British insects: the families of Coleoptera
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. Beetles 22–39 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; dorsally somewhat convex to dorsally strongly convex (of similar convexity above and below); convex beneath; not necked; somewhat waisted. Upper surfaces of body not bristly; with neither scales nor scale-like setae. Inclination of the head slight.
Eyes two; not strongly protuberant; vertically straight edged at the front; 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 patterned; dark, yellow margined either all round or at front and sides only (usually), or pale, with a broad dark bat-shaped band across the middle (in D. lapponicus). Scutellum present. Metaventrite without a transverse groove. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; 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. Hind tibia longer than wide, and longer than the relatively slender spurs. The hind claws paired; tarsal claws more or less equal in length. 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; each having the three proximal segments expanded and approximated to form a conspicuous rounded disc (in the males), or not each exhibiting a conspicuous, expanded rounded disc. Mid-leg tarsi 5-segmented; pentamerous, or pseudotetramerous, or tetramerous. 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; with conspicuous longitudinal ribs (in some females only), or not conspicuously longitudinally ribbed; when ribbed, about 8–12 ribbed. Scutellary striole absent. Elytra faintly microreticulate, with a fine impressed reticulum (at least in male in D. maginalis). The mictoreticulation regular (with punctures). Abdominal segment 8 with apparently functional spiracles.
Adults predacious (with the mandibles adapted for chewing and internal digestion). In ponds, fens, lochs, drains, sometimes in stagnant or brackish water.
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 not 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 with 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).
Worldwide and British representation. 6 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland.
Classification. Suborder Adephaga; Superfamily Caraboidea; Dytiscidae; Subfamily Dytiscinae.
General comments on this taxon. Beetles with the pronotum dark with yellowish margins, or (D. lapponicus) pale with a broad bat-shaped band across the middle; the elytra of females usually longitudinally grooved. The hind tibiae longer than wide, with relatively a relatively small pair of spurs; the hind tarsal claws paired, and the females usually with longitudinally multiply-grooved elytra.
Miscellaneous. • Dytiscus dimidiatus (Thick-horned Dytiscus: B. Ent. 099). • Dytiscus dimidiatus (details, B. Ent. 099). • Dytiscus dimidiatus: B. Ent. 099, legend+text. • Dytiscus dimidiatus: B. Ent. 099, text cont.. • Dytiscus circumcinctus, D. circumflexus, D. dimidiatus D. marginalis, D. semisulatus (from Joy, 1932). • Dytiscus marginalis (male), elytral sculpture (Balfour-Browne). • Dytiscus marginalis, larva and pupa.
To view the illustrations with detailed captions, go to the interactive key. This also offers 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.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.