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| Insects of Britain and Ireland: water beetles |
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; probably replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron (? - being ventrally hairy); perhaps incorporating bubbles of oxygen directly into the plastron (?); 0.75–2.5 mm long. Body length/maximum body width 1.6–2.57. Beetles elongate; not necked; conspicuously waisted. Upper surfaces of body not bristly; with neither scales nor scale-like setae.
Eyes two; not strongly protuberant; without bristles. Mandibles with a well developed mola. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short, or short, or about half the insect's head to tail length; not strongly asymmetric; 9 segmented. Antennal scape not swollen. Antennae clubbed. Antennal clubs 5 segmented (the club hairy); preceded by a cupule. Antennal insertions hidden from above.
Prothorax without notopleural sutures. Pronotal length/maximum pronotal width 0.42–1.05. The pronotum not narrowing towards the rear; without large pits. Scutellum present. Metaventrite without a transverse groove. The prosternal process complete; slightly overlapping the mesoventrite. Mid-and hind-legs not as in Gyrinidae (q.v.). The front tibia more or less club-shaped, not or scarcely emarginate near the attachment of the tarsus. The fore-leg coxal cavities narrowly separated. Hind coxae not extended laterally to meet the elytra; not shaped posteriorly to receive the femur. The hind coxae not produced behind into flat plates partly concealing the hind femora. Hind coxae without the steep transverse declivity characteristic of Dryopoidea. 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. Some of the tarsi with a tiny basal segment that is hard to detect. The front tarsi with as many segments as the mid-tarsi; 5-segmented; apical segment longer than the rest together, or about as long as the rest together. Mid-leg tarsi 5-segmented; pentamerous, or pseudotetramerous. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple; without an associated empodium. Hind tarsi with at least as many segments as the mid-tarsi; 5-segmented; not flattened oar-like. Hind tarsi equipped with ‘swimming hairs’. Hind tarsi with the apical segment longer than the second; with the apical segment about equal to segments 3+4, with segment 2 very short. Swimming hairs confined to the tarsi.
Elytral length/maximum width across the elytra 1.18–2. Elytra exposing no more than part of the terminal tergite. Scutellary striole absent. Wings well developed, or absent or much reduced; without an anal lobe; without a medial fleck; fringed with long hairs on the hind margin, or not fringed. Exposed abdominal sternites 7; all articulated and movable. Abdominal segment 8 with apparently functional spiracles. The male external genitalia staphylinoid.
Adults not predacious; phytophagous (on algae). In fens and acid pools, marshes and drains, with the commonest species (L. truncatellus) in running water.
Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; only very lightly pigmented. The antennae 3 segmented; 0.15–0.5 x the head width, or more than 0.5 x the width of the head. Stemmata fewer than 6. 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 non-aquatic (living in wet places, but easily drowned). The abdomen exhibiting functional abdominal spiracles; having functional spiracles on anterior segments; with the spiracles on the eighth segment borne at the ends of spiracular tubes. The larvae without gills. Abdominal tergum 8 without amature. The larvae not predacious; phytophagous (on algae?).
Larvae subcylindric, active.
Representation in Britain and Ireland, and worldwide. 5 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland.
Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Staphilinoidea; Hydraenidae.
Miscellaneous. • Limnebius truncatellus (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’.
