PLEGADERINI Portevin, 1929
POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
ABRAEINAE MacLeay, 1819
Plegaderus Erichson, 1834
P. dissectus Erichson, 1839
P. vulneratus (Panzer, 1796)
This small tribe includes about 40 species in 3 genera and is most diverse in the Holarctic region. Eubrachium Wollaston, 1862 contains 5 mostly western Palaearctic species including one from Morocco and one from the Canary Islands. The 5 species of Phloeolister Bickhardt, 1916 are restricted to South Africa (Zimbabwe). Plegaderus Erichson, 1834 includes about 30 species in 2 subgenera and is Holarctic with several recorded from northern South America. Subgenus Hemitrichoderus Reichardt, 1941 is monotypic; P. h. Adonis Marseul, 1876 occurs in Asia Minor. Plegaderus s.str. includes about 27 species, 14 occur in North America and 9 in central Europe of which 2 extend north to the UK.
Among the histerids this tribe is recognized by the form of the pronotum which has a sub-lateral groove and, in most species, a transverse groove in the posterior half which may or may not reach the lateral margins. The prosternum is also distinctive; there is a wide median cavity edged with dense setae and a pair of variously developed wide and deep sulci either side, the median cavity divides the prosternum into 2 lobes, the anterior of which extends to the front margin and the posterior lobe divides the widely spaced coxae. In most species there are well-developed sub-lateral antennal cavities.
Species occur under the bark of both broadleaf and coniferous trees, and many inhabit the galleries of other wood-boring beetles where they prey on larvae and eggs of other insects as well as other arthropods. In Europe some species are attracted to scolytid pheromone traps.
The two UK species are readily identified:
Transverse pronotal furrow extending to the lateral margins. Elytra without an oblique stria under the anterior lateral angles. Larger, 1.2-1.8mm.
Transverse pronotal furrow extending only to the sub-lateral groove. Elytra with an oblique stria under the anterior lateral angles. Smaller, 1.0-1.4mm.
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.
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.
Plegaderus vulneratus (Panzer, 1796)
Added to our list in 1968 from specimens taken in 1963 (Allen, 1968), P.vulneratus Panzer is now a very local species of south and central England and South Wales. On the continent it occurs throughout Europe extending north to Scandinavia and east to western Russia, and in many countries e.g. Poland and Slovakia, it is the most common member of the genus. Mostly associated with pine in plantations and wooded areas the species also occurs on other conifers and has been recorded widely from oak and poplar, adults are active through the spring and summer until October and will generally be found in the galleries of other wood-boring beetles. Both larvae and adults are thought to be exclusively predatory, the larvae develop over 2 years and the winter is passed as either a final instar larva or an adult.
Very small and distinctive among our fauna; compared with the more common P. dissectus, the present species is a little larger and darker; usually completely shiny black. The pronotal side margins are straight in the basal half and produced into a sharp oblique tooth at the posterior angles, the front and hind margins are strongly sinuate and the transverse furrow, which is only weakly impressed, reaches the side margin anterior to the middle. In outline the elytra are evenly and gently curved from base to apex and lack the abrupt basal widening present in P.dissectus, they also lack the oblique dorsal striae at the base and are not depressed adjacent to the suture. The front tibiae are evenly curved and broadened from base to apex; the outer margin a single curve and the apical third to half with a series of short and stout spines.
Allen, A.A. 1968. Plegaderus vulneratus, A histerid beetle new to Britain. Ent.Mon.Mag. 104:110-112