Stenus fornicatus Stephens, 1833
Widespread and locally common across Europe and parts of North Africa and also recorded from the eastern Palaearctic region, this species extends from the Mediterranean, including many of the islands, to the Black Sea and north to the UK, Denmark and the Baltic countries where it reaches into southern provinces of Norway, Sweden and Finland. In the UK it occurs locally in south-eastern England and East Anglia and there are scattered records from Hampshire, Somerset, the midlands and North Wales, it is absent from Scotland but widespread though very local in Ireland. The species is strongly hygrophilous and usually occurs in densely vegetated marshes, swamps, wet reed beds and permanently wet parts of fens and drainage ditches, but it also favours disturbed sites such as ponds in parks, fields and urban situations and old watercress beds etc. Adults are active in bright sun, climbing stems and foliage of sedges and reeds etc., often in company with other climbing species such as S. solutus Erichson, 1840 or S. cicindeloides (Schaller, 1783), but they otherwise remain hidden low down among litter and moss etc., they can move quickly when necessary, both on the ground and on the water surface, and may drop from vegetation quickly when disturbed. During late summer when ponds tend to dry out they may remain under damp matted vegetation or debris until the water returns, they overwinter under bark or among marginal moss or tussocks etc., and we have found them under bark on small floating branches in a local parkland pond during February, and among bark on fully submerged logs during April. Little is known of the biology but adults are present year-round and peak in abundance during April and May and again in late summer. They may be sampled by searching or sweeping among dense vegetation, floating debris is always worth careful examination, and they sometimes occur among winter extraction samples.
Stenus fornicatus 1
From Tottenham, 1954
Among the smaller but more distinctive of our Stenus species; the broad elytra and strongly tapering abdomen should be easily recognised in the field. 2.5-3.0 mm. Body shiny black, strongly punctured and with very fine pale pubescence throughout, antennae brown with the clubs mostly black, palps substantially dark but usually pale towards the base, legs dark with the tibial bases narrowly pale and the tarsi dark to pale brown. Head slightly wider than the pronotum, flat and uneven but not distinctly ridged between large eyes that occupy most of the lateral margin, temples short and horizontal before a wide and bulging neck. Pronotum quadrate, smoothly curved laterally to obtuse or rounded angles and smoothly convex, without impressions or a median furrow. Elytra much wider than the pronotum; distinctly transverse (about 3:2.5), broadest behind the middle and smoothly curved from rounded shoulders to recurved apical margins, punctures on the disc less dense than those on the pronotum. Abdomen unbordered and strongly tapering, four basal tergites strongly punctured, apical tergites less strongly so and appearing dull due to very fine microsculpture. Legs relatively short with middle and hind tarsi distinctly shorter than the corresponding tibiae. Fourth tarsomere strongly bilobed, the narrow lobes reaching forward to about halfway along the terminal segment. Aedeagus characteristic.