Colymbetes fuscus (Linnaeus, 1758)

A widespread and often common water beetle occurring from the east of France, including the Mediterranean regions and North Africa, to Western Siberia, and north through Russia and Scandinavia to the Arctic Circle.  It is common throughout Britain and Ireland, with the exception of highland areas, north to the Orkney and Shetland Isles. Adults occur throughout the year and the typical habitat is stagnant standing or slow moving water, often among dense aquatic or marginal vegetation. They fly, sometimes being attracted to light, and are often found in temporary pools. The life-cycle takes one year, with larvae in the spring and summer. Adults are usually found in numbers where they occur and are soon obvious in the water net; the large size and pale colouration are distinctive as other species of a similar shape and size are entirely black, or nearly so. The smaller Dytiscinae; Graphoderus, Acilius and Hydaticus are distinctly shaped and obvious with experience but also, as with all the U.K. species, they have the front margin of the eyes entire.

​

15-18mm. Head black with the clypeus and two marks on the vertex red. Antennae pale at the base becoming dark towards the apex. Palps brown with the last segment dark. Pronotum transverse with projecting front angles and a line of strong punctures behind the anterior margin; pale with the vertex darker, or extensively dark. Surface with very fine, irregular microsculpture and very fine and sparse punctures. Elytra dark brown or dark yellow with numerous small black markings (x10), with two rows of punctures on the disc and with very wide transverse microsculpture, between this are scattered fine punctures and very fine reticulation on a flat surface. Ventral surface mostly dark, elytral epipleurs pale. Hind coxal lines widely separated.  Legs pale with the posterior femur dark. Posterior femur without an apical ‘comb’ of setae. Second sternite with a stridulatory file on either side. Basal segments of the protarsus expanded in the male and with round scales beneath. Female elytra broader, especially in the apical half. Pro- and masotarsal claws unequal; the anterior a little longer and expanded towards the base.