Trichonyx sulcicollis (Reichenbach, 1816)
This generally rare species occurs from lowland to low mountain altitudes throughout most of Europe except for the Iberian Peninsula and the north of Fennoscandia, it extends east into northern Asia and has become established in North America following introductions from Europe. In the UK it is a very local and rare species which is thought to have declined in recent decades; prior to 1970 it was recorded from about a dozen sites across southern England from Somerset to West Sussex but since that time there have been fewer records and it seems to be much more restricted and local. Adults are predatory, feeding on mites and springtails etc, they occur through the summer and have also been recorded in December, they are associated with decaying wood, especially oak and elm but also beech in ancient broad-leaved woodland, wooded pasture and parkland. They typically occur among soil and debris or in old dry wood or under bark at the base of dead stumps, often along with various ants and occasionally within their nests; in the UK with Lasius brunneus (Lat.), less often with L. niger (L.), and on the continent also with L. fuliginosus (Lat.) and Formica polyctena Foerster.
Our largest species of Pselaphinae, Trichonyx is easily identified by its broad form coupled with short maxillary palpi relative to the antennae; in all other 'broad' species the palps are at least half the antennal length. 2.5-3.5mm. entirely reddish-brown, often with the appendages a little paler, dorsal surface with long, pale pubescence. Head quadrate and distinctly narrower than the pronotum, vertex with a deep horseshoe-shaped impression, eyes small, strongly convex and coarsely faceted, and temples broadly rounded to a truncate basal margin. Anterior clypeal margin bordered, labrum weakly emarginate and mandibles produced forward. Antennae inserted laterally in front of the eyes, basal segment elongate and broad, 2-8 quadrate to elongate, 9 and 10 larger and broader and the terminal segment long and broadly oval, longer than 9 and 10 combined. Maxillary palps about as long as antennomeres 1-3, penultimate segment small and quadrate, terminal segment long and acutely pointed. Pronotum broadest in front of the middle and strongly narrowed to a short neck and obtuse posterior angles, surface with scattered very fine punctures and a median longitudinal impression which joins a roughly-sculptured transverse impression across the basal margin. Elytra evenly curved and broadened from narrow rounded shoulders to slightly obtuse posterior angles, basal margin almost straight but very strongly sinuate just before the margin, lateral margins strongly bordered, surface finely punctured throughout, each with a strongly-impressed sutural stria and a curved stria inside a humeral prominence extending into the basal third or half. Three basal abdominal tergites with strongly raised lateral margins, all tergites finely punctured and with long pale pubescence. Tarsi 3 segmented; the basal segment very small and the second and third long and sub-equal, all tarsi with a single claw.
In the male the middle tibiae have a small spur at the internal apical angle and all femora are slightly thicker than those of the female.