British insects: the families of Coleoptera
Adults. Beetles aquatic (P. caesus); 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; respiring under water via air which is collected posteriorly and stored directly under the elytra; sometimes collecting air at the water surface by exserting the tip of the abdomen through the surface film (?). Beetles 3.5–4 mm long; body length/maximum body width 1.45–2.5; elytral length/pronotal length 3.24–4.05; 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, or distinctly narrower than greatest elytral width. Beetles oval; conspicuously necked to not necked; not waisted, or somewhat waisted; yellowish. Upper surfaces of body not bristly; with neither scales nor scale-like setae. Inclination of the head slight.
Eyes two; not strongly protuberant; without bristles; coarsely facetted. Mandibles without a mola. The mandibular apices simple. The maxillae with distinct galea and lacinia apically to the palp. The maxillae with an outer palpiform lobe (the galea) additional to the 4-segmented maxillary palp. The maxillary palps not especially elongated. The apical segment of the maxillary palps cylindrical to fusiform; longer than the penultimate one. The apical segment of the labial palps not expanded apically. Antennae very short, or short; not strongly asymmetric; 11 segmented. Antennal scape not swollen. Antennae filiform. 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; with notopleural sutures. Pronotal length/maximum pronotal width 0.43–0.66. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; circular to longer than wide; without lateral extensions; broadly closed internally. Hind coxae immovably fixed to the metasternum and dividing the first abdominal sternite; much expanded and extended laterally to meet the elytra; not shaped posteriorly to receive the femur. The hind coxae much enlarged and produced behind into large flat plates which partly conceal the hind femora. The hind coxal plates each with a point on the posterior margin, and covering part of the sixth abdominal sternite. 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. None of the tarsi with hidden segments. The front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented (and modified for swimming, cf. the hind pair); pentamerous. 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; 5-segmented; not flattened oar-like. Hind tarsi equipped with swimming hairs.
Elytral length/maximum width across the elytra 1.15–1.52. Elytra exposing no more than part of the terminal tergite; darkly, conspicuously longitudinally striate (each with about 13 rows of punctured striae); not exhibiting a single, strong, median-longitudinal ridge; without a microreticulum. Exposed abdominal sternites 4–6. Abdominal segment 8 with apparently functional spiracles.
Adults not predacious; phytophagous (mainly on green algae). In fenland drains and quarry ponds.
Larvae. The larvae campodeiform; elongate and more or less parallel-sided. The antennae 4 segmented; 0.15–0.5 x the head width. 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 3 segmented. The labium without ligula between the palps, or with a short ligula between the palps. The labial palps 3 segmented. The mesothoracic legs 6 segmented (including the pretarsus). The tarsi 1-clawed. The mesothoracic legs with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal, or extending to the underside.
The larvae aquatic; without gills (breathing dissolved oxygen through the body wall, which is increased in area segmental processes). Abdominal tergum 8 without amature. The last abdominal segment without cerci. The larvae predacious (with short, curved, hollow suctorial mandibles).
The larvae with 4-segmented antennae, legs with single claws, and an elongate 10th segment with cerci.
Worldwide and British representation. 1 species in Britain. Recorded from South-west England and Wales and South-eastern England (commonest in the southeast).
Classification. Suborder Adephaga; Superfamily Caraboidea; Haliplidae.
General comments on this taxon. The elytra each with about 10 longitudinal rows of large punctures, with at least five of them on the dorsal side inside the shoulder. The last segment of the maxillary palps longer than the preceding one, and the hind coxal plates each with a point on the hind margin and obscuring all but the last abdominal sternite.
Miscellaneous. • Peltodytes caesus (from Joy, 1932).
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Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.