Insects of Britain and Ireland: 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.

General appearance. 1.5–38 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. The underside without a plastron of hydrofuge hairs.

Detailed morphology. Inclination of the head slight. Eyes not strongly protuberant; not accommodating the antennae in a notch (but characteristically indented above them in the Colymbetinae); without bristles; finely facetted. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. 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 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.

Cervical sclerites absent. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.2–0.68. The pronotum with lateral keels (pronotal carinae); keels complete. Prothorax at its widest not markedly narrower than the adjoining part of the abdomen. Prothorax with notopleural sutures. Scutellum conspicuous, or highly reduced, or absent; when applicable, not elevated; anteriorly simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process present; complete; moderately or strongly overlapping the mesoventrite, or concealing most or all of the mesoventrite. Metaventrite without a transverse groove. The fore-leg coxae countersunk in ‘procoxal cavities’. The fore-leg coxal cavities open behind externally; broadly open; narrowly separated, or quite widely separated; circular to longer than wide; without lateral extensions; internally closed by a slender bar (some phreatic Hydroporinae), or broadly closed internally. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by less than the shortest diameter of the cavity. The mid-leg coxal cavities variously contiguous, or narrowly separated, or moderately to widely separated; not or scarcely oblique; open laterally. Hind-leg coxae contiguous or narrowly separated; much enlarged (but without the longitudinal plates characteristic of Noteridae); extending laterally to meet the elytra; immoveably fixed to the metasternum and dividing the first abdominal sternite. Tarsal segmentation formula 5, 5, 5, or 4, 4, 5. The tarsi exhibiting bilobed segments, or without bilobed segments; with a tiny penultimate segment hidden by distal lobing of the fourth and fused to the fifth, or without ‘hidden’ segments. 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). Mid-leg tarsi 4-segmented (Hydrporinae), or 5-segmented; pentamerous, or pseudotetramerous, or tetramerous; the penultimate segment not distinctly shorter than the antepenultimate one. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple; without an associated empodium. Hind tarsi equipped with ‘swimming hairs’; with as many segments as the mid-tarsi; 5-segmented; flattened and oar-like for swimming.

Elytral length/maximum width across the elytra 0.95–2. Elytra exposing no more than part of the terminal tergite. Elytra with six or more impressed striae, or apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae (Hydroporinae); the lines per elytron when more than five, 6–7, or 8 to 11. Scutellary striole absent. Wings well developed, or absent or much reduced. Wings without an anal lobe. Wings with a medial fleck; the medial fleck bisected by a vein, or not bisected by a vein. Abdominal sternites 4–6; all articulated and moveable, or comprising both fused and moveable components. Basal abdominal sternites immovably joined when present, 2–3. Abdominal segment 8 with apparently functional spiracles. The male external genitalia adephagan.

Adult habitat, ecology. Water-beetles. Beetles free-swimming and diving strongly, with the main thrust delivered by the specially adapted hind legs, moving clumsily on land. Diving and ‘Rowing’ by parallel-simultaneous leg movements. Beetles respiring under water via air which is collected posteriorly and stored directly under the elytra (respiring via the terminal pair of spiracles); regularly posing tail first at the water surface to replenish air; collecting air at the water surface by exserting the tip of the abdomen through the surface film. Predacious (on a variety of aquatic animals, including tadpoles and fish, with the mandibles adapted for chewing and internal digestion).

Larvae. Mature larvae minute (less than 3 mm long) to relatively large. The larvae campodeiform; elongate and more or less parallel-sided, or oblong to ovate. Body circular in cross-section, or somewhat flattened, or strongly flattened. Vestiture restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized, or dorsally only very lightly pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 4 segmented, or 5–15 segmented (? - “five or more”); more than 0.5 x the width of the head. Stemmata present, or absent; on either side of the larval head when present, 6. The larval fronto-clypeus much extended forwardly, or not extended forwardly. The 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. Labial palps present and segmented; 2 segmented, or 3 segmented, or 4 segmented. Mesothoracic legs present and segmented; 6 segmented; the tarsi 2-clawed; with 2 moveable claws. Visible abdominal segments 8, or 9. Tergum 9 of the abdomen completely ventral. The abdomen having functional spiracles on anterior segments (these annular), or with functional spiracles confined to the the eighth segment; without spiracular tubes. The larvae without abdominal gills. The abdominal apex without a respiratory chamber. 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, or without cerci.

Larvae aquatic (like the adults, periodically taking air at the water surface via the modified, terminal pair of spiracles); 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).

Classification. Suborder Adephaga; Superfamily Caraboidea.

Representation in Britain and Ireland, and worldwide. 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. 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. Boat-shaped and flattened beneath, the scutellum nearly always visible.

Illustrations. • 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 in Britain or Ireland. 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).


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. Insects of Britain and Ireland: the families of Coleoptera. Version: 16th May 2016. delta-intkey.com’.

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