Lathrobium elongatum (Linnaeus, 1767)
This locally common Western Palaearctic species occurs throughout northern and central Europe extending south to northern Italy and the north of the Balkan Peninsula and north to the UK and Fennoscandia, to the east it reaches Lake Baikal but records from further east have proved to be other species. Here it is generally common across Wales and central and southern England but generally rare in the West Country and further north to the Scottish border, beyond this there are scattered records from the Scottish Highlands and the west of Northern Ireland. Adults occur in most permanently damp situations e.g. marshland, well-vegetated wetland margins, floodplains and damp woodland, often with fluctuating water levels and usually on clay or fine silty soil. They are present year-round and generally common in the spring and autumn, during the winter and early spring they may be found they may be found in grass tussocks or under debris on damp soil that is not prone to flooding although they also occur in flood refuse, and during the warmer months among matted vegetation or reed litter etc beside shallow water or on permanently wet soil. Pairs will often be found together but they usually occur only in small numbers, they are diurnally active and during the warmest weather will sometimes be seen running on damp soil but they are also active at night. Sampling simply involves sieving or searching through marginal debris but specimens will need to be taken for close examination or dissection in order to be sure of their identity. So far as is known adults are always fully-winged. Reproduction is thought to occur in spring and early summer with larvae developing through the summer to produce new-generation adults in the autumn.
This medium-sized species is suggestive of certain Staphylininae but the reduced terminal maxillary palpomere and the lateral antennal insertions are diagnostic for the Paederinae, it is a very distinctive species by virtue of its size and (usually) bicoloured elytra but several other members of the genus e.g. L. geminum Kraatz, 1857 and L. pallidipenne Hochhuth, 1851are closely similar and so males will need to be dissected and the terminal abdominal tergite of the female examined. 8-9mm, long and rather parallel-sided with the pronotum about as wide as the head but narrower than the elytra, entirely shiny black with the appendages and the apical half to two-thirds of the elytra red, the elytral colour may be very dark and individuals with entirely dark elytra occur but they are rare (see below). Legs red with darker coxae. Dorsal surface finely pubescent, forebody and elytra moderately strongly but not densely punctured, abdomen much more finely punctured. Head with small eyes and long, parallel temples that curve to a broad neck which is about half the pronotal width, vertex weakly and evenly convex and with weak cellular microsculpture. Mandibles curved and projecting forward between the antennal insertions, apical margin of the labrum deeply excised. Antennae and palps entirely pale, all antennal segments elongate. Pronotum elongate and parallel-sided with rounded anterior and posterior angles, surface evenly and only weakly convex and lacking larger punctures or sculpture. Elytra elongate with broad shoulders and curved lateral margins, without striae but longitudinally impressed beside the suture, typically bicoloured but entirely dark specimens can be recognised by the size, broad neck and well-developed shoulders. Abdomen shiny black and finely pubescent; females may be identified among our UK species by the short and truncate eighth sternite, the deeply and roundly excised ninth tergite and the flat, apically rounded tenth tergite. Males may be recognized by the very distinctive form of the aedeagus; in side view apically truncated and acutely pointed, from below rounded at the base and evenly tapering to an acute apex, the dorsal process extending beyond the ventral process. Legs long and robust; middle and hind femora slender, front femora broadly expanded with a strong internal tooth, femora gradually widened from a slender base to a truncate apex, tarsi 5-segmented, and the front tarsi widely dilated in both sexes.