Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz


Crawling Water-beetles.

General appearance. 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. The underside without a plastron of hydrofuge hairs.

Detailed morphology. Inclination of the head slight. Eyes not strongly protuberant; without bristles; coarsely facetted. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. 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 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.

Cervical sclerites absent. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.43–0.66. 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 absent. The prosternal process complete; concealing most or all of the mesoventrite. Metaventrite with a transverse groove, or 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; broadly closed internally. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by less than the shortest diameter of the cavity to more than the shortest diameter of the cavity. The mid-leg coxal cavities moderately to widely separated; not or scarcely oblique; open laterally. Hind-leg coxae contiguous or narrowly separated; much enlarged; extending laterally to meet the elytra; immoveably fixed to the metasternum and dividing the first abdominal sternite; produced behind into flat plates which conceal the hind femora when these are retracted (and also concealing the basal abdominal sternites). Tarsal segmentation formula 5, 5, 5. The tarsi without ‘hidden’ segments. 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 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. Hind tarsi equipped with ‘swimming hairs’; with as many segments as the mid-tarsi; 5-segmented (somewhat modified for swimming); not flattened and oar-like (but with swimming hairs).

Elytral length/maximum width across the elytra 1.15–1.52. Elytra exposing no more than part of the terminal tergite; darkly, conspicuously striate. Elytra with six or more longitudinal lines of punctures, or apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae; the lines per elytron when more than five, 10. Scutellary striole present, or absent. Wings well developed, or absent or much reduced. Wings with an anal lobe. Wings without a medial fleck. Abdominal sternites 4–6; all articulated and moveable, or comprising both fused and moveable components. Basal abdominal sternites immovably joined when present, 2, or 3. Abdominal segment 8 with apparently functional spiracles. The male external genitalia adephagan.

Adult habitat, ecology. Water-beetles. Beetles 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. Beetles respiring under water via air which is collected posteriorly and stored directly under the elytra (presumably? - taking in air at the water surface, storing it both under the elytra and in the post-coxal cavities); regularly posing tail first at the water surface to replenish air, or not noticeably posing regularly at the water surface to replenish air; sometimes collecting air at the water surface by exserting the tip of the abdomen through the surface film (?). Not predacious, or predacious and not predacious; phytophagous (mainly on green algae - Chara, Spirogyra, etc. - and aquatic angiosperms such as Ceratophyllum, but perhaps also consuming insect eggs and hydrozoans?).

Larvae. Mature larvae small to medium-sized. The larvae campodeiform; elongate and more or less parallel-sided. Body circular in cross-section to somewhat flattened. Vestiture restricted to fine hairs or setae, or not restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized, or dorsally only very lightly pigmented or sclerotized; ventrally more or less heavily pigmented, or ventrally only very lightly pigmented. The antennae 4 segmented; 0.15–0.5 x the head width. Stemmata present; on either side of the larval head 6. The larval fronto-clypeus 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. The labium without ligula between the palps, or with a short ligula between the palps. Labial palps present and segmented; 2 segmented. Mesothoracic legs present and segmented; 6 segmented; the tarsi 1-clawed; with 1 moveable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal, or extending to the underside. 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 without amature. The last abdominal segment with cerci, or without cerci.

Larvae aquatic; predacious (with short, curved, hollow suctorial mandibles (Britton, 1970)).

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

Classification. Suborder Adephaga; Superfamily Caraboidea.

Representation in Britain and Ireland, and worldwide. About 200 species worldwide; genera 5. 18 species in Britain; genera in Britain 3; Brychius, Haliplus, Peltodytes. E.g., Haliplus obliquus.

General comments. Boat-shaped beetles, the scutellum not visible; crawling under water, and swimming, via alternate leg movements. The thorax is laterally sinuous and stria 3 of the elytra strongly raised in Brychius; the 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.

Illustrations. • 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).

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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.’.