Oreodytes Seidlitz, 1887
This small Holarctic genus includes about 25 species, 13 of which occur in North America and 6 occur in Europe, and of the European species only O. sanmarkii (Sahlberg, 1826) is Holarctic. In some systems (see here) various species are included within Nectoporus Guignot, 1950, which from our point of view is awkward because this includes the widespread O. sanmarkii, in a general sense these include the short-oval species (see below) while the long-oval species are retained in the present genus. For the sake of sanity we discuss the genus in the wider sense, which corresponds to our presently accepted checklists (although the situation is likely to change, see here) Of the European species only 3, O. sanmarkii, O. septentrionalis (Gyllenhal, 1826) and O. davisii (Curtis, 1831) are widespread, O. meridionalis Binaghi & Sanilippo, 1971 is endemic to Italy, O. angelinii Fery, 2015 is endemic to Greece, and O. alpinus (Paykull, 1798) is restricted to higher latitudes.
The species occur mostly in colder freshwater habitats, often on lake margins or slow-moving sections of streams and rivers, but some e.g. O. septentrionalis prefer fast moving streams. Many are more common in upland or mountain situations and most prefer exposed gravel or sandy substrates, often with little vegetation. Most occur over a long season from early spring until the autumn and peak in abundance during the summer. In flowing water particularly several species may occur together.
Among our UK fauna the genus is easily recognized by the short longitudinal sub-lateral groove on the pronotum coupled with a pale elytral base. All are small water beetles, 2.8-5.0 mm, although some foreign species are slightly larger, the shape varies from short and broadly-oval to elongate and almost parallel-sided, and the form may be continuous or not. All are glabrous and finely reticulate above and all are pale with darker markings but these can be very variable, especially on the pronotum. The elytra usually have dark lines on the disc and blotches towards the margins but the lines may be expanded in places or even fused to give a maculate pattern, the basal margin is always pale albeit sometimes interrupted by dark lines. The ventral surface is usually extensively black. With the exception of O. alpinus, in which the female elytra have a small sub-apical tooth, the sexes are very similar, males have slightly dilated front tarsi but this is often only obvious by comparison. Immature stages of various species have been described, including that of the Japanese endemic O. kanoi (Kamiya, 1938) (Okada & Alarie, 2020).
Our UK species are readily identified as follows and so dissection should not be necessary.
Larger species, body length at least 3.8 mm.
Smaller species, body length at most 3.6 mm.
On average larger, 4.2-5.0 mm. Lateral pronotal margins almost straight towards the base, pronotal base narrower than the elytral base. Elytra densely darkened about the base, leaving only a narrow pale band. Female elytra with a sharp subapical expansion.
On average smaller, 3.8-4.5 mm. Lateral pronotal margins evenly curved, elytral base not wider than the pronotal base. Elytral base more extensively pale, usually only the inner stripes extending narrowly forward. Elytral margin smooth in both sexes.
On average larger, 3.2-3.6 mm. Body more elongate and less rounded, elytra with random larger punctures in addition to those in regular longitudinal series. Abdomen extensively black.
On average smaller, 2.9-3.3 mm. Body broader and more rounded, elytra with larger punctures confined to more-or-less regular longitudinal series. Abdomen extensively black or reddish.