Insects of Britain and Ireland: the families of Coleoptera
General appearance. 3.5–7.8 mm long. Body length/maximum body width 1.4–1.98. Elytral length/pronotal length 3.1–4.95. Base of prothorax not or scarcely 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 oval; dorsally somewhat convex to dorsally strongly convex; not necked; not waisted; very dark, blue or olive green. Upper surfaces of body glabrous or subglabrous (glabrous in Gyrinus), or non-glabrous (Orectochilus with thick, short pubescence); not bristly; with neither scales nor scale-like setae. The underside without a plastron of hydrofuge hairs.
Detailed morphology. Inclination of the head slight. Eyes ostensibly four; not strongly protuberant; without bristles. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. Mandibles without a mola; without prosthecae. The mandibular apices bidentate or bilobed. The incisor edges of the mandibles with a single tooth. The maxillae with distinct galea and lacinia apically to the palp, or with a single apical structure additional to the palp. The apical segment of the maxillary palps cylindrical to fusiform, or somewhat expanded and truncate to subtriangular. The apical segment of the labial palps more or less expanded apically. Antennae very short; 8–11 segmented; clubbed. Antennal clubs preceded by a cupule, or without a cupule. Antennal insertions visible from above, or hidden from above.
Cervical sclerites absent. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.3–0.53. The pronotum with lateral keels (pronotal carinae); keels complete. Prothorax at its widest not markedly narrower than the adjoining part of the abdomen. Prothorax with notopleural sutures. Scutellum conspicuous to absent; when applicable, not elevated; anteriorly simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process present; incomplete; falling short of the mesoventrite. Metaventrite with a transverse groove, or without a transverse groove. Mid- and hind-legs oar-like and much shorter than the fore-legs, which are elongated and modified for grasping prey. The fore-leg coxae countersunk in procoxal cavities. The fore-leg coxal cavities open behind externally; broadly open; medianly confluent, or narrowly separated; slightly transverse; without lateral extensions; internally closed by a slender bar. 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, or markedly oblique; open laterally. Hind-leg coxae contiguous or narrowly separated; much enlarged; extending laterally to meet the elytra; immoveably fixed to the metasternum and dividing the first abdominal sternite. Tarsal segmentation formula 5, 5, 5. The tarsi without bilobed segments; without hidden segments. Front tarsi with as many segments as the mid-tarsi; 5-segmented (large). Mid-leg tarsi 5-segmented (but much shorter than those of the fore-legs, flattened and oar-like); pentamerous; 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. Hind tarsi equipped with swimming hairs; with as many segments as the mid-tarsi; 5-segmented; flattened and oar-like for swimming (like the mid-leg tarsi: much shorter than those of the fore-legs).
Elytral length/maximum width across the elytra 1.15–1.54. Elytra exposing no more than part of the terminal tergite to at least one but fewer than three complete abdominal tergites; glossy; striate (Gyrinus with punctured striae), or without striae (confusedly punctured in Orectochilus). 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; the lines per elytron when more than five, 9 to 11. Scutellary striole absent. Wings well developed. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites 6–7; comprising both fused and moveable components. Basal abdominal sternites immovably joined 2, or 3. Abdominal segment 8 with apparently functional spiracles. The male external genitalia adephagan.
Adult habitat, ecology. Water-beetles (conspicuously gregarious, storing air under the elytra). Beetles surface swimmers with the main thrust delivered by the specialised middle legs, moving clumsily on land (Whirligigs). Rowing by parallel-simultaneous leg movements (on the surface of the water, in colonies, with characteristic whirling motion). Beetles respiring under water via air which is collected posteriorly and stored directly under the elytra; not noticeably posing regularly at the water surface to replenish air. Largely predacious (preying on insects and other small aquatic animals, finding food on the water surface with the aid of their specialized antennae).
Larvae. Mature larvae small to medium-sized. The larvae campodeiform; elongate and more or less parallel-sided. Body somewhat flattened. Vestiture restricted to fine hairs or setae. The larvae dorsally only very lightly pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 4 segmented; more than 0.5 x the width of the head. Stemmata present; on either side of the larval head 6. The larval fronto-clypeus much extended forwardly. The 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 maxillary palps 4 segmented. The labium without ligula between the palps. Labial palps present and segmented; 3 segmented. Mesothoracic legs present and segmented; 6 segmented; the tarsi 2-clawed; with 2 moveable claws. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal. The abdomen having functional spiracles on anterior segments (annular), or with functional spiracles confined to the the eighth segment; without spiracular tubes. The larvae with abdominal gills; with long and narrow lateral gills on abdominal segments 1–9. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The posterior segments without swimming hairs (?). The last abdominal segment with cerci, or without cerci.
Larvae aquatic (the early stages living underwater, and breathing dissolved oxygen via gills; subsequently developing spiracles and pupating on land); entirely predacious (feeding on a variety of insects and other small aquatic animals); bottom dwellers until they emerge from the water to pupate.
Classification. Suborder Adephaga; Superfamily Caraboidea.
Representation in Britain and Ireland, and worldwide. About 700 species worldwide. 13 species in Britain; genera in Britain 3; Aulonogyrus, Gyrinus, Orectochilus. E.g., Gyrinus bicolor (Elongated Whirl-wig or Whirligig Beetle).
General comments. Smooth, boat-shaped, surface-swimming beetles, with divided eyes which are adapted for simultaneously seeing above and below the water, and characteristically swimming rapidly in circles when alarmed. Gyrinus differing from Orectochilus in the elytral striation and puncturing (q.v.).
Illustrations. • Gyrinus bicolor (Elongated Whirl-wig or Whirligig beetle: B. Ent. 079). • Gyrinus bicolor (details, B. Ent. 079). • Gyrinus bicolor: B. Ent. 079, legend+text. • Gyrinus bicolor: B. Ent. 079, text cont..
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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’.