sampling is easily achieved by searching among marginal substrate and debris, on warm days they are active but they can run rapidly and will vanish into cracks or under stones when disturbed, they often occur in numbers in pitfall traps but these can be very destructive in marginal situations.
Stenus boops 1
Stenus boops 2
From Tottenham, 1954
4-5 mm. A rather nondescript species, narrow, strongly punctured and entirely black, it soon becomes familiar with experience but specimens should be taken for critical examination – and a series of males should be dissected – as there are several superficially similar species in the same habitats. Head and pronotum strongly and densely but discretely punctured, elytral punctation similar but confluent in places, especially towards the apical and lateral margins, dorsal pubescence short and sparse, often extensively inconspicuous. Head transverse and distinctly wider than the pronotum, with massive eyes that occupy the lateral margin and the labrum at most as wide as the interocular distance, surface broadly but only weakly depressed between the eyes and a weakly-raised central area, maxillary palps pale towards the base, antennae substantially dark, and often a little paler from the fourth or fifth segment. Pronotum elongate, broadest in front of the middle and narrowed to a curved apical margin and near-perpendicular posterior angles, surface evenly convex and usually without a longitudinal median depression towards the base but a short or irregular channel is sometimes present. Elytra quadrate or slightly elongate, much broader than the pronotum and about as broad as the head, with sloping shoulders and curved lateral margins (these are more developed in macropterous specimens), surface distinctly uneven due to lateral and apical depressions. Abdomen tapering from the base and strongly bordered, basal tergite with four incomplete longitudinal keels from the base, tergites 1-4 moderately strongly but not densely punctured, apical tergites more finely and sparsely punctured, all tergites with distinct microsculpture, in females the eighth tergite is longer and narrows to a point. Legs long and slender, the posterior tarsi much shorter than the tibiae and the tarsi without strongly or deeply lobed segments. Aedeagus distinctive; the parameres narrow and extending only a little beyond the median lobe which has a median ridge within the strongly narrowed apical part.