British insects: the families of Coleoptera


L. Watson and M. J. Dallwitz

Cercyon Leach

Adults. Beetles terrestrial, or sub-aquatic, or aquatic; walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; moving in the water by alternate, walking leg movements; when aquatic, replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron; when aquatic, collecting air at the water surface by exserting one antenna to achieve a continuum of air with the ventral plastron, or collecting air at the water surface by exserting one antenna to achieve a continuum of air with the ventral plastron and incorporating bubbles of oxygen directly into the plastron. Beetles 1.3–4.4 mm long; body length/maximum body width 1.3–2.55. Beetles somewhat waisted.

Eyes two; not strongly protuberant; without bristles. Mandibles with a well developed mola; with well developed prosthecae. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short; not strongly asymmetric. Antennal scape not swollen. Antennae clubbed. Antennal clubs 3 segmented (the club hairy); without a cupule. Antennal insertions visible from above, or hidden from above.

Prothorax without notopleural sutures. The pronotum not narrowing towards the rear. Scutellum present; not much longer than broad. Mid-and hind-legs not as in Gyrinidae (q.v.). The front tibia more or less club-shaped, not or scarcely emarginate near the attachment of the tarsus. Hind coxae not extended laterally to meet the elytra; not shaped posteriorly to receive the femur. The hind coxae not produced behind into flat plates partly concealing the hind femora. Hind coxae without the steep transverse declivity characteristic of Dryopoidea. The inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel. Tarsal segmentation formula 5, 5, 5. None of the tarsi with ‘hidden’ segments. The front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented; pentamerous. Hind tarsi equipped with ‘swimming hairs’. Swimming hairs confined to the tarsi.

Elytra striate or with regular rows of punctures.

Adults not predacious. In diverse (including brackish) aquatic and damp habitats, often associated with live or decaying vegetation, occasionally under decaying seaweed.

Larvae. The larvae elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; only very lightly pigmented or sclerotized; only very lightly pigmented. The antennae 3 segmented. Frontoclypeal suture between frons and clypeus indistinct or absent. The labrum and head capsule completely fused, with no suture apparent. Apices of the mandibles with a single lobe or tooth. The labial palps 4 segmented. The mesothoracic legs with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal.

Abdominal tergum 8 without amature. The larvae predacious.

Worldwide and British representation. About 21 species in Britain (mostly terrestrial and associated with dung, but 9 species generally found in vegetation in or near water). Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland.

Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea; Hydrophilidae; Subfamily Sphaeridiinae.

General comments on this taxon. Maxillary palps longer than the antennae.

Miscellaneous. • Cercyon, Helophorus, Hydrochus, Laccobius and Sphaeridium (with Hydraenidae and Staphylinidae): Fowler Suppl. 3, 1913. • Fowler Suppl. 3, 1913: original legend.. • 10 Cercyon species (from Joy, 1932).

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Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: water beetles. Version: 18th September 2012.’.