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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; 2.4–4.7 mm long. Body length/maximum body width 1.3–2.55. Beetles somewhat waisted; without a rostrum.
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 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; 7 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 narrowing towards the rear (with characteristic depressions); with large pits. The prosternal process incomplete; falling short of the mesoventrite to slightly overlapping the mesoventrite. Mid-and hind-legs not as in Gyrinidae (q.v.). 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 4, 4, 4. 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; pseudotetramerous (with the first segment reduced). Hind tarsi without ‘swimming’ hairs. Hind tarsi with the apical segment longer than the second; with the apical segment at least as long as all the rest together. Swimming hairs confined to the tarsi.
Wings with an anal lobe, or without an anal lobe; without a medial fleck. The male external genitalia trilobate.
Adults not predacious. In muddy water in marshes, fens, heaths, ponds and drains.
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. Stemmata 6. 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 labium without ligula between the palps. The labial palps 4 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 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. 6 species in Britain. Recorded from Northern England, South-west England and Wales, South-eastern England, and Ireland (generally scarce, especially in the North).
Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea; Hydrophilidae; Subfamily Hydrochinae.
General comments. The pronotal pits arranged in a triangle with its base posterior.
Miscellaneous. • Hydrochus elongatus (Elongated Hydrochus: B. Ent. 359). • Hydrochus elongatus: B. Ent. 359, legend+text. • Hydrochus elongatus: B. Ent. 359, text cont.. • Cercyon, Helophorus, Hydrochus, Laccobius and Sphaeridium (with Hydraenidae and Staphylinidae): Fowler Suppl. 3, 1913. HYDROPHILIDAE. 1, Laccobius atrocephalus. 2, Helophorus tuberculatus; 3, Helophorus rufipes. 4, Hydrochus nitidicollis. HYDRAENIDAE. 5, Ochthebius subinteger ssp. lejolisi. 6, Hydraena britteni. HYDROPHILIDAE. 7, Sphaeridium lunatum. 8, Cercyon littoralis var. binotatum. STAPHYLINIDAE. 9, Aleochara discipennis; 10, Aleochara moesta; 11, Aleochara maculata. 12, Oxypoda longipes; 13, Oxypoda perplexa. From Fowler's plate, with the names from the original legend (q.v.) updated. • Fowler Suppl. 3, 1913: original legend..
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’.
