PAEDERINAE Fleming, 1821
With the exception of the distinctive Paederus species, these rove beetles tend to lead concealed lives among decaying organic matter in a wide range of habitats.
Although morphologically very variable, this group of mostly quite distinctive genera can usually be distinguished by the antennae being inserted outside the outer margin of the mandibles, usually laterally in front of the eyes but sometimes on the anterior angle of the frons, the insertions generally hidden by a narrow dorso-lateral expansion of the frons. The subfamily is divided into two tribes according to the structure of the maxillary palpi; in the Paederini Fleming, 1821-which includes all our UK species-the terminal segment is small and either rounded or tapering to a point, whereas in the Pinophilini Nordmann, 1837 it is securiform and generally at least as long as the penultimate segment. Worldwide the subfamily includes more than 6000 species in about 220 genera, Pinophilini includes about 26 genera and is mostly tropical in distribution with very few extending into temperate regions; 4 genera occur in the Nearctic region and 6 species of 4 genera occur in southern Europe, none of which have been recorded from the UK. Paederini includes the majority of the subfamily, it is cosmopolitan in distribution and diverse in temperate regions; more than 500 species of 31 genera and 8 subtribes occur in Europe and of these 62 species of 14 genera and 7 subtribes extend to the UK. Our species tend to be representatives of large and widespread genera e.g. of the more than 440 species of the near-cosmopolitan genus Astenus Dejean, 1833, about 85 occur in Europe and only 5 of these extend to the UK, Paederus Fabricius, 1775 is a cosmopolitan genus of more than 600 mostly tropical species which is only poorly represented in northern temperate regions; 15 occur in North America and 16 in Europe of which 4, including the Holarctic P. riparius (Linnaeus, 1758), occur in the UK.
Species occur in a wide range of habitats, all are predaceous and most lead concealed life-cycles among decaying vegetation in terrestrial situations although adults of Paederus and Paederidus are unusual in being diurnally active, climbing vegetation in search of prey etc. Paederus is a huge worldwide genus with most occurring in tropical regions and many island endemics, at least some are known to contain haemolymph toxins that cause skin irritations in humans and so the bright colours of the adults may be aposematic, a warning to potential predators, and mimicked by other non-toxic members of the subtribe. More generally adults of the subfamily may be found under logs and debris in well-vegetated situations or among compost or decaying fungi and leaf-litter but they are very rarely found in dung, on the continent some are associated with subterranean mammal nests and some have been found in ant nests but it is not known whether there are any specific associations. Many of our UK species tend to occur either in very dry or in wetland marginal situations.
The group is reasonably well-defined, belonging to the Staphylinine group of subfamilies and being most closely related to the Staphylininae, the two tribes are also well-defined and generally accepted but it seems very likely that some of the very large genera will be split, relevant to our fauna this has already begun with Lathrobium, but in a wider sense such large genera as Palaminus Erichson, 1839 (300 spp.) and Oedichirus Erichson, 1839 (> 320 species and almost cosmopolitan, about 110 of these endemic to Madagascar) among the Pinophilini and Astenus and Paederus, already split into many subgenera, are likely to be revised. Morphology and size vary greatly, even in restricted faunas such as that of the UK; our species vary from 2.5 to almost 10.0mm in size and the genera are mostly very distinctive, the colourful Paederus and Paederidus cannot be mistaken while species of Rugilus, Astenus and Scopaeus are rather ant-like with long legs and a large truncate head attached to a proportionally narrow pronotum by a very slender neck. Others are more conventionally staphylinid in appearance; elongate, sub-parallel and rather convex although our 2 species of Achenium Leach, 1819 are very flattened, an adaptation to their subterranean way of life. Certain larger species of Lathrobium might be confused with various Staphylininae, especially the more primitive groups such as Othiini or Xantholinini, but the two characters defining all our species; antennal placement and the diminutive terminal palpomere, will separate the present subfamily. Omaliinae are readily distinguished by the presence of ocelli, and Oxytelinae by the form of the hind coxae which are transverse, in Paederinae they are round to elongate, usually rounded at the base and, while they may be large and occupy much of the raised margin, they do not extend along the apical margin of the metasternum towards the epipleura. Despite being morphologically very diverse, members of the subfamily soon become familiar from the general habitus. The following very general description applies to much of the northern temperate fauna. Most are rather drab, black to pale brown throughout, but there are many bicoloured species with contrasting head, pronotum and/or elytra, some have striking bicoloured elytra and members of Paederus have aposematic colouration. The head is usually near-quadrate, smoothly convex, punctured and finely pubescent to almost glabrous, in some there are depressions or distinct fovea near the eyes but they lack well-defined frontal furrows or sexual modifications, the eyes are very variable but often relatively large and placed towards the anterior margin. In most the temples are long; they may be continuously curved from the eyes to the neck, diverging to a truncate or weakly curved basal margin or almost parallel-sided to rounded posterior angles. The maxillary palps are usually long with well-defined segments, the penultimate expanded towards a truncate apex and the terminal segment tiny, antennae 11-segmented and filiform, often with only the basal segment(s) elongate, most obviously so in Ochthephilum where the basal segment forms a distinct scape, mandibles usually prominent and with various internal teeth. The neck varies from short and very thin (almost ridiculously so when viewed against the broad head in species of Rugilus) to robust and wide. Pronotum quadrate to elongate and generally of two distinct forms, simply oblong with rounded angles or parallel-sided laterally and strongly narrowed in an oblique angle to a narrow anterior margin and much wider posterior margin. Pronotal surface simply convex or with various basal fovea, smooth and very finely punctured to densely and coarsely punctured throughout, in some with a smooth and narrow median longitudinal area that may be complete or partial. Lateral margins usually bordered but in many this is reflexed under the margin, especially towards the front, and so not visible from above. Elytra quadrate to elongate and usually at most only a little wider than the pronotum, with rounded shoulders and near parallel-sided to a truncate posterior margin, usually randomly and finely to moderately strongly punctured throughout, in a few e.g. Lobrathium multipunctum (Gravenhorst, 1802) with rows of stronger punctures, the suture is usually joined throughout its length and the adjacent cuticle may be slightly raised or wrinkled. The abdomen is always long and relatively narrow, at most only weakly dilated about the middle, and strongly bordered laterally, the tergites are variously punctured and pubescent but otherwise unmodified. Males sometimes have dilated pro-tarsal segments but in all cases can be distinguished by the excised apical margin of the terminal sternite. In some species e.g. Paederus and Rugilus the legs are long and slender and the beetles can move very quickly but in most the legs are shorter and robust with well-developed femora, especially on the front legs, this is most developed in those with a subterranean way of life e.g. Achenium and Lathrobium. Tibiae usually long and gradually expanded to a truncate apex, smooth or with various spines externally and a small spur on the inner apical angle. Tarsi 5-segmented and usually short, meso- and meta-tarsal segments simple and variously elongate although usually only moderately so, in some e.g. Paederus, the fourth segment is bilobed, pro-tarsi simple or variously expanded, in Achenium and some Lathrobium strongly so.
