Rugilus orbiculatus (Paykull, 1789)
This is a generally common species occurring from lowlands to about 1900m throughout Europe north to the UK and central Fennoscandia, it also occurs across Mediterranean Africa including the Azores, Canary Islands and Madeira, the Caucasus, Asia Minor and extends into southern Siberia, it has been recorded from Australia and, following introductions, is now established and widespread across North America, having been first recorded in the early twentieth century. Here it is common throughout Wales and southern England, though more local and rare in the West Country, and extends sporadically north to the Scottish Highlands. Adults occur throughout the year and peak in abundance from April to June, they may be found in a wide range of habitats including woodland margins, grassland, agricultural land and gardens, they also occur among debris and litter on wetland margins and damp environments generally. In northern Europe they occur under debris and among litter in salt marshes and may be very abundant following storm surges and high tides. During the warmer months they may be swept from grass and herbage in just about any situation but otherwise they occur among decaying plant material such as deep leaf litter, compost and hay stacks; they may be abundant in decaying grass cuttings and stored dung and straw mix, during the autumn we have often found them among decaying fungi and they also occur among winter flood refuse. This is generally our most frequently occurring member of the genus and should soon be found when sieving suitable material, adults superficially resemble ants, though they tend to move very slowly, and while the genus will quickly become familiar they will need to be examined carefully as there are several closely similar species.
4-5mm. Body dark brown to black with yellow elytral apices, antennae pale to dark brown, often paler apically, legs pale with the femoral apices and tibiae variously darker. Habitus very distinctive; with a broad head and very narrow neck, strongly tapering pronotum and broad elytra. Head slightly transverse with large eyes that occupy about half the lateral margin and long temples converging to a straight or weakly curved basal margin, mouthparts pale; mandibles curved and prominent, each with a prominent internal tooth, vertex smooth and weakly-convex, densely and moderately strongly punctured throughout or, occasionally, with a small unpunctured area at the centre. Pronotum quadrate or slightly elongate and broadest about the middle, tapering to a straight basal margin and strongly tapering to a very narrow anterior margin, surface densely and quite strongly punctured but for a smooth and shiny central longitudinal strip, sometimes with a weakly-defined longitudinal furrow towards the base. Elytra distinctly longer than the pronotum; quadrate with broad shoulders, an almost straight basal margin and weakly curved lateral margins, widest across the middle and at least as wide as the head measured across the eyes, surface finely and diffusely punctured and pubescent, bark brown to black and always with a distinct pale macula at the posterior angle that generally extends along the base. Abdomen broadest about the middle, tergites 1-4 with strongly raised margins, all tergites finely and densely punctured. Male anterior tarsi slightly broader than those of the female, apex of sternite 8 notched at centre, simple in the female.