MICROPEPLINAE Leach, 1815
These tiny rove beetles are widespread but not usually abundant. They develop in decaying organic material and can often be found dispersing in the summer months.
This is a small group of about eighty species included in (variously) five genera. They were previously classified as a separate family but are now included in the Omaliinae group of subfamilies within the Staphylinidae. Most of the species, about sixty, are included in the genus Micropeplus Latreille, 1809, which is distributed in the Holarctic, Oriental, Neotropical and northern and central African regions, it is most diverse in the eastern Palaearctic; 14 species occur in North America and 19 in Europe, of which five extend north to the U.K. Members are sometimes divided into six species groups or subgenera or may be included in distinct genera e.g. our UK species M. tesserula Curtis, 1828 is often included in Arrhenopeplus Koch, 1937 and is so listed in the latest Palaearctic checklist (Löbl & Löbl, 2015). Regarding the European fauna, five species are sometimes included in Arrhenopeplus according to the form of the penultimate abdominal tergite; in Arrhenopeplus it is smooth and evenly convex to oblique sub-lateral grooves whereas in Micropeplus it is carinate and often deeply impressed medially between sub-lateral grooves, but this generic distinction is not universally accepted and remains controversial. Cerapeplus Löbl & Burckhardt, 1988 includes two species, from Thailand and China. The monotypic Kalisus LeConte, 1874 occurs in North America. Peplomicrus Bernhauer, 1928, formerly included in Micropeplus, includes about 12 species and is widespread in tropical Africa and Central and South America. The subfamily is absent from Australasia and South Africa. All genera include very distinctive species that are immediately recognized within the family; small <3mm, broad and flattened with distinct ridges to the head, pronotum and elytra, the antennae are 8 or 9 segmented with a one segmented club, the elytra are short, leaving five or six articulated abdominal segments exposed, and the tarsi are four segmented. In some groups e.g. the monotypic Nearctic genus Kalissus LeConte, 1874 the ridges are confined to the elytra and are variously developed, often convex with fine longitudinal ridges. They occur among decaying vegetation, leaf-litter, fungi, mammal and bird nests etc. in a wide variety of situations from lowlands to alpine meadows and wetland marginal situations and all are thought to be saprophagous or mould-feeders. Adults of various species-and probably the majority- occur year-round; during the winter they may be extracted from decaying vegetation or fungi and in the warmer months may be swept from grassland etc or may be found crawling on the surface of damp soil, many have been swept in flight during the evening and they occur regularly in flight-interception traps.
The subfamily is represented in the UK by five species of Micropeplus, as listed in the 2018 checklist. They are distinguished among our fauna by the following characters although all are readily recognized by the general form. Small, shiny and drab-coloured species, 1.0-2.5mm, broadly-oval and flattened with a strongly sculptured dorsal surface. Head transverse, variously and strongly impressed, with prominent convex eyes and 9-segmented antennae. Antennae moderately long; two basal segments enlarged, 3-8 progressively less elongate towards the apex and the terminal segment forming an abrupt and broad club. Pronotum transverse with deep and wide impressions, often delimited by raised ridges, lateral margins explanate; smooth or strongly angled and finely to strongly toothed. Prosternum with deep grooves for the reception of the antennae. All coxae widely separated, tarsi 4-segmented but may appear 3-segmented due to the very small first segment. Elytra transverse and sub-parallel, exposing at least four abdominal tergites, with distinct longitudinal carinae, interstices vary from smooth to very strongly punctured. Abdomen broad and short, the tergites with variously developed but always with distinct longitudinal carinae. Males of all UK species are distinguished by the head being produced into a point anteriorly. Our species provide a reasonable insight to the subfamily as most of the variation concerns the development of the dorsal structure e.g. the elytral and abdominal carinae are sometimes represented by broad and shiny tubercles. The only possible confusion might be with Pseudopsis sulcata Newman, 1834 (our only member of the Pseudopsinae Ganglbauer, 1895) which is only slightly larger and has longitudinal ridges to the pronotum and elytra but here the antennae are 11-segmented and lack a club. The five UK species may be distinguished by the following key, adapted from Tottenham, 1954.
