British insects: the families of Coleoptera


L. Watson and M. J. Dallwitz


= Dytiscidae part; including Phreatodytidae.

Burrowing Water-beetles, Diving 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; 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 3.5–5 mm long; body length/maximum body width 1.38–2.1; elytral length/pronotal length 2.05–3.75; base of prothorax not or scarcely narrower than the combined elytral bases, or distinctly 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 not necked; yellowish- or reddish-brown. 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; without bristles; finely facetted. Mandibles without a mola; with reduced prosthecae, or without prosthecae. The mandibular apices bidentate or bilobed, or multidentate or multilobed. The incisor edges of the mandibles simple. 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 apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps more or less expanded apically. Antennae short; 11 segmented; more or less filiform (but somewhat thickened, the segments varying in shape and somewhat produced on the inner side and the fifth somewhat dilated in the male). 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 0.4–0.65. Metaventrite without a transverse groove. The fore-leg coxal cavities broadly open; quite widely separated; circular to longer than wide; without lateral extensions; 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 (with characteristic, conspicuous median longitudinal processes covering the articulations of the trochanters). The inner parts (‘processes’) of the hind coxae constituting longitudinal plates that are fused to one another and joined with the metasternum, endowing the beetle ventrally with a flat, median longitudinal keel. The hind coxal processes very broad, taken together at least as wide as long and forming a distinctive plate with a W-shaped hind margin (see illustrations). 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. Mid-leg tarsi 5-segmented; pentamerous. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple. 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’. Swimming hairs on both middle and hind legs; present on both tibiae and tarsi.

Elytral length/maximum width across the elytra 0.92–1.56. Elytra exposing no more than part of the terminal tergite; glossy. Scutellary striole absent. Elytra microreticulate, with a fine impressed reticulum. The mictoreticulation longitudinal (and traversed by large, eccentric compound punctures). 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, or 3. Abdominal segment 8 with apparently functional spiracles.

Adults predacious, or not predacious (?).

Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; only very lightly pigmented or sclerotized (in the British species); only very lightly pigmented. The antennae 4 segmented; 0.15–0.5 x the head width. Stemmata 6. 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. The labium without ligula between the palps. The labial palps 3 segmented. The mesothoracic legs 6 segmented (including the pretarsus); with 1 movable claw (in Noterus). Visible abdominal segments 9. Tergum 9 of the abdomen completely ventral.

The larvae aquatic. The abdomen exhibiting functional abdominal spiracles; having functional spiracles on anterior segments, or with functional spiracles confined to the the eighth segment. The larvae without gills (some species seeming to obtain air by tapping air from plants via a pointed apical process, while others visit the surface). Abdominal tergum 8 bearing a single median process with a pair of spiracles at its apex (in Noterus), or without amature. The larvae probably predacious (but not digesting extra-orally).

The larvae lack the neck constriction seen in Dytiscidae, and unlike them do not digest prey extra-orally.

Worldwide and British representation. About 230 species worldwide; genera 12 (especially tropical). 2 species in Britain; genera in Britain 1; Noterus.

Classification. Suborder Adephaga; Superfamily Caraboidea. E.g., N. clavicornis (Marsham's Noterus Water-beetle).

General comments on this taxon. Beetles dorsally very convex and boat-shaped, the scutellum invisible; the rear of the head about as wide as the front of the pronotum; burrowing in ponds and marshes..

Miscellaneous. • Noterus clavicornis (Marsham's Water-beetle: B. Ent. 236). • Noterus clavicornis (details, B. Ent. 236). • Noterus clavicornis: B. Ent. 236, legend+text. • Noterus clavicornis: B. Ent. 236, text cont.. • N. clavicornis, elytral sculpture (Balfour-Browne).

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.’.