British insects: the families of Coleoptera


L. Watson and M. J. Dallwitz

Riolus Mulsant and Rey

Adults. Beetles 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; replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron; incorporating bubbles of oxygen directly into the plastron. Beetles 1.6–2.3 mm long; body length/maximum body width 1.8–3.1. Beetles oval to elongate-oval; without ventral body cavities for reception of the legs; not necked; somewhat waisted; conspicuously long-legged. All the legs long, but none longer than the entire beetle. Upper surfaces of body not bristly; with neither scales nor scale-like setae.

Eyes two; not strongly protuberant; without bristles. Mandibles with a well developed mola. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps not especially elongated. Antennae short; not strongly asymmetric; 7–11 segmented (?). Antennal scape not swollen. Antennae filiform. Antennal insertions visible from above; not in fossae.

Prothorax without notopleural sutures. The pronotum without sharply defined ridges (though with a distinct lateral border). The scutellum elevated. Metaventrite with a transverse groove. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; quite widely separated; without lateral extensions; internally open. Hind coxae not extended laterally to meet the elytra; posteriorly shaped to receive the retracted femur. The hind coxae not produced behind into flat plates partly concealing the hind femora. Hind coxae with a steep transverse declivity against which the femur retracts. 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. Mid-leg tarsi 5-segmented; pentamerous. The claws of the mid-leg tarsi not appendaged. Hind tarsi with at least as many segments as the mid-tarsi; 5-segmented (the elongate last segment bulbous at the tip).

Elytra exposing no more than part of the terminal tergite; not truncate; ribbed; with conspicuous longitudinal ribs; 3 ribbed. Exposed abdominal sternites 5–6; comprising both fused and movable components; immovably joined 3.

Adults not predacious; phytophagous; in living vegetation. In base-rich running water and lakes.

Larvae. The larvae campodeiform; elongate and more or less parallel-sided; only very lightly pigmented or sclerotized; more or less heavily pigmented. The antennae 3 segmented; 0.15–0.5 x the head width. Stemmata fewer than 6. The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate, or trilobed or tridentate (?). The maxillary palps 4 segmented. The labium with a short ligula between the palps, or with a ligula at least as long as the palps. The labial palps 4 segmented. The mesothoracic legs 5 segmented (including the pretarsus); with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen extending to the underside.

The larvae aquatic; with anal gill tufts. Abdominal tergum 8 without amature. The larvae not predacious; phytophagous; on living vegetation.

Worldwide and British representation. 2 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, and South-eastern England (but nowhere common).

Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea; Elmidae.

General comments on this taxon. Each elytron with three raised longitudinal ridges; the elongate last segment of the hind tarsi bulbous at the tip.

Miscellaneous. • Riolus cupreus, R. subviolaceus (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.’.