Description

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.