British insects: the families of Coleoptera


L. Watson and M. J. Dallwitz


Crawling Water-beeetles.

Adults. Beetles aquatic; walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; moving in the water by alternate, walking leg movements; respiring under water via air which is collected posteriorly and stored directly under the elytra (with additional storage in the post-coxal cavities); sometimes collecting air at the water surface by exserting the tip of the abdomen through the surface film (?). Beetles 2–4.5 mm long; body length/maximum body width 1.45–2.5; elytral length/pronotal length 3.24–4.05; 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; conspicuously necked to not necked; not waisted to somewhat waisted; yellowish. 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; coarsely facetted. Mandibles without a mola; with well developed prosthecae, or without prosthecae. The mandibular apices simple. 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 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 visible 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 0.43–0.66. Metaventrite with a transverse groove, or 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; 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 much enlarged and produced behind into large flat plates which partly conceal the hind femora (and also concealing the basal abdominal sternites). Tarsal segmentation formula 5, 5, 5. 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 (and modified for swimming, cf. the hind pair); 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; not flattened oar-like. Hind tarsi equipped with ‘swimming hairs’. Swimming hairs on both middle and hind legs (at least in Haliplus); present on both tibiae and tarsi (at least in Haliplus).

Elytral length/maximum width across the elytra 1.15–1.52. Elytra exposing no more than part of the terminal tergite; darkly, conspicuously longitudinally striate (each with 10–13 rows of punctured striae). Scutellary striole present, or absent. Elytra without a microreticulum (according to Balfour-Browne). 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 not predacious; phytophagous (mainly on green algae).

Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae, or not restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized, or only very lightly pigmented or sclerotized; more or less heavily pigmented, or 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, or with a short ligula between the palps. The labial palps 3 segmented. The mesothoracic legs 6 segmented (including the pretarsus). The tarsi 1-clawed. The mesothoracic legs with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal, or extending to the underside.

The larvae aquatic. The abdomen exhibiting functional abdominal spiracles, or without functional spiracles; having functional spiracles on anterior segments, or with functional spiracles confined to the the eighth segment. The larvae without gills (breathing dissolved oxygen through the body wall, which is increased in area segmental processes). Abdominal tergum 8 without amature. The last abdominal segment without cerci. The larvae predacious (with short, curved, hollow suctorial mandibles).

The larvae with 4-segmented antennae, legs with single claws, and an elongate 10th segment with cerci.

Worldwide and British representation. About 200 species worldwide; genera 5. 18 species in Britain; genera in Britain 3; Brychius, Haliplus, Peltodytes.

Classification. Suborder Adephaga; Superfamily Caraboidea. E.g., Haliplus obliquus.

General comments on this taxon. The elytra each with about 10 longitudinal rows of large punctures, with at least five of them on the dorsal side inside the shoulder. Boat-shaped beetles, the scutellum not visible; crawling under water, and swimming, via alternate leg movements. The thorax laterally sinuous and stria 3 of the elytra strongly raised in Brychius; thorax broadest at the base and not sinuate in the other genera, in which stria 3 of the elytra is not raised. In Peltodytes, the last joint of the maxillary palps is as broad as, and as long as, the penultimate one, while in Haliplus it is much narrower and shorter than it.

Miscellaneous. • Haliplus fulvus: B. Ent. 730. • Haliplus fulvus: B. Ent. 730, legend+text. • Haliplus fulvus: B. Ent. 730, text cont.. • Haliplus fulvus (Janson 43). • Haliplus obliquus (Rye & Fowler III4). • Haliplidae - typical underside (fom Joy, 1932).

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.’.