PLEGADERINI Portevin, 1929 

Plegaderus dissectus Erichson, 1839

This widespread Western Palaearctic species occurs throughout Europe from the Mediterranean north to Scandinavia and the UK and east to western Russia, Asia Minor and Iran; throughout most of this range it is rare and of very local occurrence e.g. in Poland it is very rare and sporadic in the south and west, in Germany it is classified as vulnerable and in Sweden as threatened. In the UK it has been considered historically as very rare but nowadays id locally common across south and central England and there are a few scattered records from North Somerset and eastern Wales. The typical habitat is deciduous or mixed woodland or parkland etc. where they inhabit decaying stumps and logs, often where the wood is damp. The preferred wood seems to be oak and beech but they also occur on elm and locally we find them on dead horse-chestnut and lime stumps, and they only very rarely occur on conifers. Adults are active from March to October and may be found under bark or logs during the day but they are active on the surface at night and are easily seen by torchlight, they also occur among accumulated wood debris in hollow stumps etc. and locally we have recorded them from bark and tussock samples from around old stumps through the winter. Locally we often find them on stumps and trees hosting the ant Lasius brunneus (Latreille, 1798), and in the UK there may be an association with Dorcus burrows, in Sweden they are associated with the monotomid Rhizophagus brancsiki Reitter, 1905 and in Poland with the ants Lasius fuliginosus (Latreille, 1798) and Formica canicularia Latreille, 1798. Both adults and larvae are predators of small larvae and eggs etc. under bark or among wood debris.

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Our two species of Plegaderus Erichson, 1834 are distinct among the UK histerid fauna in having a transverse furrow across the pronotum, and P. vulneratus (Panzer, 1796) is distinct from the present species as outlined above. 1.0-1.5mm. Elongate and broadly oval, dark brown to almost black, or with the lateral margins and (usually) appendages lighter and characteristically broadened behind the base of the elytra. Head smooth and shiny, finely punctured (X40) and densely microsculptured towards the base and with small, moderately convex eyes. Antennae inserted by the anterior margin of the eyes; scape broad and curved, segments 2 and 3 elongate, 4-8 quadrate and 9-11 form an abrupt and slightly elongate club. Pronotum weakly transverse with sharply obtuse anterior angles, acute, obliquely-produced posterior angles, and sinuate lateral margins that are broadest before the middle. The median transverse furrow meets the sub-lateral furrows but does not extend to the lateral margin. Punctation fine anteriorly, as the head, becoming stronger towards the base and outside the lateral furrows. Scutellum very small but visible. Elytra transverse; broadest before the middle and narrowed to a truncate apex leaving the pygidium and propygidium exposed, the surface shiny with scattered large punctures similar to those at the base of the pronotum. Disc weakly but distinctly depressed adjacent to the suture, with two obvious striae; one from the near the middle of the base and one below the shoulders, and there may be others, especially towards the lateral margins, that sometimes appear as series of confluent punctures. Pygidium and propygidium strongly punctured, the cuticle shiny and often with a faint blue metallic lustre. Legs long and flattened, pro-tibiae characteristically and abruptly widened towards the apex and with a series of stiff setae along the external margin. Meso- and meta-tibiae gradually widened from the base and straight or only weakly curved externally.