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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; 6–7 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; dubiously 6 segmented, or 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 prosternal process present; complete; if present, 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. 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 swamps and muddy water.
Larvae. The larvae oblong to ovate; 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 bilobed or bidentate. The maxillary palps 4 segmented. The labium with a short 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 non-aquatic (?). The abdomen exhibiting functional abdominal spiracles; with functional spiracles confined to the the eighth segment. Abdominal tergum 8 bearing a single median process without apical spiracles. The larvae predacious.
Representation in Britain and Ireland, and worldwide. 1 species in Britain (S. emarginatus). Recorded from formerly South-eastern England (supposed extinct).
Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea; Hydrophilidae; Subfamily Spercheinae.
Miscellaneous. • Spercheus emarginatus (Notch-headed Hydrophilus: B. Ent. 394). • Spercheus emarginatus: B. Ent. 394, legend+text. • Spercheus emarginatus: B. Ent. 394, legend+text. • Spercheus emarginatus (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’.
