Agabus undulatus (Schrank, 1776)
This widely distributed Western Palaearctic species extends from France to southern Austria and on to Kyrgystan in the south and north to the UK. Denmark and into some southernmost provinces of Sweden and Norway, it tends to be locally abundant in central regions but becomes much more local and scarce to the north. In the UK it is more-or-less restricted to a few sites in the east; in south-west Yorkshire, Northamptonshire, Cambridgeshire and Huntingdonshire; it was formerly much more widespread in the south with records from Dorset to Kent, Wales and the Severn catchment and the west midlands, but there seem to be no modern records from these sites. The species is associated with natural as well as artificial habitats; usually in permanent well vegetated still or slow-moving stagnant water in open situations or in shaded ponds in open deciduous woodland. Adults are present year-round; they are active from March until September and peak in abundance during early spring and late summer. Breeding occurs in the spring, after the adults have overwintered, and larvae develop through the warmer months to new-generation adults from late summer. Both adults and larvae are predatory, larvae are free-swimming and leave the water to pupate, and so far as is known the species is univoltine throughout the range. Adults are usually described as flightless, and this is no doubt the case in general, but they have appeared in remote artificial habitats and so this is questionable. Sampling usually involves dipping among dense vegetation in reed-beds, marshes, ponds or drains on heavy clay or peaty soils or sorting through samples of decaying leaf-litter from shaded ponds or drains, they usually appear as single specimens and rarely in numbers.
7.0-7.8mm. Very distinctive among our fauna due to the elytral pattern. Broadly-oval and continuous in outline, head dark brown or reddish with two pale spots (these may greatly enlarged), pronotum dark brown, sometimes almost black, with paler lateral margins, elytral ground colour usually concolorous with the pronotum; each with a pale lateral border in the basal half which continues as a narrow and irregular transverse band behind the basal margin, this does not reach the suture and may be fragmented, and with pale lateral spots behind the middle and before the apex. Antennae and palps pale brown, in each case with the terminal segment darkened apically. Legs pale brown. Head almost flat between large and convex eyes that are emarginate behind the antennal insertions. Pronotum broadest across perpendicular posterior angles and smoothly narrowed to projecting anterior angles, lateral borders narrow throughout, surface finely microsculptured and with a band of larger punctures behind the apical and basal margins. Prosternal process narrow and sharp, metasternal wings narrow. Scutellum widely transverse and obtusely pointed. Elytra broadest at or behind the middle, surface with fine punctures and mesh-like microsculpture (which tends to be more strongly impressed in females), and several longitudinal rows of stronger setiferous punctures. Metafemora with row of fine setae towards the base of the ventral margin, metatibiae coarsely punctured dorsally. Basal meta- and protarsomeres dilated in males.