Agabus didymus (Olivier, 1795)

This is a mostly south-western European species; it is common in Portugal, Spain, France and Italy but more local and sporadic further north where it reaches the UK and a few southern provinces of Fennoscandia, to the east it extends to Ukraine, Greece, Lebanon and Israel and it is present in some Mediterranean countries of North Africa; it occurs from lowlands to about 1700 m but the distribution is discontinuous and it is absent from many central European countries. In the UK it is generally common throughout England with the exception of the northwest, mostly coastal in Wales and Anglesey and in Scotland known from only a few records in the south. Typical habitats are well-vegetated slow-moving ditches and streams as well as ponds, lake margins and peat bogs, often with clay or loamy substrate but also in sandy areas, and often where they are exposed to the sun. Adults occur year-round; they overwinter in soil or under logs or debris away from the water margin and are active over a long season from early spring, peaking in abundance during June and August, they usually occur in numbers and often among other numbers of other common diving beetles. This is among the more eurytopic of our diving beetles; locally we find them in abundance in deep, fast-flowing drainage ditches on chalky-clay substrates and in vegetated gravel pit outflows, and in some areas of southern Europe they are common in fast-flowing streams devoid of vegetation. Adults may be sampled by sweeping among marginal vegetation but they often occur in deeper water than other common species and they rarely occur in shaded situations, during the summer they have also been recorded at light UV light traps.

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7.5-8.0 mm. Easily identified even in the field due to the bronze colour and characteristic elytral markings. Elongate-oval and broadest about the middle, body black with dense, near-isodiametric microsculpture and a distinct metallic-bronze reflection, legs dark brown or reddish-brown, palpi red with the apical half of the terminal segment dark, antennae clear yellow with the apex of the terminal segment darkened. Head with two foveate punctures on the clypeus inside the anterior margin of the eyes which are strongly incised anteriorly. Pronotum dark but usually with the lateral and sometimes the base narrowly reddish, indistinctly punctured behind the anterior margin and either side before the basal margin. Elytra each with three rows of setiferous punctures but otherwise without any hint of striae, entirely dark but for a round or broadly U-shaped pale mark before the apex and a translucent, creamy N-shaped marking behind the middle. Front and middle legs usually paler than the dark brown hind legs, external margin of hind femora with a short series of stiff setae towards the apex, hind tibiae short; about twice as long as broad and with the longer apical spur more than half the tibial length. All claws more or less equal in length.