Gyrinus aeratus Stephens, 1835

A widespread, almost Holarctic, species with a rather patchy and mostly northern Palaearctic distribution, it is locally common across central and northern Europe from France eastwards but generally absent from the Mediterranean countries; it extends to the far north of Fennoscandia and the UK but has not been recorded from Iceland, and east across northern Russia to eastern Siberia, in North America it occurs from Alaska to Ontario, extending south to New York. In the UK it is locally common across northern and western Ireland and throughout Scotland including most of the islands north to Shetland, but in England and Wales it has suffered a decline over recent decades and is now very local and scarce. Adults are present year-round and are active over a long season from early in the spring, they are thought to be late spring and early summer breeders and adult populations peak during mid and late-summer, typical habitats in the UK are lakes, canals and large drainage ditches and they sometimes occur in slow moving sections of rivers, in Europe they also occur in reservoirs and gravel pits and further east on the tundra and in artificial lakes and volcanic ponds. The species sometimes occurs in very large aggregations which may include other whirligigs such as G. marinus Gyllenhal, 1808, they gyrate rapidly but when alarmed tend to swim away from any disturbance rather than diving, they are predaceous and may sometimes be seen in numbers feeding on floating carrion. Little is known of the life-cycle but it is thought to be typical of the family with larvae developing in the summer and new-generation adults appearing in late-summer and autumn.

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Distinguished among our UK species by the broadly-oval shape and medium size, 4.5-6.3mm (females are larger than males), entirely dark elytra and black middle and hind tarsal claws; our only other species sharing these characters is G. marinus but here the explanate elytral border becomes wider towards the apex while in the present species it is either parallel to the elytral margin or slightly narrower apically. Difficult specimens may be separated by the form of the male genitalia; in aeratus the median lobe is rounded or subtruncate while in marinus it tapers to a point. Dorsal surface with a bronze reflection, ventral surface metallic black with the hypomera, mesosternum, and epipleura brownish (the gonocoxosternites may also, rarely, be paler). Head, pronotum and elytra with fine but distinct punctation and very fine reticulation; females generally have much stronger reticulation, especially towards the elytral apex. Elytral striae and subapical transverse series’ of punctures distinct and complete although the inner striae tend to be weaker than the others. The sexes are sometimes given as being dimorphic in size and shape e.g. Oygur and Wolfe (1991) give the male length as 4.7-5.4mm and width as 2.5-3.0mm, and the female length as 5.8-6.2mm and width as 3.0-3.3mm.