Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz


= Pelobiidae, Paelobiidae.


General appearance. 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 to distinctly narrower than the combined elytral bases. Greatest prothoracic width distinctly narrower than greatest elytral width. Beetles oval to elongate-oval (widening posteriorly); 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. The underside without a plastron of hydrofuge hairs.

Detailed morphology. Inclination of the head slight. Eyes strongly protuberant (by contrast with those of other water beetles); without bristles; finely facetted. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. 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 apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short; 11 segmented; filiform. Antennal insertions visible from above, or hidden from above.

Cervical sclerites absent. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.4–0.5. The pronotum with lateral keels (pronotal carinae); keels complete. Prothorax at its widest not markedly narrower than the adjoining part of the abdomen. Prothorax with notopleural sutures. Scutellum conspicuous, or highly reduced; when applicable, not elevated; anteriorly simple; posteriorly narrowly rounded or acute. The prosternal process complete; concealing most or all of the mesoventrite. Metaventrite with a transverse groove. The fore-leg coxae countersunk in ‘procoxal cavities’. The fore-leg coxal cavities open behind externally; broadly open; quite widely separated; slightly transverse; without lateral extensions; broadly closed internally. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by less than the shortest diameter of the cavity. The mid-leg coxal cavities contiguous, or narrowly separated; not or scarcely oblique; open laterally. Hind-leg coxae contiguous or narrowly separated; extending laterally to meet the elytra; immoveably fixed to the metasternum and dividing the first abdominal sternite. Tarsal segmentation formula 5, 5, 5. The tarsi without bilobed segments; without ‘hidden’ segments. Front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented; pentamerous; the penultimate segment not distinctly shorter than the antepenultimate one. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple. Hind tarsi equipped with ‘swimming hairs’; with as many segments as the mid-tarsi; 5-segmented; longer than the tibiae, flattened and oar-like for swimming.

Elytral length/maximum width across the elytra 1.22–1.26. Elytra exposing no more than part of the terminal tergite; glabrous. Elytra apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae. Scutellary striole absent. Wings well developed. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites 6; comprising both fused and moveable components. Basal abdominal sternites immovably joined 3. Abdominal segment 8 with apparently functional spiracles. The male external genitalia trilobate.

Adult habitat, ecology. Water-beetles. Beetles 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. Beetles respiring under water via air which is collected posteriorly and stored directly under the elytra (and respiring via the terminal pair of spiracles); regularly posing tail first at the water surface to replenish air; collecting air at the water surface by exserting the tip of the abdomen through the surface film. Predacious (bottom feeders, e.g. on small animals such as tubificid worms?).

Larvae. Mature larvae small to medium-sized. The larvae campodeiform; oblong to ovate. Body somewhat flattened. Vestiture restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 4 segmented; 0.15–0.5 x the head width. Stemmata present; on either side of the larval head 6. The larval fronto-clypeus not extended forwardly. The 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. Labial palps present and segmented; 2 segmented. Mesothoracic legs present and segmented; 6 segmented; the tarsi 2-clawed; with 2 moveable claws. Visible abdominal segments 9. Tergum 9 of the abdomen completely ventral. The abdomen having functional spiracles on anterior segments (then these annular), or with functional spiracles confined to the the eighth segment; without spiracular tubes. The larvae with abdominal gills; with ventral abdominal gill tufts on abdominal segments 1–3. The abdominal apex without a respiratory chamber. Abdominal tergum 8 bearing a single median process without apical spiracles. The last abdominal segment with cerci; with a long, median spine.

Larvae aquatic; predacious (? - family members observed feeding on insect larvae and Tubifex worms); inhabiting fine mud and ooze under still waters.

Classification. Suborder Adephaga; Superfamily Caraboidea.

Representation in Britain and Ireland, and worldwide. 5 species worldwide; genera 1 (a small monogeneric family, recorded from Europe, N. Africa, China and Australia). 1 species in Britain; genera in Britain 1; Hygrobia. E.g., H. hermanni.

General comments. Beetles stridulating loudly, by rubbing the apex of the abdomen on a file inside the elytra. Associated mainly with stagnant water..

Illustrations. • Hygrobia hermanni (Rye & Fowler III5).

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. Insects of Britain and Ireland: the families of Coleoptera. Version: 16th May 2016.’.