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| Insects of Britain and Ireland: water beetles |
Adults. Beetles terrestrial to sub-aquatic; walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; perhaps replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron (?); incorporating bubbles of oxygen directly into the plastron (?); 2.5–5 mm long. Body length/maximum body width 1.7–3.05; elytral length/pronotal length 2.2–3.53; 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. Beetles elongate; without ventral body cavities for reception of the legs; not necked; conspicuously waisted. Upper surfaces of body non-glabrous; not bristly; with neither scales nor scale-like setae. Beetles prognathous. Inclination of the head slight.
Eyes two; not strongly protuberant; without bristles; finely facetted. Mandibles with a well developed mola. The mandibular apices simple, or bidentate or bilobed. The incisor edges of the mandibles with a single tooth, or with two or more teeth. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps not especially elongated. The apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short; strongly asymmetric; (9–)11 segmented. Antennal scape not swollen. Antennae clubbed. Antennal clubs 7 segmented (serrate); preceded by a cupule, or without a cupule. 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; without notopleural sutures. Pronotal length/maximum pronotal width 0.52–0.82. Metaventrite with a transverse groove. The prosternal process complete; moderately or strongly overlapping the mesoventrite. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; narrowly separated; strongly transverse; without lateral extensions; internally open. Hind coxae not extended laterally to meet the elytra; posteriorly shaped to receive the retracted femur. The hind coxae not produced behind into flat plates partly concealing the hind femora. Hind coxae with a steep transverse declivity against which the femur retracts. The inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel. Tarsal segmentation formula 4, 4, 4. None of the tarsi with conspicuously bilobed segments. None of the tarsi with ‘hidden’ segments (but the terminal segment about half as long as the basal three together). The front tarsi with as many segments as the mid-tarsi; 4-segmented. Mid-leg tarsi 4-segmented; tetramerous. 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; 4-segmented.
Elytral length/maximum width across the elytra 1.18–2.3. Elytra exposing no more than part of the terminal tergite; not truncate; hard. Scutellary striole absent. Elytra non-glabrous (densely hairy). Wings well developed, or absent or much reduced; without an anal lobe; without a medial fleck. Abdominal tergites 10. Exposed abdominal sternites 5; comprising both fused and movable components; immovably joined 3. Abdominal segment 8 with apparently functional spiracles. The male external genitalia not classified.
Adults not predacious; phytophagous; in living vegetation, or in decaying plant material (found in stiff mud at the sides of ponds and streams). In mud in damp places or at the sides of ponds.
Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized; only very lightly pigmented. The antennae 3 segmented; less than 0.15 x the with of the head. Stemmata fewer than 6 (five). Frontoclypeal suture between frons and clypeus distinct. The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate. The maxillary palps 3 segmented. The labium with a short ligula between the palps. The labial palps 3 segmented. The mesothoracic legs 5 segmented (including the pretarsus); with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal.
The larvae seemingly non-aquatic. The abdomen exhibiting functional abdominal spiracles; having functional spiracles on anterior segments. The larvae without gills. Abdominal tergum 8 without amature. The larvae not predacious; phytophagous; on living vegetation, or in decaying plant material (in mud).
The larvae elongate, campodeiform, with rather distended thoracic segments, and well developed legs.
Representation in Britain and Ireland, and worldwide. 8 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland.
Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea; Heteroceridae.
Miscellaneous. • Heterocerus obsoletus Curtis (Largest Heterocerus: B. Ent. 224). • Heterocerus obsoletus (details, B. Ent. 224). • Heterocerus obsoletus (B. Ent. 224, legend+text). The original description of this species. • Heterocerus obsoletus (B. Ent. 224, text cont.). • Heterocerus fenestratus, H. flexuosus, H. fusculus and H. maritimus: (with Elmidae and Dryopidae): Fowler 3, 98 (1889). HYDROPHILIDAE. 1, Georyssus crenulatus. ELMIDAE. 2, Elmis aenea. 3, Limnius volkmari. 4, Esolus parallelepipedus. 5, Oulimnius tuberculatus. 6, Macronychus quadrituberculatus. DRYOPIDAE. 7, Potaminus substriatus; 7a, Potaminus substriatus, antenna. 8 and 9, Dryops auriculatus. HETEROCERIDAE. 10–13, Heterocerus. 10, H. flexuosus; 11, H. fenestratus; 12, H. fusculus; 13, H. maritimus. From Fowler's plate, with the names from the original legend (q.v.) updated. • Fowler 3, 98 (1889): original legend.. • H. flexuosus, H. marginatus, H. obsoletus (from joy, 1932).
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, 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. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2012 onwards. Insects of Britain and Ireland: water beetles. Version: 14th April 2022. delta-intkey.com’.
