Ilybius fuliginosus (Fabricius, 1792)
This species is widespread and generally common throughout the Palaearctic and Oriental regions, occurring throughout Europe north to Scandinavia and eastward to Russia and China*. It is common throughout the U.K, including all the islands, north to Orkney and Shetland. Adults occur year round in a wide range of wetland habitats including still and moving water, and peaty and brackish environments. They are strong fliers and quick to invade new areas; they invariably turn up in the spring in numbers in flooded tyre ruts in woodland areas and in cattle troughs etc. and sometimes come to light. The adults leave the water in September or October and overwinter in nearby soil, among matted vegetation or under logs etc. and become active again by March or April. Egg laying occurs in the summer; the eggs being inserted into plant stems. Beetles ovipositing in early summer are likely to have overwintered; those laying later are likely to have overwintered as larvae and eclosed in the spring. During the summer adults from both generations will occur together. Adults are often, or usually, found in numbers and often among numbers of other common water beetles and are generally distinctive; the elongate and rather parallel form coupled with the dark, sometimes metallic bronze, colouration with strongly contrasting pale margins will soon become familiar.
9.5-11.5mm. The shape and colouration are distinctive; the pale margins are not sharply delimited and there may be additional small pale marks on the elytra. The underside is mostly dark testaceous. The entire upper surface has fine, mostly quadrate or isodiametric, microsculpture. Eyes emarginate behind the antennal insertions. Antennae and palps pale. The front of the head and two spots on the vertex are red. The pronotal anterior and lateral margins are finely bordered. Scutellum broadly triangular. The three longitudinal series of punctures on the elytra are generally faint and rather randomly arranged. Legs red. The hind femora have a short ‘comb’ of stiff setae on the inner margin near the apex. Meso- and metatibiae have dense, long swimming hairs along the upper surface. The male basal pro- and masotarsal segments are dilated and equipped with pale sucker hairs underneath and the protarsal claws are unequal. The metatarsal claws are unequal in both sexes; the inner claw being longer and less curved.
* A second subspecies, I. f. turcestanicus Gschwendtner, 1934, occurs in Europe and northern Asia.