Oreodytes davisii (Curtis, 1831)
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ADEPHAGA Clairville, 1806
HYDROPORINI Aubé, 1836
With the exception of recent records from Turkey (Erman & Erman, 2002) this species is known only from montane areas of Europe; the distribution is patchy, it is absent from the Mediterranean islands and is represented in Spain by ssp. rhianae Carr, R. 2001. The nominate subspecies is widespread but sporadic, being more or less confined to upland streams and lakes and reaching the alpine zone throughout the range; it occurs from the Pyrenees to Italy and Ukraine and is known sporadically further south through the Balkans to Greece, to the north it reaches the UK, Germany and Poland but it is generally very infrequent and rare e.g. in Poland it is known only from the Tatra Mountain and their valleys and foothills. In the UK it is widespread across Wales and Northern England, although largely absent from the Lake District, and there are scattered records from south to the midlands, it occurs locally throughout Scotland and parts of Northern Ireland and there are records from the Inner Hebrides and the east of Ireland. Adults have been recorded throughout the year; winter is passed exclusively in this stage and they are active from March until October, peaking in abundance during June and July. There is a single generation each year, breeding occurs early in the season and larvae develop in spring and early summer. The typical habitat is areas of cold still or slow moving water beside fast streams although adults also occur on reservoirs and lake margins at lower altitudes, they seem to prefer shallow water with stony or gravelly substrates, often where silt accumulates in areas devoid of vegetation. Adults may be netted from open water or found by turning boulders etc. in fast-flowing stretches of shallow water, they also fly and have been recorded at flight-interception traps. The species is sensitive to eutrophication as well as changes to water quality in general and is thought to have declined by more than 30% in the UK over the last 25 years.
3.8-4.5 mm. Broadly-oval and discontinuous in outline, glabrous and finely reticulate dorsally, colour variable from extensively dark to pale with well-defined dark markings but typically the dark elytral stripes do not reach the basal margin, leaving a pale base that is obvious even in the field, underside black except for the pale prosternal epipleura, femora dark the apex broadly pale, legs otherwise mostly pale. Apical maxillary palpomere darkened, antennae darkened from the fifth or sixth segment. Head broad with large convex and slightly protruding eyes, surface smoothly convex and without structure, usually extensively pale with the base and a V-shaped mark which extends to the antennal insertions darkened. Pronotum transverse, broadest about the middle and curved to protruding anterior angles and obtuse posterior angles, surface finely punctured throughout, more strongly so along the apical and basal margins, and with a short longitudinal impression either side towards the base. Scutellum not visible. Elytra at most only slightly wider across the base than the base of the pronotum, this junction forming an obtuse angle, gently curved laterally and pointed apically, without preapical teeth or lobes, surface finely punctured and sculptured, without definite striae, each with seven longitudinal black lines, the third of which usually approaches the base and the outermost may be abbreviated or form several blotches. Tarsi apparently 4-4-5; front tarsi with bilobed basal segments which are only slightly wider in males, and the basal segment of the hind tarsi expanded internally at the apex in both sexes. Aedeagus smoothly curved from the base, without a subapical constriction.