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| Insects of Britain and Ireland: water beetles |
Adults. Beetles sub-aquatic, or 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; when aquatic, replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron; when aquatic, collecting air at the water surface by exserting one antenna to achieve a continuum of air with the ventral plastron; 3.7–7 mm long. Body length/maximum body width 1.3–2.55. Beetles somewhat waisted.
Eyes two; not strongly protuberant; without bristles. Mandibles with a well developed mola; with well developed prosthecae. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The maxillary palps less than twice as long as the antennae; 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; not strongly asymmetric; 9 segmented. Antennal scape not swollen. Antennae clubbed. Antennal clubs 3 segmented (the club hairy); preceded by a cupule. Antennal insertions visible from above, or hidden from above.
Prothorax without notopleural sutures. The pronotum not narrowing towards the rear. Scutellum present; not much longer than broad. The mesoventrite keeled. The prosternal process present, or absent; incomplete; if present, falling short of the mesoventrite to 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 medianly confluent to 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 5, 5, 5. Some of the tarsi with a tiny basal segment that is hard to detect, or none of the tarsi with ‘hidden’ segments (?). The front tarsi with as many segments as the mid-tarsi; 5-segmented; apical segment shorter than the rest together. Mid-leg tarsi 5-segmented; pentamerous. Hind tarsi 5-segmented. Hind tarsi equipped with ‘swimming hairs’. Hind tarsi with the second segment longest; with the second and apical segments almost equal in length. Swimming hairs confined to the tarsi.
Elytra without longitudinal striae; each with a well marked sutural stria (these not punctured). The rear halves of the elytra each with a distinct groove alongside the rear of the midline. Wings with an anal lobe, or without an anal lobe; without a medial fleck. The male external genitalia trilobate.
Adults not predacious. In diverse still water habitats - acid, alkaline, brackish and fresh.
Larvae. The larvae elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; only very lightly pigmented or sclerotized; only very lightly pigmented. The antennae 3 segmented. 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 4 segmented. The labial palps 4 segmented. The mesothoracic legs with 1 movable claw. Tergum 9 of the abdomen entirely dorsal.
The larvae aquatic. The abdominal apex with a respiratory chamber, formed from the 8th and 9th terga and enclosing a pair of enlarged spiracles. Abdominal tergum 8 without amature. The larvae predacious.
Representation in Britain and Ireland, and worldwide. 10 species in Britain. Recorded from Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland.
Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea; Hydrophilidae; Subfamily Hydrophilinae.
General comments. Maxillary palps longer than the antennae.
Miscellaneous. • Enochrus fuscipennis, E. halophilus, E. melanocephalus (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’.
