Colymbetes fuscus (Linnaeus, 1758)

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ADEPHAGA Clairville, 1806

DYTISCIDAE Leach, 1815

COLYMBETINAE Erichson, 1837

Colymbetes Clairville, 1806

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.

COLYMBETES Clairville, 1806

Most species of Colymbetes occur in the eastern Palaearctic, with only 5 in Europe and one in the U.K; C. fuscus (Linnaeus, 1758) Characters common to members of the genus include:

Size varies from 11 to 20mm. Outline continuous; broadly elongate and rather depressed, black to brown or yellow, the pronotum may have darker markings. Eyes notched behind the antennae. Labial palps elongate and entire. The clypeus generally has a small fovea on each side near the anterior margin. Pronotum without lateral margins, prosternal process short and the metasternal ‘wings’ are broadly triangular. Metacoxal lines are not strongly convergent. Elytral microsculpture widely transverse in most species, a character which separates this from other genera within the tribe. Apical angle of the metafemur glabrous or with randomly spaced setae. Pro and meso tarsal segments 1-3 with round scales and/or adhesive setae underneath in the male. Claws unequal.

Larval characters. These have the typical dytiscid form with a large head and powerful mandibles, long legs and a tapering abdomen. The head lacks a frontal horn. Abdominal segments 7 and 8 are glabrous or only sparsely pubescent. The third and fourth antennal segments are subequal in length. The terminal cerci are long, with two groups of hairs but no spine-like setae (this character separates the genus from Rhantus; in that genus there are many short, spine-like setae on the outer margin of the cerci).

Similar Species
Rhantus suturalis
  • Smaller (10-13mm)
  • Elytra lacking transverse reticulation.
  • Generally paler appearance.
Acilius sulcatus
  • Broader.
  • Elytra lacking transverse reticulation.

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