PROTEININAE Erichson, 1839
All our species are small and very distinctive, most are common and should soon be found when working compost, dung or other decaying vegetable matter and most can be found year-round.
This small subfamily includes about 200 species in 11 genera and 5 tribes, it is a primitive subfamily classified within the Omaliine group and is probably most closely related to Micropeplinae Leach, 1815 and Omaliinae Macleay, 1825, the classification has remained relatively stable over the years although Metopsia Wollaston, 1854 has often been included within its own subfamily, the Metopsiinae Tottenham, 1954. Proteinini Erichson, 1839 occurs throughout the world with the exception of Australasia while the other tribes, which include only a few mostly monotypic genera, occur primarily in the Southern Hemisphere. Anepiini Steel, 1966 includes 2 monotypic genera; Eupsorus Broun, 1904 from New Zealand, and Anepius Blackburn, 1902 from Nepal, but the Palaearctic fauna otherwise includes only members of the single tribe Proteinini. This group includes 3 genera, all of which are represented in Europe. Proteinus Latreille, 1797 is an Holarctic genus of about 40 species and is most diverse in the Palaearctic region; 10 species occur in North America of which one is introduced from Europe and one, P. brachypterus (Fabricius, 1792) is Holarctic in distribution, 34 species are recorded from the Palaearctic region of which 13 occur in Europe and of these 5 extend to the UK. Megarthrus Stephens, 1829 is by far the largest genus with about 120 species and is almost cosmopolitan in distribution, being absent from Australasia and temperate parts of South America, 12 species are recorded from North America while about 80 occur in the Palaearctic region, they are most diverse in central and eastern Asia and only 14 occur in Europe of which 5 extend to the UK. Metopsia is a Western Palaearctic genus of 12 species, most of which occur on various Atlantic islands, Gran Canaria, Tenerife and La Palma, or in north west Africa, only 2 are widespread in Europe and one of these, M. clypeata (Müller, P.W.J., 1821) extends to the UK.
They are small beetles <3mm, characteristically broad with medium-length truncate elytra that cover at least the first abdominal tergite. The head, pronotum and elytra are usually smoothly convex although most species are distinctly microsculptured, the head is sometimes foveate or otherwise indented and in Metopsia there is a single ocellus on the vertex, the pronotum may have a median longitudinal impression. The antennae are 11-segmented with the two basal segments expanded and remainder filiform or at most only weakly thickened towards the apex giving the impression of a very indistinct 3- or 4-segmented club. The tarsi are 5-segmented or, in some Southern Hemisphere groups, 4-segmented. In some, especially Megarthrus, the pronotum is variously indented, angled or produced laterally or at the anterior or posterior angles, and the clypeus is often characteristically widely emarginate and angled. Among the UK fauna they are quite distinctive and should soon become obvious from the general habitus; distinctive features include the single ocellus in Metopsia and the notched pronotal angles in Megarthrus, Proteinus are broadly-oval, dark coloured and very finely pubescent, they have characteristic antennae which are inserted laterally in front of the eyes and outside the base of the mandibles, these are gradually thickened towards the apex and have 2 basal segments enlarged. All species have large convex and protruding eyes, a strongly transverse pronotum and broad elytra which are curved laterally.
All species occur in fungi, dung, decaying vegetation or among leaf-litter or tussocks. Adults are present year-round and may occur in very large numbers during the spring and autumn, especially in dung (Megarthrus), or among decaying terrestrial fungi (Proteinus), and they will usually be found among numerous other species of small staphs. They are readily sampled by sieving host material or taking samples for extraction but they are nocturnal and are easily viewed by torchlight as they swarm and run on the surface of wood or soil in the vicinity of fungi, and lifting large decaying brackets from the ground in the autumn will often reveal an abundance of specimens, especially where the fungus is wet through and strongly aromatic. The larvae are thought to develop in decaying organic material while the adults may be saprophagous, mycophagous or scavengers.
Our UK species are widely distributed, though most are generally absent from The West Country, and several should quickly be found by general sampling; sieving old hay, straw or compost which has lain undisturbed and begun to decay will often yield several species in abundance. Metopsia clypeata (Müller, P.W.J., 1821) is locally common throughout the UK north to the Scottish Highlands and will sometimes occur when sweeping long dry grass in the summer. Proteinus ovalis Stephens, 1834 and P. brachypterus (Fabricius, 1792) are generally common throughout Wales and Southern England but less so further north, the latter occurring in Orkney. P. laevigatus Hochhuth, 1872 is locally common in the southeast but otherwise rare, P. atomarius Erichson, 1840 is widespread though very local in Wales and Southern England, and P. crenulatus is generally rare and known from scattered records from Wales, the west midlands and Scottish Highlands. Megarthrus denticollis (Beck, 1817) and M. depressus (Paykull, 1789) are generally common across England and Wales while M. bellevoyei Saulcy, 1862 and M. prosseni Schatzmayr, 1904 are more restricted in south and central England and Wales, and M. hemipterus (Illiger, 1794) is a rare species occurring sporadically in south east England.