British insects: the families of Coleoptera
Screech- or Squeak-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; respiring under water via air which is collected posteriorly and stored directly under the elytra; collecting air at the water surface by exserting the tip of the abdomen through the surface film. Beetles 8.5–10 mm long; body length/maximum body width 1.55–1.58; elytral length/pronotal length 3.65–4.19; 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. Beetles oval to elongate-oval; dorsally strongly convex (convex above and below); not necked. Upper surfaces of body glabrous or subglabrous; not bristly; with neither scales nor scale-like setae. Inclination of the head slight.
Eyes two; strongly protuberant (by contrast with those of other water beetles); without bristles; finely facetted. Mandibles with a reduced mola; without prosthecae. The mandibular apices simple, or bidentate or bilobed, or multidentate or multilobed. The incisor edges of the mandibles simple, or with a single tooth. 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. The apical segment of the labial palps not expanded apically. Antennae very short; not strongly asymmetric; 11 segmented. Antennal scape not swollen. Antennae filiform. Antennal insertions visible from above, or hidden from above.
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 0.4–0.5. Metaventrite with a transverse groove. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; quite widely separated; slightly transverse; without lateral extensions; broadly closed internally. Hind coxae immovably fixed to the metasternum and dividing the first abdominal sternite; 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. None of the tarsi with conspicuously bilobed segments. 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; 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; longer than the tibiae, not flattened oar-like. Hind tarsi equipped with swimming hairs. Swimming hairs on both middle and hind legs; present on both tibiae and tarsi.
Elytral length/maximum width across the elytra 1.22–1.26. Elytra exposing no more than part of the terminal tergite. Scutellary striole absent. Elytra glabrous; microreticulate, with a fine impressed reticulum. The mictoreticulation regular. Wings well developed. Exposed abdominal sternites 6; comprising both fused and movable components; immovably joined 3. Abdominal segment 8 with apparently functional spiracles.
Adults predacious. In silt and detritus in ponds.
Larvae. The larvae campodeiform; oblong to ovate; vestiture restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized; only very lightly pigmented. 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 4 segmented. The labium with a short ligula between the palps. The labial palps 4 segmented. The mesothoracic legs 6 segmented (including the pretarsus). The tarsi 2-clawed. The mesothoracic legs with 2 movable claws. Visible abdominal segments 9. Tergum 9 of the abdomen completely ventral.
The larvae aquatic. The abdomen exhibiting functional abdominal spiracles, or without functional spiracles; having functional spiracles on anterior segments, or with functional spiracles confined to the the eighth segment. The larvae with ventral abdominal gill tufts on abdominal segments 1–3. Abdominal tergum 8 bearing a single median process without apical spiracles. The last abdominal segment with cerci. The larvae predacious.
Worldwide and British representation. 1 species in Britain (H. hermanni). Recorded from Northern England, South-west England and Wales, South-eastern England, and Ireland (scarce in the North).
Classification. Suborder Adephaga; Superfamily Caraboidea; Hygrobiidae (Paelobiidae).
General comments on this taxon. Beetles stridulating loudly, by rubbing the apex of the abdomen on a file inside the elytra. Associated mainly with stagnant water.
Miscellaneous. • Hygrobia hermanni (from Joy, 1932). • Hygrobia hermanni (Rye & Fowler III5). • H. hermanni, elytral sculpture (Balfour-Browne).
To view the illustrations with detailed captions, go to the interactive key. This also offers 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.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.