British insects: the families of Coleoptera

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L. Watson and M. J. Dallwitz

Dytiscidae

True Water-beetles, Predacious Diving Beetles; the larvae known as Water Tigers.

Adults. Beetles aquatic; free-swimming and diving strongly, with the main thrust delivered by the specially adapted hind legs, moving clumsily on land; diving and ‘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 1.7–38 mm long; body length/maximum body width 1.23–2.4; elytral length/pronotal length 1.8–7.3; 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 to elongate-oval; dorsally somewhat convex to dorsally strongly convex (of similar convexity above and below); not necked; somewhat waisted. Upper surfaces of body glabrous or subglabrous, or non-glabrous; not bristly; with neither scales nor scale-like setae. Inclination of the head slight.

Eyes not strongly protuberant; vertically straight edged at the front, or with the front edge conspicuously indented above the antennal insertions; without bristles; finely facetted. Mandibles without a mola; without prosthecae. The mandibular apices bidentate or bilobed, or multidentate or multilobed. The incisor edges of the mandibles simple, or with a single tooth. 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 apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short to short; 11 segmented; 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 with a single longitudinal furrow posteriorly on each side, these located two-thirds of the way from the mid-line and adjoining a pair of short furrows at the bases of the (closed) elytra (Hydroglyphus, Bidessus), or without the pair of longitudinal furrows characteristic of Hydroglyphus, Bidessus et al. (q.v.). Metaventrite without a transverse groove. The fore-leg coxal cavities broadly open; narrowly separated, or quite widely separated; circular to longer than wide; without lateral extensions; internally closed by a slender bar, or broadly closed internally. Hind coxae immovably fixed to the metasternum and dividing the first abdominal sternite; much expanded and extended laterally to meet the elytra. The hind coxae expanded but not produced behind into flat plates partly concealing the hind femora (i.e., unlike Haliplidae). The inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel (beetles without the median longitudinal plates characteristic of Noteridae). The hind coxal processes longer than broad, the hind margin not W-shaped. Tarsal segmentation formula 5, 5, 5 (usually), or 4, 4, 5. Some of the tarsi exhibiting conspicuously bilobed segments, or none of the tarsi with conspicuously bilobed segments. Some tarsi with a tiny penultimate segment hidden by distal lobing of the fourth and fused to the fifth, or none of the tarsi with ‘hidden’ segments. The front tarsi with as many segments as the mid-tarsi; 4-segmented (Hydroporinae), or 5-segmented (sometimes the males having the first three segments dilated to form adhesive pads, used to grip the female); each having the three proximal segments expanded and approximated to form a conspicuous rounded disc (in the males of some genera), or not each exhibiting a conspicuous, expanded rounded disc. Mid-leg tarsi 4-segmented (Hydroporinae), or 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; hard. Elytra predominantly finely but rather conspicuously transversely striated across lines of punctured striae (Colymbetes only), or not predominantly transversely striated across lines of puntured striae. Scutellary striole absent. Elytra nearly always microreticulate, with a fine impressed reticulum (though often inconspicuously so), or without a microreticulum (as exemplified by Scarodytes). Wings well developed, or absent or much reduced. Exposed abdominal sternites 4–6; all articulated and movable, or comprising both fused and movable components; immovably joined when present, 2–3. Abdominal segment 8 with apparently functional spiracles.

Adults predacious (on a variety of aquatic animals, including tadpoles and fish, with the mandibles adapted for chewing and internal digestion).

Larvae. The larvae campodeiform (constricted at the neck); elongate and more or less parallel-sided, or oblong to ovate; vestiture restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized, or only very lightly pigmented or sclerotized; only very lightly pigmented. The antennae 4–10 segmented (4 to five or more); more than 0.5 x the width of the head. Stemmata 6, or fewer than 6 (then lacking). The larval fronto-clypeus much extended forwardly (hydroporines), or 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 maxillary palps 3 segmented, or 4 segmented. The labium without ligula between the palps, or with a short ligula between the palps, or with a ligula at least as long as the palps. The labial palps 3 segmented, or 4 segmented. The mesothoracic legs 6 segmented (including the pretarsus). The tarsi 2-clawed. The mesothoracic legs with 2 movable claws. Visible abdominal segments 8, or 9. 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). The abdomen having functional spiracles on anterior segments, or with functional spiracles confined to the the eighth segment. The larvae without gills. Abdominal tergum 8 bearing a single median process with a pair of spiracles at its apex. The posterior segments with swimming hairs, or without swimming hairs. The last abdominal segment with cerci. The larvae predacious (on a variety of aquatic animals, including tadpoles and fish, and 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. About 4000 species worldwide; genera about 160 (in 7 subfamilies). 113 species in Britain; genera in Britain 27; Acilius, Colymbetes, Dytiscus, Graphoderus, Hygrotus, Oreodites, Platambus, etc.

Classification. Suborder Adephaga; Superfamily Caraboidea. E.g., Acilius canaliculatus ((Pale-thighed Acilius); Colymbetes cicur (Tuther's Dyticus Water-beetle: from southern Africa); Dytiscus dimidiatus (Thick-horned Dyticus Beetle); Graphoderus cinereus (Orange-striped Hydaticus Water-beetle); Oreodites davisi (Davisian Water-beetle); Hygrotus decoratus (Ornamented Hygrotus); Platambus maculatus. (The genus Cybister Curtis, exmplified by his illustration of C. roeselii, seemingly does not occur in Britain).

General comments on this taxon. Beetles boat-shaped and flattened beneath, the scutellum nearly always visible; the back of the head about as wide as the front of the pronotum.

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 marginalis, larva and pupa. • Acilius canaliculatus (Pale-thighed Acilius: B. Ent. 063). • Acilius canaliculatus (details, B. Ent. 063). • Acilius canaliculatus: B. Ent. 063, legend+text. • Acilius canaliculatus: B. Ent. 063, text cont.. • Colymbetes cicur (Tuther's Dytiscus: not British. B. Ent. 207). • Cybister roeselii (Roesel's Diving-beetle: B. Ent. 151. Mainland European?). • Cybister roeselii: B. Ent. 151, legend+text. • Cybister roeselii: B. Ent. 151, text cont.. • Graphoderus cinereus (Orange-striped Hydaticus: B. Ent. 095). • Graphoderus cinereus: B. Ent. 095, legend+text. • Graphoderus cinereus: B. Ent. 095, text cont.. • Oreodites davisi (Davisian Water-beetle: B. Ent. 343). • Oreodites davisi: B. Ent. 343, legend+text. • Oreodites davisi: B. Ent. 343, text cont.. • Hygrotus decoratus (Ornamented Hygrotus: B. Ent. 531). • Hygrotus decoratus: B. Ent. 531, legend+text. • Hygrotus decoratus: B. Ent. 531, text cont.. • Graptodytes hoffgarteni and Platambus maculatus, with Carabidae and Hydrophilidae: Fowler Suppl. 2, 1913. • Fowler Suppl. 2, 1913: original legend. • Platambus maculatus (Rye & Fowler III2). • Larvae of Colymbetes, Dytiscus: head details.


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’.

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