British insects: the families of Coleoptera


L. Watson and M. J. Dallwitz

Orectochilus Dejean

Adults. Beetles aquatic; 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; respiring under water via air which is collected posteriorly and stored directly under the elytra. Beetles 5.5–6.5 mm long; body length/maximum body width 1.4–1.98; base of prothorax not or scarcely narrower than the combined elytral bases. Beetles oval; dorsally somewhat convex to dorsally strongly convex; not necked; not waisted; very dark, blue or olive green. Upper surfaces of body non-glabrous; 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 an outer palpiform lobe (the galea) additional to the 4-segmented maxillary palp. The maxillary palps not especially elongated. The apical segment of the labial palps more or less expanded apically. Antennae very short; not strongly asymmetric; 8–11 segmented (?). Antennal scape not swollen. Antennae clubbed. 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. The pronotum and elytra covered with fine hair. 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; 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; 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 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’. Swimming hairs on both middle and hind legs; present on both tibiae and tarsi.

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; without longitudinal striae (confusedly punctured, by contrast with Gyrinus). Scutellary striole absent. Elytra faintly microreticulate, with a fine impressed reticulum. The mictoreticulation regular (with eccentric compound punctures). 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). In running water and wave-washed like edges.

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); 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.

Worldwide and British representation. 1 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland (scarce in the southeast).

Classification. Suborder Adephaga; Superfamily Caraboidea; Gyrinidae.

General comments on this taxon. Further differing from Gyrinus in having the labrum protruding as viewed from above.

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