Plegaderus dissectus Erichson, 1839

Formerly considered a great rarity (Allen, 1968) this tiny histerid is now very local but widespread across southern England with modern (post 1980) records west to north Somerset and north to south Lancashire and Nottinghamshire (NBN). There are also a few records from eastern Wales; Dyfed-Powys (Hyman and Parsons). The adults, which are probably predatory, inhabit ancient broad-leaved woodland, wooded pasture and wooded fens, typically living within ancient standing or fallen timber in varying stages of decay and occasionally under sappy bark (Alexander 2002) They have been recorded from March to October and recorded hosts include elm, poplar, horse chestnut and a red rotten birch log but most records are from beech and oak. They are rarely, if ever, found on conifers. To date our only record is from Cassiobury park (August 2008) when a single adult was extracted from a grass tussock taken from the base of a beech stump among numerous ants (Lasius brunneus), the stump was completely devoid of bark, hollow and powder dry. Samples taken from this site earlier in the year, as well as two subsequent samples, failed to reveal the species.

We have recorded another specimen from a sample of oyster mushroom fruiting upon fallen beech in Whippendell wood (Jan 2009).

A further species, Plegaderus vulneratus Panz. is a recent addition to the British list and so is not included in Joy or Halstead, it keys out with P.dissectus in both works and is described by Allen (1968). See ID Aids.

1-1.5mm. Our Plegaderus spp. are distinct from other British histerids by the possession of a transverse pronotal furrow. Elongate and broad, upper surface dark pitchy brown to black with margins, especially to pronotum lighter. Legs and segment one of antennae dark rufous, remainder of antennae lighter. Head smooth and shining (X40), finely punctate, base behind posterior margin of eyes densely microsculptured. Antennae inserted beside eyes, scape strongly curved, segments 2 and 3 elongate, 4-8 quadrate, with a rounded and slightly elongate club. Pronotum transverse (29:21), front angles sharp and obtuse, sides strongly sinuate and broadest before middle, hind angles produced to a sharp oblique point. Strongly furrowed inside lateral margins and transversely furrowed near middle, this meets the lateral furrow but does not extend beyond it to the side margin. Finely and quite densely punctured in front of furrow, as head, much more sparsely behind. Puncturation along side border stronger. Scutellum tiny but visible. Elytra transverse (33:28), broadest behind middle and narrowed to apex, truncate leaving pygidium and propygidium exposed. Surface shining, randomly and quite densely punctured, punctures much stronger than those on pronotum. Surface slightly depressed alongside weakly raised suture, with two obvious striae; an outwardly oblique stria to about middle and a smaller oblique stria outside humerus, traces of other laterally before middle sometimes appear as a series of confluent punctures. pygidium and propygidium strongly and randomly punctured, cuticle shining with a weak blue metallic lustre. Front tibiae characteristic; broadened and parallel in anterior half with a series along outer edge. Meso and Meta-tibiae gradually widened from base to apex, sides straight.

Allen, A.A. 1968. Plegaderus vulneratus, A histerid beetle new to Britain. Ent.Mon.Mag. 104:110-112