Insects of Britain and Ireland: the families of Coleoptera
General appearance. (0.75–)1.1–1.7(–2.5) mm long. Body length/maximum body width 1.6–2.57. Elytral length/pronotal length 1.77–3.62. Base of prothorax not or scarcely narrower than the combined elytral bases, or distinctly narrower than the combined elytral bases. Greatest prothoracic width not narrower or only slightly narrower than the greatest elytral width, or distinctly narrower than greatest elytral width. Beetles elongate; not necked; conspicuously waisted. Upper surfaces of body glabrous or subglabrous, or non-glabrous; not bristly; with neither scales nor scale-like setae. The underside without a plastron of hydrofuge hairs.
Detailed morphology. Eyes not strongly protuberant; without bristles. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. Mandibles with a well developed mola. The mandibular apices simple, or bidentate or bilobed. The incisor edges of the mandibles simple, or with a single tooth. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps conspicuously elongated, sometimes longer than the antennae; apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short to about half the insect's head to tail length (perhaps with a respiratory function?); 7–11 segmented; clubbed. Antennal clubs 5 segmented (the club hairy); preceded by a cupule, or without a cupule. Antennal insertions hidden from above.
Cervical sclerites present (mostly), or absent (Limnebius). Pronotal length/maximum pronotal width 0.42–1.05. The pronotum with lateral keels (pronotal carinae); keels complete. Prothorax without notopleural sutures. Scutellum conspicuous, or conspicuous to highly reduced; when applicable, not elevated; anteriorly simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process complete; slightly overlapping the mesoventrite. Metaventrite without a transverse groove. The fore-leg coxae countersunk in procoxal cavities. The fore-leg coxal cavities open behind externally, or closed behind externally (most Hydraeninae); broadly open to narrowly open, or broadly closed (most Hydraeninae); narrowly separated; strongly transverse, or slightly transverse; with narrow lateral extensions (most Hydraeninae), or without lateral extensions; internally open, or broadly closed internally (some Hydraeninae). The mid-leg coxae countersunk in mesocoxal cavities; separated by less than the shortest diameter of the cavity. The mid-leg coxal cavities narrowly separated; not or scarcely oblique; open laterally. Hind-leg coxae contiguous or narrowly separated; extending laterally to meet the elytra. Tarsal segmentation formula 4, 4, 4. The tarsi without bilobed segments; with a tiny basal segment that is hard to detect. Front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented; pentamerous, or pseudotetramerous (the basal one), or tetramerous; the penultimate segment not distinctly shorter than the antepenultimate one. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple; without an associated empodium. Hind tarsi with as many segments as the mid-tarsi; 5-segmented; not flattened and oar-like.
Elytral length/maximum width across the elytra 1.18–2. Elytra exposing no more than part of the terminal tergite. Elytra with six or more longitudinal lines of punctures, or apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae (Limnebius); the lines per elytron when more than five, 6–7, or 8 to 10, or 12 or more. Scutellary striole absent. Wings well developed, or absent or much reduced; fringed with long hairs on the hind margin, or not fringed. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites (6–)7; all articulated and moveable. Abdominal segment 8 with apparently functional spiracles. The male external genitalia staphylinoid.
Adult habitat, ecology. All or mostly? water-beetles. Beetles 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. Beetles replenishing air for respiration beneath the elytra indirectly from that acquired and held in the ventral plastron; not noticeably posing regularly at the water surface to replenish air; incorporating bubbles of oxygen directly into the plastron. Occurring in various aquatic, semi-aquatic, riparian and littoral habitats; phytophagous, or phytophagous and consuming decaying plant material (feeding principally on algae?).
Larvae. Mature larvae minute (less than 3 mm long). The larvae campodeiform; elongate and more or less parallel-sided. Body somewhat flattened. Vestiture restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized, or dorsally only very lightly pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 3 segmented; 0.15–0.5 x the head width, or more than 0.5 x the width of the head. Stemmata present; on either side of the larval head 3, or 5. The larval fronto-clypeus not extended forwardly. The frontoclypeal suture between frons and clypeus distinct. The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate. The maxillary palps 3 segmented. The labium with a short ligula between the palps. Labial palps present and segmented; 2 segmented. Mesothoracic legs present and segmented; 5 segmented; with 1 moveable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal. The abdomen having functional spiracles on anterior segments (these annular); with the spiracles on the eighth segment borne at the ends of spiracular tubes, or without spiracular tubes. The larvae without abdominal gills. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The last abdominal segment with cerci.
Larvae non-aquatic (living in wet places, but easily drowned); not predacious, or predacious and not predacious (?); found in assorted aquatic and littoral habitats, cf. the adults; phytophagous (perhaps mainly on algae, but some predacious and saprophagous behaviour has been reported in the family).
Larvae subcylindric, active, with 10 abdominal segments.
Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Staphilinoidea.
Representation in Britain and Ireland, and worldwide. Genera at least 20. 29 species in Britain; genera in Britain 3; Hydraena, Limnebius, Ochthebius. E.g., Hydraena testacea (Testaceous Hydraena); Ochthebius punctatus (Irish Ochthebius); Ochthebius exsculptus (Gibson's Enicocerus).
Illustrations. • Hydraena testacea (Testaceous Hydraena: B. Ent. 307). • Hydraena testacea (details, B. Ent. 307). • Hydraena testacea: B. Ent. 307, legend+text. • Ochthebius punctatus (Irish Ochthebius: B. Ent. 250). • Ochthebius punctatus: B. Ent. 250, legend+text. • Ochthebius punctatus: B. Ent. 250, text cont.. • Ochthebius exsculptus (Gibson's Enicocerus): B. Ent. 291. • Ochthebius exsculptus: B. Ent. 291, legend+text. • Ochthebius exsculptus: B. Ent. 291, text cont.. • Hydraena britteni and Ochthebius subinteger, with Hydrophilidae and Staphylinidae: Fowler Suppl. 3, 1913. • Fowler Suppl. 3, 1913: original legend..
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Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. Insects of Britain and Ireland: the families of Coleoptera. Version: 16th May 2016. delta-intkey.com’.