Each elytron with four longitudinal ridges, including the sutural ridge, the cuticle between shiny and very finely punctured. Penultimate tergite evenly convex across the middle. [Head with two raised elongate tubercles separated by a V-shaped impression. Pronotum densely and strongly punctured, with a wide and shallow impression either side on the disc and rather evenly rounded laterally; anterior angles sharp and perpendicular, posterior angles obtuse, lateral margin with four small teeth.] Shiny dark brown to black with the lateral margins of the pronotum, antennal bases and legs pale red to yellow. Generally the smallest species at 1.5mm
Each elytron with four or five ridges, including the sutural ridge, the cuticle between strongly and densely punctured. Penultimate tergite medially sculptured.
Each elytron with five ridges, including the sutural, together wider than the pronotum which is distinctly angled laterally. Head with a median basal furrow.
Each elytron with four longitudinal ridges, including the sutural, together narrower, or at most only as wide as, the pronotum which is smoothly curved laterally. Head with two or more basal furrows.
Dorsal surface shiny; vertex and pronotal disc punctured and densely rugose so that the longitudinal carinae are obscured. Dark brown to black with the lateral pronotal margins dark red. Smaller species, 1.7-2.0mm.
Dorsal surface dull, vertex and pronotal disc punctured and finely rugose so that the longitudinal carinae are distinct. Body entirely black or very dark brown, sometimes with a small pale macula towards the lateral margin of the pronotum. Larger species, 2.0-2.5mm.
Elytral base distinctly narrower than the pronotum measured across the posterior angles. Vertex with two indistinct furrows which converge anteriorly and are flanked each side by a round depression. Central carina of the penultimate tergite produced into an acute tooth which projects above the apical declivity when viewed from the side.2.0mm.
Elytral base only slightly narrower than the pronotum measured across the posterior angles. Vertex with five often indistinct converging furrows. Central carina of the penultimate tergite simply obtuse, not at all projecting, when viewed from the side. 2.0mm.
Micropeplus tesserula Curtis, 1828
Micropeplus tesserula is a mostly boreal-mountain species considered to be an ice-age relict across much of western Europe; it is generally rare and sporadic, reaching France in the west and Asia Minor and Russia in the east, and extending from northwest Africa north to the UK and above the Arctic Circle in Scandinavia. It is also quoted as occurring in both the Nearctic and Neotropical regions in the Palaearctic catalogue. Typical habitats are forest margins and clearings where they occur under decaying bark or among moss, leaf-litter or decaying plant remains. Here it is widespread in southern England and Wales, extending north to south Yorkshire but absent from the West Country; records are widely scattered and it is generally very local and rare.
Micropeplus caelatus Erichson, 1839
Micropeplus caelatus is a very rare European species with a mainly northern distribution; it occurs sporadically across the north to the south of Sweden and the UK and there are a few widely scattered records from further south including France, Italy and the Caucasus. In many parts of this distribution it has declined over recent decades, presumably due to the draining of many of its habitats, e.g. it has not been recorded from Denmark since 1950 and the modern UK distribution, which formerly included the Somerset levels, is restricted to south-west Ireland where it is very rare. Typical habitats are wetland margins on acid moorland and sphagnum bogs with areas of overgrown vegetation, adults are mainly recorded in the autumn among accumulated decaying plant remains or under debris in wet habitats, and during warmer spells may be found in the open crawling on exposed wet soil.
Micropeplus porcatus (Paykull, 1789)
Micropeplus porcatus is the most common member of the family in Europe, it occurs from North Africa to southern Fennoscandia and the UK, and east into western Russia, here it is widespread with records from the south coast to Orkney and the Western Scottish Islands but they are sporadic and widely spaced and it is generally rare or perhaps under-recorded. The species occurs from lowland to low mountain altitudes among decaying vegetation, often in wetland or along river valleys but it is also synanthropic, occurring among decaying straw and hay in barns and haystacks or among compost heaps in domestic gardens.