Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz

Elmidae

= Elmididae, Elminthidae, Helmidae, Helminthidae, Limniidae.

Riffle Beetles, Water-beetles.

General appearance. 1.3–4.75 mm long. Body length/maximum body width 1.8–3.1. Elytral length/pronotal length 1.8–4.7. 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 oval to elongate-oval; without ventral body cavities for reception of the legs; not necked; somewhat waisted; conspicuously long-legged. Upper surfaces of body glabrous or subglabrous, or non-glabrous; not bristly; with neither scales nor scale-like setae. The underside exhibiting a plastron of hydrofuge hairs, detectable as a bubble when the insect is submerged (in all British representatives).

Detailed morphology. Inclination of the head slight to strong. Eyes not strongly protuberant; without bristles. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. Mandibles with a well developed mola. The mandibular apices bidentate or bilobed, or multidentate or multilobed. The incisor edges of the mandibles simple, or with a single tooth (some Elminae). The maxillae with distinct galea and lacinia apically to the palp. The apical segment of the maxillary palps cylindrical to fusiform (Elminae), or somewhat expanded and truncate to subtriangular, or securiform to cultriform. The apical segment of the labial palps more or less expanded apically (Larainae), or not expanded apically. Antennae short; 7–11 segmented. Antennal scape not swollen. Antennae filiform. Antennal insertions visible from above; not in fossae.

Cervical sclerites present. Pronotal length/maximum pronotal width 0.6–1.1. The pronotum with lateral keels (pronotal carinae), or without lateral keels (some Elminae); keels when present, complete, or incomplete (some Larainae). Prothorax without notopleural sutures. Scutellum conspicuous; elevated above the mesoscutum in lateral view; anteriorly simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process complete; slightly overlapping the mesoventrite to moderately or strongly overlapping the mesoventrite. Metaventrite with a transverse groove. The fore-leg coxae countersunk in ‘procoxal cavities’. The fore-leg coxal cavities open behind externally; broadly open; quite widely separated; strongly transverse (Larainae), or slightly transverse to circular to longer than wide (Elminae); without lateral extensions; internally open. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by less than the shortest diameter of the cavity, or 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, or widely separated; extending laterally to meet the elytra, or not markedly extended laterally; posteriorly shaped to receive the retracted femur; with a steep transverse declivity against which the femur retracts. Tarsal segmentation formula 5, 5, 5. The tarsi without bilobed segments; without ‘hidden’ segments. Front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented; 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, or one-toothed or bifid. Hind tarsi with as many segments as the mid-tarsi; 5-segmented.

Elytral length/maximum width across the elytra 1.3–2.5. Elytra exposing no more than part of the terminal tergite; not truncate. Elytra with six or more longitudinal lines of punctures, or with six or more impressed striae, 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, 6–7, or 8 to 10. Scutellary striole present (some Larainae), or absent. Wings well developed, or absent or much reduced. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites 5–6; comprising both fused and moveable components. Basal abdominal sternites immovably joined 3. Abdominal segment 8 with apparently functional spiracles, or apparently without functional spiracles. The male external genitalia trilobate.

Adult habitat, ecology. Water-beetles (in running water, under stones and logs). 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 replenishing air for respiration beneath the elytra indirectly from that acquired and held in the ventral plastron; not noticeably posing regularly at the water surface to replenish air; incorporating bubbles of oxygen directly into the plastron. Often in submerged sand, gravel, or wet wood in streams; fully aquatic, phytophagous, or consuming rotting wood.

Larvae. Mature larvae minute (less than 3 mm long), or small to medium-sized. The larvae campodeiform; elongate and more or less parallel-sided, or oblong to ovate. Body circular in cross-section to strongly flattened. Vestiture restricted to fine hairs or setae, or not restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized; ventrally more or less heavily pigmented. The antennae 3 segmented; 0.15–0.5 x the head width. Stemmata present; on either side of the larval head 1, or 5. The larval fronto-clypeus not extended forwardly. The frontoclypeal suture between frons and clypeus indistinct or absent, or distinct. The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate, or trilobed or tridentate. The maxillary palps 4 segmented. The labium 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. Mesothoracic legs present and segmented; 5 segmented; with 1 moveable claw. Visible abdominal segments 10. Tergum 9 of the abdomen extending to the underside. The abdomen having functional spiracles on anterior segments (biforous or bilabiate), or with functional spiracles confined to the the eighth segment; without spiracular tubes. The larvae with abdominal gills; with anal gill tufts. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The posterior segments without swimming hairs. The last abdominal segment without cerci.

Larvae aquatic (in a variety of habitats with shallow moving water, in gravel or waterlogged wood, or among roots and moss at the water’s edge); phytophagous, or consuming decaying plant material (apparently feeding on decaying plant material, diatoms and other algae).

Larvae woodlouse-shaped or cylindric, with three tufts of retractive anal gills.

Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea.

Representation in Britain and Ireland, and worldwide. Genera about 30 (?). 11 species in Britain; genera in Britain 8; Elmis, Esolus, Limnius, Macronychus, Normandia, Oulimnius, Riolus, Stenelmis. E.g., Limnius volkmari (Volckmar's Elmis Water-beetle).

General comments. The distal hind tarsal segment elongate, and bulbous at its tip.

Illustrations. • Limnius volckmari (Volckmar's Water-beetle: bent294). • Limnius volckmari (details, B. Ent. 294). • Limnius volckmari: B. Ent. 294, legend+text. • Elmis aenea, Esolus, Limnius, Macronychus, Oulimnius (with Dryopidae, Heteroceridae and Hydrophilidae): Fowler 3, 98 (1889). • Fowler 3, 98 (1889): original legend..


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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. delta-intkey.com’.

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