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| Insects of Britain and Ireland: water beetles |
Adults. Beetles aquatic; free-swimming and diving strongly, with the main thrust delivered by the specially adapted hind legs, moving clumsily on land; ‘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; collecting air at the water surface by exserting the tip of the abdomen through the surface film; 3.9–5.3 mm long. Body length/maximum body width 1.23–2.4; base of prothorax not or scarcely narrower than the combined elytral bases. Beetles oval to elongate-oval; only about 2 to 2.5 times as long as the maximum depth in lateral view: globular, less elongate and the elytra more rounded laterally than is usual in Hydroporus etc.; dorsally strongly convex (almost globular, of similar convexity above and below); convex beneath; with the closed elytra not drawn out to form small point at the rear; not necked; somewhat waisted; with sharply defined elytral spots and/or blotches, or without sharply defined stripes, spots or blotches. Upper surfaces of body not bristly; with neither scales nor scale-like setae. Beetles without a rostrum. Inclination of the head slight.
Eyes two; not strongly protuberant; without bristles; finely facetted. Mandibles without a mola; without prosthecae. The maxillae with distinct galea and lacinia apically to the palp. The maxillae with an outer palpiform lobe (the galea) additional to the 4-segmented maxillary palp. The maxillae without stylet-like lobes. The maxillary palps not especially elongated. The apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short, or short; not strongly asymmetric; 11 segmented. Antennal scape not swollen. Antennae filiform. Antennal insertions hidden from above; not in fossae.
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 0.2–0.68. The pronotum without a single, short median-longitudinal furrow towards the outer margin on each side; without a short longitudinal furrow on each side. Scutellum absent. Metaventrite without a transverse groove. The prosternal process complete; moderately or strongly overlapping the mesoventrite to concealing most or all of the mesoventrite. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; narrowly separated to quite widely separated; circular to longer than wide; without lateral extensions. 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. The hind claws paired; tarsal claws very unequal in length. Tarsal segmentation formula 4, 4, 5 (the deep lobes of the expanded third segment obscuring the diminutive fourth one). Some of the tarsi exhibiting conspicuously bilobed segments. Some tarsi with a tiny penultimate segment hidden by distal lobing of the fourth and fused to the fifth. The front tarsi with as many segments as the mid-tarsi; 5-segmented (but pseudotetramerous, like the mid-leg ones). Mid-leg tarsi 5-segmented; pseudotetramerous. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple; without an associated empodium. Hind tarsi with at least as many segments as the mid-tarsi; 5-segmented; flattened oar-like for swimming. Hind tarsi equipped with ‘swimming hairs’.
Elytral length/maximum width across the elytra 0.95–2. Elytra exposing no more than part of the terminal tergite; subquadrate. Scutellary striole absent. Elytra with epipleura. The elytral epipleura divided into two parts by a distinct ridge at the outer corner of the elytron. Elytra microreticulate, with a fine impressed reticulum. The mictoreticulation regular, or longitudinal (with scattered, simple punctures). Wings without an anal lobe; with a medial fleck; the medial fleck bisected by a vein, or not bisected by a vein. Abdominal segment 8 with apparently functional spiracles. The male external genitalia adephagan.
Adults predacious (with the mandibles adapted for chewing and internal digestion). In still or slow-running water with vegetation.
Larvae. The larvae campodeiform; vestiture restricted to fine hairs or setae; only very lightly pigmented. The antennae more than 0.5 x the width of the head. 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 mesothoracic legs 6 segmented (including the pretarsus). The tarsi 2-clawed. The mesothoracic legs with 2 movable claws. Tergum 9 of the abdomen completely ventral.
The larvae aquatic (like the adults, periodically taking air at the water surface via the modified, terminal pair of spiracles); without gills. Abdominal tergum 8 bearing a single median process with a pair of spiracles at its apex. The posterior segments without swimming hairs. The last abdominal segment with cerci. The larvae predacious (conducting digestion externally, with digestive enzymes and the products of digestion transported via the hollow mandibles; finally leaving the water to pupate in damp soil).
Representation in Britain and Ireland, and worldwide. 2 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland (scarce in Scotland).
Classification. Suborder Adephaga; Superfamily Buprestoidea; Dytiscidae; Subfamily Hydroporinae.
General comments. The hind tarsal claws very unequal.
Miscellaneous. • Hyphydrus ovatus, elytral sculpture (Balfour-Browne).
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2012 onwards. Insects of Britain and Ireland: water beetles. Version: 14th April 2022. delta-intkey.com’.
