British insects: the families of Coleoptera
British insects: the families of Coleoptera | |
True Water-beetles, Predacious Diving Beetles; the larvae known as Water Tigers.
General appearance. 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.
Detailed morphology. Inclination of the head slight. Eyes not strongly protuberant; without bristles; finely facetted. The maxillae with an outer palpiform lobe (the galea) additional to the 4-segmented maxillary palp. Antennae very short to short; 11 segmented; filiform. Antennal insertions hidden from above; not in fossae. Mandibular prosthecae absent. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.2–0.68. Prothorax at its widest not markedly narrower than the adjoining part of the abdomen. Prothorax with notopleural sutures. Metaventrite without a transverse groove. Hind coxae immovably fixed to the metasternum and dividing the first abdominal sternite; much expanded and extended laterally to meet the elytra (without the longitudinal plates characteristic of Noteridae). 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 claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple; without an associated empodium. Hind tarsi with as many segments as the mid-tarsi; 5-segmented; modified for swimming (often), or not modified for swimming. Elytral length/maximum width across the elytra 0.95–2. Elytra exposing no more than part of the terminal tergite. Scutellary striole absent. 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.
Adult habitat, ecology. Water-beetles (taking in air periodically at the water surface via the terminal pair of spiracles); diving and ‘Rowing’ by parallel-simultaneous leg movements; predacious (on a variety of aquatic animals, including tadpoles and fish, with the mandibles adapted for chewing and internal digestion).
Larvae. 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.
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. 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.. • 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).
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: the families of Coleoptera. Version: 2nd January 2012. http://delta-intkey.com’.
