EUASTHETINAE Thomson, C.G., 1859
This obscure group is widespread but rarely encountered. All our species are associated with decaying vegetation, often in wetland habitats.
This is a large group of more than 750 species included in 26 genera and 6 tribes which has variously been considered as a tribe of the Steninae. Our U.K. species are included in the largest tribe, the Euaesthetini Thomson, C.G., 1859, a cosmopolitan group of 12 genera. Two more tribes occur in northern temperate regions; Fenderiini Scheerpeltz, 1974 with one Nearctic and one Holarctic genus, and Nordenskioldiini Bernhauer & Schubert, 1911 with 3 Holarctic genera and one confined to the Himalayas. Stanaesthetini Bernhauer & Schubert, 1911 includes four tropical genera, and both the monogeneric Alzadaesthetini Scheerpeltz, 1974 and the Austroesthetini Cameron, 1944, with four genera, occur in southern hemisphere temperate regions. Many regions include endemic genera; Australia has the most with 5, 3 occur in Chile, 2 in the Mediterranean region and one each in New Zealand, Madagascar, East Africa, The Himalayas and North America. By far the greatest diversity is in tropical areas but northern temperate regions are well-represented; 22 species in 5 genera are known from The United States, and almost 70 species in 5 genera are known from Europe although most of these, about 60 species, are in the single genus Octavius Fauvel, 1873, a pantropical group of about 220 species which extends into South Africa and Europe. The U.K. fauna includes four species in two genera although one of these, Edaphus beszedesi Reitter, 1914, is adventive from mainland Europe. Our two genera represent part of a much wider diversity; Euaesthetus Gravenhorst, 1806 is Holarctic and includes about 40 species, 20 of which are Nearctic, and Edaphus Motschulsky, 1857 includes more than 350 species and is almost cosmopolitan, being present on many oceanic islands but absent from various parts of South America. The species are all small, generally <2mm and rather parallel-sided, the head often with various impressions between the eyes and the pronotum cordate, or at least broadest in the anterior half, and variously impressed towards the base. The eyes are relatively small and placed towards the base of the head and so the cheeks are long and the temples often absent or nearly so, in many exotic species the eyes are greatly reduced or absent. The antennae are short, slender and have a two-segmented club (three-segmented in some non-U.K. species); they are inserted on the dorsal surface well in front of the eyes and within the base of the mandibles. This antennal placement along with the small size and antennal club will identify the group within the U.K. fauna. The abdomen is strongly bordered and generally tapering from the basal third to the apex. The tarsi are 4-4-4 or 5-5-5 although in some tropical genera it is 5-5-4. They occur among decaying vegetation in a wide range of permanently damp situations, often among wetland margins, including salt marshes, but also among leaf-litter and tussocks in woodland situations, and they often turn up in samples of flood refuse. Many species have reduced wings or are apterous.
Edaphus lederi Eppelsheim, 1878 was only recently (2006) discovered in the UK but has since been found at several sites in the south east and is likely to be spreading; it is otherwise widespread across the Palaearctic region and has been accidentally introduced into Australia, New Zealand and North America. Among our fauna it may be distinguished by its small size, about 1mm, and body form and sculpture; there are two longitudinal furrows on the clypeus, the pronotum is slightly transverse, finely punctured and has a transverse series of foveate punctures along the base and the elytra are transverse. The eyes are convex and large, occupying about half the lateral margin of the head, and the temples (when visible beyond the pronotum) are long and parallel-sided. The overall form and sculpture is suggestive of some Pselaphinae e.g. small Euplectus, but the anterior antennal insertions are diagnostic. Typical habitats are compost and decaying vegetation, and while the species is very local and sporadic it may be common where it occurs.
Our 3 species of Euaesthetus are broader and larger, 1.5-2.0mm, have small eyes placed towards the back of the head so that the temples are absent or only tiny, a strongly punctured pronotum and transverse elytra. Interesting features of the genus are the series of spines along the anterior margin of the labrum and the form of the mandibles; the inner margin has a sharp median tooth below which the margin is serrate. The maxillary palpi are about half the antennal length while the labial palpi are small and generally not visible from above. All our species are widespread but very local, or perhaps under-recorded, and occur among litter or in tussocks in wetland or permanently damp situations. E. laeviusculus Mannerheim, 1844 occurs throughout England and Wales and there are a few records from Southern Scotland, it usually occurs among vegetation in mires and bogs on acid moorland. E. bipunctatus (Ljungh, 1804) is widespread in Wales and across the midlands to East Anglia and in Kent but otherwise very rare and sporadic; it is usually associated with permanently damp grassland and salt marsh vegetation. E. ruficapillus Lacordaire, 1835 occurs locally throughout Wales, Northern Ireland and the south of England except for The West Country and very rarely further north to Southern Scotland, it is usually associated with wetland habitats and lake margins etc.