Oreodytes sanmarkii (Sahlberg, C.R., 1826)
This very widespread species has been recorded throughout the Palaearctic region from Europe to the far east of Russia and Japan, it has long been known from two sites in Canada: Hudson Bay (Southampton Island) and in the northern Yukon, but has recently been found to be common at several sites in the Northwest Territories, which might suggest a native Holarctic distribution. The species is generally common across Europe, extending from Portugal to France and northern parts of Italy and the Balkans and north to the UK, Denmark reaching the Arctic Circle in Scandinavia. The typical form occurs west to the Pyrenees while single subspecies, O. s. alienus (Sharp, 1873) is endemic to Portugal and Spain. With the exception of Anglesey, East Anglia and much of the southeast, it is generally common throughout England, Wales and Scotland north to Orkney and the Inner Hebrides, in Ireland it is widespread though generally local except for the extreme southwest and across the north, where it is common. Adults are present year-round; they have been found under waterside debris and among moss during the winter and are active over a long season from early March until November, peaking in abundance during the spring and again in August. The species occurs from lowlands to middle mountain altitudes and in Northern Europe it is often common in cold and clear mountain streams, the usual habitat is open stretches of streams and rivers, especially slow-moving water over hard substrates with exposed sand or gravel, but it also occurs on exposed wave-washed lake margins. It seems that most adults have reduced wings although specimens have been recorded in flight and very occasionally at suction traps. Little is known of the life cycle but both adults and larvae are predaceous and it is likely that development occurs during the spring and summer with winter being passed mostly in the adult stage. According to some sources the main food is chironomid larvae. Adults can be swept from sparsely-vegetated shallow stretches of rivers etc, especially over exposed gravel, but they have been found at depths down to 5m. on lake margins leading to suggestion that they may be able to breathe without visiting the surface.
Oreodytes sanmarkii 1
Oreodytes sanmarkii 2
© Lech Borowiec
2.5-3.3 mm. Broadly oval and only weakly convex, glabrous and very finely microsculptured above, body pale brown with dark markings to the head and pronotum and longitudinal stripes on the elytra, these for the most part discrete although the outermost may be fused about the middle, antennae pale with two or three apical segments darkened, legs pale or with the terminal tarsomere darker towards the apex. Ventral surface black or sometimes with the abdomen pale, contrasting with the pale epipleura. Head proportionally large; rounded anteriorly (from above) and broadest across large and weakly convex eyes. Pronotum widely transverse, curved and narrowed laterally from rounded posterior angles to slightly projecting anterior angles, apical margin almost straight, basal margin sinuate and produced about the centre. Surface finely and rather sparsely punctured and with a longitudinal impression towards each lateral margin, the dark colour varies from a partial and sometimes diffuse transverse median mark to a deep and broad median mark that may join medially with a darkened basal margin. Scutellum not visible. Elytra slightly broader across the base than the base of the pronotum, widest about the middle and evenly curved to a slightly acuminate apical margin, surface very finely punctured throughout, although these may be indistinct among the microsculpture, and with larger punctures forming weak striae. Elytral colour varies but typically there are six dark stripes which do not reach the basal or apical margins, the outer ones are often abbreviated and the outermost is often, or usually, interrupted behind the middle. Legs long and slender, with front and middle tarsi apparently 4-segmented, the tiny fourth segment concealed by the bilobed third segment, hind tarsi with five simple elongate segments. Hind trocanters densely punctured and pubescent (unique among our members of the genus), hind tibia with two irregular rows of punctures. The sexes are very similar but males have slightly more dilated front tarsi and slightly longer claws when compared with females.