Five species of the huge genus Astenus occur in the UK, they are mostly associated with dry habitats and most are very local and rare although A. pulchellus (Heer, 1839), which occurs in compost and dung heaps, is widespread and may be locally common in the southeast, and A. lyonessius (Joy, 1908) occurs locally throughout the southeast. A. immaculatus Stephens, 1833 is associated with damp habitats in the southeast. Our three species of Ochthephilum are associated with wetlands; two are very rare and restricted to the south coast while O. fracticorne (Paykull, 1800) occurs locally throughout England and Wales, extending into southern Scotland. Lobrathium multipunctum (Gravenhorst, 1802) is locally common in a variety of wetland habitats across Wales and England north to the midlands, becoming sporadic and rare as far as the Scottish border, it is medium sized and distinctive and should soon be found when working wetlands. Platydomene angusticolle Boisduval & Lacordaire, 1835 is a very local riparian insect of Wales, southern Scotland and the Highlands. Five species of Tetartopeus Czwalina, 1888 occur in the UK; all are wetland insects and with the exception of the widespread and generally common T. terminatum Gravenhorst, 1802, all are very rare and local. The previous three genera were formerly included as subgenera of Lathrobium Gravenhorst, 1802, but now this genus includes only eleven UK species, several are widespread and common and most occur in wetland situations but specimens will also occur in damp grassland, agricultural margins and along woodland borders, some have been recorded at light and we have recorded the common L. brunnipes (Fabricius, 1793) from a domestic garden. They are among the largest members of the subfamily and some should soon become familiar. Hypomedon debilicornis (Wollaston, 1857) is a recent addition to the UK fauna, it is associated with dung and so far remains a very local insect of the southeast. Our two species of Lithocharis Dejean, 1833 are widespread and generally common, both occur in garden compost and other synanthropic situations and often occur together. Two species of Pseudomedon Mulsant & Rey, 1878 occur in the UK; both are widespread but very local and rare; P. obscurellus (Erichson, 1840) is associated with dry dung heaps while P. obsoletus (Nordmann, 1837) is a wetland insect. Eight species of Medon Stephens, 1833 occur in the UK; all are very local or rare and most occur in only in Wales and the south of England. M. ripicola (Kraatz, 1854) is a wetland species and M. castaneus (Gravenhorst, 1802) is associated with subterranean mammal nests but otherwise they occur in among decaying vegetation in a range of habitats. Our two species of Achenium are subterranean and so only rarely recorded; they are widespread though very local and rare across the south of England and may be found among the roots of grass in damp places (Joy, 1932). Two species of Sunius Stephens, 1829 are widespread locally common in open and well-vegetated habitats across Wales and the south of England while a third, S. bicolor (Olivier, 1795) is very rare and mostly coastal in the southeast, around the Wash and in South Wales, this species also occurs in salt-marshes. The large and colourful Paederus will soon be encountered in wetland habitats; three species are confined to wetlands while a fourth, P. littoralis Gravenhorst, 1802, occurs more generally. With the exception of P. caligatus Erichson, 1840, which is very local and rare (although we have found it in abundance in the New Forest), they are widespread and locally common in the south of England and Wales. Two species of Peaderidus Mulsant & Rey, 1878 are included in the UK checklist but both are thought to be long extinct here. Species of Rugilus Leach, 1819 are small ant-like beetles associated with decaying vegetation in a range of habitats including domestic gardens, seven species occur in the UK; several are widespread and common and should soon appear when working compost and leaf litter. Our five species of Scopaeus Erichson, 1840 occur in riparian and marginal habitats; only two are widespread across England but both are very local and rare, S. gracilis (Sperk, 1835) is confined to Wales while S. ryei Wollaston, 1872 has not been recorded in recent decades and S. minutus Erichson, 1840 is known from only a few coastal records. For UK species Identification is generally straightforward although some species will require dissection; our list, including those considered to be extinct is keyed by and substantially illustrated in the RES handbook by Derek Lott and Roy Anderson.