Agabus affinis (Paykull, 1798)
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A transpalaearctic species extending to the far east of Russia, although it is not known from Korea and an older record from northern China is based on a misidentification. In Europe it occurs sporadically from the Pyrenees through northern Italy to the Black Sea in the south, although it is generally absent from the Balkan Peninsula and does not occur in Greece, and extends north to the UK and far above the Arctic Circle in Scandinavia; it is generally common in the north but otherwise very local and scarce. In the UK the distribution is largely western and northern, extending from Cornwall to the far north of Scotland and the Inner Hebrides, it is widespread in Surrey and Hampshire and there are groups of records from Norfolk but it is otherwise very local and scarce in the southeast. Adults occur year-round; they overwinter among moss or marginal substrate etc. and are active from March until November, and occasionally during mild winter spells, peaking in abundance during June and July. Typical habitats are moorland and heathland pools, acid bogs and sheltered lake margins with dense vegetation, usually among Sphagnum or Calligiergon mosses, but they may also be common in temporary pools on peat cuttings and in small woodland pools containing plenty of moss. Breeding occurs in the spring and larvae develop through the summer, pupating among marginal substrate to produce adults in late summer and autumn that will overwinter. Adults may be sampled by sweeping among moss etc., especially in acidic pools, they generally occur in numbers and often along with other water beetles; we found them in abundance among grass and matted vegetation on the margins of flooded pools on the Surrey heathlands during 2018. It is thought that the majority of adults are flightless but fully-winged specimens with normal flight muscles occur.
6.0-7.2mm. Elongate, continuous in outline and rather parallel-sided, body shiny black or dark reddish-black and sometimes faintly reddish on the vertex and towards the posterior pronotal angles, elytra with obscure lateral and subapical spots. Legs reddish-brown, antennae pale with the apex of the terminal segment dark. Dorsal surface without distinct microsculpture (X30, at higher magnification a very fine microsculpture composed of tiny meshes can be seen). Head broad and smooth with relatively large, weakly-convex eyes that protrude only slightly. Pronotum transverse, broadest about the base and evenly curved and narrowed to projecting anterior angles, lateral margins very narrow throughout, surface weakly convex and with a complete transverse series of punctures behind the apical margin and a series of stronger punctures, which may be interrupted or fade about the middle, in front of the base. Prosternal process ridged medially and triangular in cross section at its broadest point. Elytra without striae but with several irregular series of moderately strong punctures. Hind femora with a small ‘comb’ of short setae before the outer apical angle and hind tarsal claws equal in size. Basal front and middle tarsomeres dilated and anterior front tarsal claw toothed at the base in males. The parallel body form is distinctive in the field and the above combination of characters should provide a reliable identification but the form of the median lobe of the aedeagus is distinctive, there is a tooth-like swelling on the ventral margin and the apex is narrowly pointed.