British insects: the families of Coleoptera
Adults. Beetles aquatic (on the water's surface, conspicuously gregarious); surface swimmers with the main thrust delivered by the specialised middle legs, moving clumsily on land (Whirligigs); rowing and diving in water by parallel-simultaneous leg movements (especially on the surface of the water, in colonies, with characteristic whirling motion); respiring under water via air which is collected posteriorly and stored directly under the elytra. Beetles 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. Inclination of the head slight.
Eyes ostensibly four; not strongly protuberant; without bristles. 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 maxillae with an outer palpiform lobe (the galea) additional to the 4-segmented maxillary 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.
Prothorax shorter than wide; at its widest not markedly narrower than the adjoining part of the abdomen; with notopleural sutures. Pronotal length/maximum pronotal width 0.3–0.53. 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 coxal cavities broadly open; medianly confluent, or narrowly separated; slightly transverse; without lateral extensions; internally closed by a slender bar. Hind coxae immovably fixed to the metasternum and dividing the first abdominal sternite; much expanded and extended laterally to meet the elytra. Tarsal segmentation formula 5, 5, 5. None of the tarsi with conspicuously bilobed segments. None of the tarsi with hidden segments. The front tarsi with as many segments as the mid-tarsi; 5-segmented (large). Mid-leg tarsi 5-segmented (much shorter than those of the fore-legs, flattened and oar-like); pentamerous. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple. Hind tarsi with at least as many segments as the mid-tarsi; 5-segmented; flattened oar-like for swimming (much shorter than those of the fore-legs, flattened and oar-like). Hind tarsi equipped with swimming hairs.
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; longitudinally striate (Gyrinus with punctured striae), or without longitudinal striae (confusedly punctured in Orectochilus). Scutellary striole absent. Elytra microreticulate, with a fine impressed reticulum. Wings well developed. Exposed abdominal sternites 6–7; comprising both fused and movable components; immovably joined 2, or 3. Abdominal segment 8 with apparently functional spiracles.
Adults largely predacious (finding food on the water surface with the aid of their specialized antennae).
Larvae. The larvae campodeiform; 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 4 segmented; more than 0.5 x the width of the head. Stemmata 6. The larval fronto-clypeus much extended forwardly. 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. The labial palps 4 segmented. The mesothoracic legs 6 segmented (including the pretarsus). The tarsi 2-clawed. The mesothoracic legs with 2 movable claws. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal.
The larvae aquatic (the early stages living underwater, and breathing dissolved oxygen via gills; subsequently developing spiracles and pupating on land). The abdomen exhibiting functional abdominal spiracles, or without functional spiracles; having functional spiracles on anterior segments, or with functional spiracles confined to the the eighth segment. The larvae with long and narrow lateral gills on abdominal segments 1–9. Abdominal tergum 8 without amature. The last abdominal segment without cerci. The larvae entirely predacious (the prey digested extra-orally).
Worldwide and British representation. About 700 species worldwide. 13 species in Britain; genera in Britain 2, or 3; Aulonogyrus (A. striatus, doubtfully recorded from the Outer Hebrides), Gyrinus, Orectochilus.
Classification. Suborder Adephaga; Superfamily Caraboidea. E.g., Gyrinus bicolor (Elongated Whirl-wig or Whirligig Beetle).
General comments on this taxon. Smooth, boat-shaped, surface-swimming beetles, with divided eyes which are adapted for simultaneously seeing above and below the water. Characteristically swimming rapidly in circles when alarmed. Gyrinus differing from Orectochilus in the finely hairy pronotum and elytra and protruding labrum of the latter, and in elytral striation and puncturing (q.v.).
Miscellaneous. • 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. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.