Laccornis oblongus (Stephens, 1835)
This species is generally quoted as Holarctic in distribution but this may be contentious as its presence in North America is based on a single record from northwest Canada, it is widespread in northern and Central Europe from France to Russia including the UK and parts of Fennoscandia although it may have declined as it was known from only a single locality in Norway and its recent presence needs confirmation, it was formerly widespread in Sweden, extending north to the Arctic Circle, but may now be regionally extinct; it is seems to be widespread in eastern Fennoscandia, Denmark and various Baltic countries but is generally very local and uncommon, to the east it extends through northern Russia into eastern Siberia. Here it is widespread though very local and scarce in England and southern Scotland with records centred around East Anglia and North Somerset, is absent from the islands and known from only a single Welsh locality but is widespread through Ireland with many records from the north east. Adults occur year-round and peak in abundance during May, they breed from early spring and larvae develop rapidly, passing through three instars, all of which have been recorded in June on the continent, new-generation adults occur from early summer and will go on to overwinter. Typical habitats in the UK are shallow weedy or mossy areas of fens and marshes, small mossy pools and peat cuttings and they are only rarely found in flowing water, but in northern Europe they also occur in woodland pools rich in organic matter, in the north of Fennoscandia it is confined to seasonal pools in conifer forests or to marshes rich in Carex and Sphagnum. Both adults and larvae are thought to be predatory and adults may be flightless although the distribution might suggest otherwise. Our only record is of a single specimen swept from a small peaty pool near Shapwick, North Somerset on 27/5/2013.
At 4.5-5.0mm this species is among the largest of our Hydroporinae, this large size, rather parallel-sided habitus and (usually) distinctly bicoloured appearance is usually sufficient to identify the species in the field. Head and pronotum extensively black or dark grey, the anterior margin of the head and lateral margins of the pronotum paler, usually brown, elytra brown and usually lighter across the base, sometimes with the suture and lateral margins narrowly and obscurely darker, legs entirely pale brown, antennae pale with at least the apices of the distal segments darkened. Dorsal surface finely microsculptured and punctured; the pronotum with larger punctures towards the margins and the elytra with moderately large punctures throughout. Distinguished among our water beetle fauna by the lack of a visible scutellum or oblique epipleural line under the elytral humerus and having a broad and flat prosternal process finely bordered around the apex. Males may be distinguished by their dilated basal front and middle tarsal segments and a longitudinally-impressed apical abdominal sternite.