Rugilus Leach, 1819
Rugilus includes more than 200 species and is almost cosmopolitan in distribution. Various subgeneric divisions are proposed but these are incomplete and in this sense the genus needs to be revised, similarly modern research has produced many new species, especially from the Eastern Palaearctic and Oriental regions, and so any subgeneric groupings are likely to be changed. The Palaearctic and Oriental regions include about 95 species, 76 of which are included in the predominantly western nominate subgenus, 16 eastern species are included in the predominantly eastern subgenus Eurystilicus Fagel, 1953 and 2 are of uncertain placement. The European fauna includes 16 species of Rugilus s.str. as well as R. nematideus Gistel, 1857 and R. salicetorum Gistel, 1857, both endemic to Germany and of uncertain subgeneric placement. Further afield 13 species are known from North America, of which 4 are adventive, and 39 species are known from South and Central America. The Afrotropical region, including Madagascar, is very diverse with about 117 species, but only 2 non-native species are known from Australia although Papua New Guinea. Of the European species only 7 (R. angustatus (Geoffroy, 1785), R. rufipes Germar, 1836, R. subtilis (Erichson, 1840), R. orbiculatus (Paykull, 1789), R. erichsoni (Fauvel, 1867), R. similis (Erichson, 1839) and R. geniculatus (Erichson, 1839)) are widespread and extend north to the UK although of these R. similis is absent from much of Western Europe. The only other widespread species is R. mixtus (Lohse, 1956) which occurs throughout Central Europe but is not known from the UK. Of the rest; R. rossi (Zanetti, 1977) is known from Italy and the Near East, and R. festivus (Mulsant & Rey, 1853) occurs in south west Europe and North Africa, the others are endemic to certain areas: R. sardous (Lohse, 1956) to Sardinia, R. dilutipes (Reitter, 1884) to Greece, R. graditanus (Peyerimhoff, 1937) and R. ibiricus (Fagel, 1959) to Spain, R. pecticensis (Coiffait, 1939) to France and R. maltzevi Gusarov, 1991 to Ukraine. Most species occur in organic detritus, moss and decaying wood etc. in a wide variety of permanently damp or wet habitats. Adults of most species are present year-round and many species are capable of flight. A few species e.g. R. orbiculatus and R. rufipes have become widely dispersed across the world with trade in horticultural and agricultural products etc.
The species are very distinctive due to the narrow and discontinuous body shape and very narrow neck (see below). Head large and broad, densely punctured and without structure, eyes variable in size, generally with long and rounded temples to a very narrow neck. Labrum with a small tooth either side of a small central incision, mandibles long, curved and asymmetric; the right with four internal teeth and the left with three. Penultimate maxillary palpomere long and broadened from the base, terminal segment diminutive and narrow. Gular sutures substantially fused. Pronotum elongate and narrow, usually distinctly narrower than the head and pronotum, rounded or straight laterally and strongly converging to narrow basal and apical margins, surface without sculpture, strongly and densely punctured but for a smooth median strip that is often partly impressed. Abdomen quadrate to slightly elongate with rounded shoulders and recurved apical margins, surface punctured throughout, often densely so, without striae but often depressed along the suture. Abdomen elongate and usually dilated about the middle, basal tergites strongly bordered, the fifth much longer than the others and less strongly bordered, all tergites finely punctured throughout, the basal tergites strongly punctured within transverse basal depressions. Apical sternites variously depressed or excised in males. Legs long and slender, tibiae without obvious apical spurs. Tarsi with 5 simple segments, the front tarsi at most only slightly broader in males. Among our UK fauna they may be distinguished by the following characters: 3.5-6.6 mm. Body not metallic, head and pronotum densely punctured, eyes situated towards the front of the head, pronotum narrow and strongly narrowed from a short lateral margin to narrow basal and apical margins, lateral margin reflexed and not visible from above. Middle and hind tarsomeres simple.
Our UK species may be identified from the following key:
Pronotum orange, contrasting with the dark body, elytra narrowly pale about the posterior angles and apical margin. Pronotal punctures less dense, with shiny cuticle visible in places. 6.0-6.5 mm.
Entirely dark or with the apical elytral margin narrowly pale. Pronotum more densely punctured.
Eyes much smaller than the temples, occupying much less than half the lateral margin, temples therefore relatively long. Elytra unicoloured, similar to the head and elytra or a little darker.
Eyes larger; occupying at least half the lateral margin and almost as long as or slightly longer than the temples. Elytra variable but almost always paler about the posterior angles.
Head transverse, temples straight behind the eyes and curved to a straight basal margin. Abdomen finely but not densely punctured. Antennae and legs entirely reddish or brown. 5.5-6.0 mm.
Head quadrate or nearly so, temples and base smoothly rounded. Abdomen moderately densely punctured. Middle and hind femora darkened towards the apex. 5.0-6.3 mm.
Elytral punctures stronger; at least half the diameter of those on the pronotum and mostly separated by about their own diameter. [Elytra usually pale only inside the posterior angles] Length more than 5 mm.
Elytral punctures finer; less than half the diameter of the pronotal punctures and mostly separated by more than their own diameter. [Elytra usually pale along the entire apical margin] 4.5 mm.
Femora entirely brownish. Fifth ventrite in males with a deep rounded depression bordered by small tubercles that do not extend beyond the posterior margin. 5.5-6.0 mm.
Middle and hind femora darkened towards the apex. Fifth ventrite in males with a broadly rounded depression bordered by sharp ridges that extend well beyond the posterior margin. 5.5 mm.
Elytra as wide as the head. 4-5 mm.
Elytra distinctly narrower than the head. 4.0-5.5 mm