Philonthus marginatus (Müller, O.F., 1764)
Through much of its continental range this is a generally scarce and sporadic rove beetle, becoming more frequent in northern regions; it is widespread throughout central and northern Europe from the Pyrenees and northern Italy, extending east into Russia and north to the Arctic Circle in Scandinavia and the Faroe Islands, it occurs from lowland to mountain altitudes; up to 2500m in the Alps. In the UK it is generally common throughout, extending north to the Western Scottish Islands and Orkney although it seems to be absent from the West Country. Adults occur year-round in a wide range of habitats e.g. open woodland, damp grassland, heathland, marginal areas and, in northern continental regions, on peat bogs, pine forests and burned areas of bogs and heaths, but they are most frequently encountered on dung pasture. Both adults and larvae are predatory, more especially on diptera larvae, and so may occur wherever the prey is abundant; decaying vegetation and fungi, carrion, mammal nests and, more typically, dung. Adults become active in the spring when they migrate by flight from overwintering quarters to their breeding sites and at this time may be seen flying above dung pasture in hot weather, they are attracted to freshly deposited dung upon which pairs will form up and mate, but once the dung has begun to dry and form a crust it is no longer attractive to them. After mating the females are no longer receptive, they become very aggressive and run rapidly over the dung searching into holes for prey, after a period of feeding they enter the pat from ground level and lay eggs singly a few cm from the margin. Larvae soon emerge and their development is very rapid, they pass through 3 instars and the cycle from egg to adult is completed within 18 to 25 days. Adults thus become common from late spring or early summer and remain so into the autumn when they leave the breeding sites to overwinter among litter and tussocks etc. in woodland, hedgerows and grassland margins etc. although they may remain active very late into the year as we have found them to be common among decaying terrestrial bracket fungi during early December in local woodlands. Sampling adults is straightforward; in hot weather they may be swept in flight above dung pasture or found by immersing dung samples, sieving compost etc. or searching through likely material and, at least in the autumn, they are nocturnally active and readily found among decaying fungi, usually along with many other rove beetles.
Identification is simple due to the bicoloured pronotum. 7-9.5mm. Head slightly elongate, the eyes large and continuous in outline with the rounded temples, vertex with linear microsculpture, scattered large punctures posterior to the eyes and a pair of punctures close to the anterior margin of the eyes. Antennae black with the first segment and often the base of the second segment pale, at least ventrally, maxillary palps extensively pale but usually darkened towards the apex. Pronotum broadest in front of rounded hind angles and narrowed to the anterior margin, black with broadly pale lateral margins and with a longitudinal series of 4 setiferous punctures either side of the middle. Elytra quadrate, much broader than the head; densely and strongly punctured and with recumbent long, pale pubescence. Abdomen with strongly raised lateral margins, shiny and sparsely punctured and pubescent. Legs pale; front tarsi strongly dilated in both sexes.