Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz

Hydrophilidae

Including Epimetopidae, Georissidae, Georyssidae, Helophoridae, Hydrochidae, Spercheidae, Sphaeridiidae.

Water-, Pond-, Marsh-beetles, Scavenger Water-beetles, Vegetarian Water-beetles.

General appearance. 1.5–38 mm long. Body length/maximum body width 1.3–2.55. Elytral length/pronotal length 1.7–4.85. Base of prothorax not or scarcely narrower than the combined elytral bases, or distinctly 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 to elongate-oval; not necked; somewhat waisted. Upper surfaces of body glabrous or subglabrous, or non-glabrous; not bristly; exhibiting scales or scale-like setae, or with neither scales nor scale-like setae. The underside usually exhibiting a plastron of hydrofuge hairs, detectable as a bubble when the insect is submerged.

Detailed morphology. Eyes not strongly protuberant; without bristles. The labrum at least partly visible in antero-dorsal view, or concealed beneath the clypeus (e.g. in Spercheinae); labrum mostly moderately to heavily sclerotized. Mandibles with a well developed mola; with well developed prosthecae. The mandibular apices simple, or bidentate or bilobed. The incisor edges of the mandibles simple, or with a single tooth. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps conspicuously elongated, sometimes longer than the antennae (apparently having taken over their sensory functions); apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short (and modified for involvement in respiration); 7–9 segmented; clubbed (the club hairy). Antennal clubs lamellate, or not lamellate; 5 segmented; preceded by a cupule, or without a cupule. Antennal insertions visible from above, or hidden from above.

Cervical sclerites present. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.25–1.08. The pronotum with lateral keels (pronotal carinae); keels complete. Posterior edge of the pronotum distinctly crenulate (some Helophorinae), or not crenulate. Prothorax at its widest not markedly narrower than the adjoining part of the abdomen. Prothorax without notopleural sutures. Scutellum conspicuous, or highly reduced to absent; when applicable, elevated above the mesoscutum in lateral view; anteriorly simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate (Helophorinae). The prosternal process present (mostly), or absent (Georyssinae and Chaetarthriinae); when present, complete, or incomplete (Helophorinae); when present, falling short of the mesoventrite, or slightly overlapping the mesoventrite. Metaventrite with a transverse groove, or without a transverse groove. The fore-leg coxae countersunk in ‘procoxal cavities’. The fore-leg coxal cavities open behind externally (mostly), or closed behind externally (Spercheinae); broadly open (mostly), or broadly closed (Spercheinae); medianly confluent, or narrowly separated (Helophorinae, Spercheinae); strongly transverse, or slightly transverse; with narrow lateral extensions (mostly), or without lateral extensions (Georyssinae); internally closed by a slender bar, or broadly closed internally. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by less than the shortest diameter of the cavity, or more than the shortest diameter of the cavity. The mid-leg coxal cavities narrowly separated, or moderately to widely separated, or contiguous (Spercheine only); not or scarcely oblique, or markedly oblique; open laterally. Hind-leg coxae contiguous or narrowly separated, or widely separated; extending laterally to meet the elytra (mostly), or not markedly extended laterally (Georyssinae, Helophorinae). Tarsal segmentation formula 5, 5, 5, or 4, 4, 4 (rarely). The tarsi without bilobed segments; with a tiny basal segment that is hard to detect, or without ‘hidden’ segments. Front tarsi with as many segments as the mid-tarsi, or with one segment fewer than the mid-tarsi; 4-segmented (rarely), or 5-segmented. Mid-leg tarsi 4-segmented (rarely), or 5-segmented; pentamerous, or pseudotetramerous (then the hasal one reduced), or tetramerous; 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, or one-toothed or bifid; with an empodium between them (this setose, sometimes with more than three setae). Hind tarsi equipped with ‘swimming hairs’, or without ‘swimming’ hairs; with as many segments as the mid-tarsi; 4-segmented, or 5-segmented; flattened and oar-like for swimming (notably in Hydrophilus piceus), or not flattened and oar-like (but often equipped with swimming hairs).

Elytral length/maximum width across the elytra 0.93–1.95. Elytra exposing no more than part of the terminal tergite; smooth; non-glabrous (e.g., in Hydrophilus), or glabrous. Elytra with six or more longitudinal lines of punctures, or with six or more impressed striae, or apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae; the lines per elytron when more than five, 6–7, or 8 to 11. Scutellary striole present, or absent. Wings well developed, or absent or much reduced. Wings variously with an anal lobe, or without an anal lobe. Wings without a medial fleck. Abdominal sternites (4–)5(–6); all articulated and moveable, or comprising both fused and moveable components. Basal abdominal sternites immovably joined when present, 2. Abdominal segment 8 with apparently functional spiracles, or apparently without functional spiracles. The male external genitalia trilobate.

Adult habitat, ecology. Mostly water-beetles (or in wet situations), or land-dwellers (e.g., many most Sphaeridiinae being found in dung or decaying vegetable matter). 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 replenishing air for respiration beneath the elytra indirectly from that acquired and held in the ventral plastron (this often conspicuous as a bubble on the underside of the submerged insect); regularly posing head first at the water surface to replenish air; collecting air at the water surface by exserting one antenna to achieve a continuum of air with the ventral plastron, or replenishing air in the ventral plastron by exserting both antennae at the water surface, or incorporating bubbles of oxygen directly into the plastron. Not predacious; when non-aquatic, in decaying plant material, or associated with dung; phytophagous, or consuming decaying plant material, or coprophagous (mostly saprophagous).

Larvae. Mature larvae minute (less than 3 mm long) to relatively large. The larvae campodeiform, or apodous; elongate and more or less parallel-sided. Body somewhat flattened (mostly), or circular in cross-section to somewhat flattened (Spercheinae). Vestiture restricted to fine hairs or setae. The larvae dorsally only very lightly pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 3 segmented; 0.15–0.5 x the head width to more than 0.5 x the width of the head. Stemmata present; on either side of the larval head 1, or 3, or 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, or bilobed or bidentate (Spercheinae only). The maxillary palps 4 segmented. The labium without ligula between the palps, or with a short ligula between the palps. Labial palps present and segmented; 1 segmented, or 2 segmented. Mesothoracic legs present and segmented, or much reduced or absent; 0 segmented, or 3 segmented (Georissinae), or 4 segmented, or 5 segmented; with 1 moveable claw. Visible abdominal segments 8–10. Tergum 9 of the abdomen entirely dorsal. The abdomen having functional spiracles on anterior segments (then these biforous or bilabiate), or with functional spiracles confined to the the eighth segment; with anterior and 8th-segmental spiracles borne at the ends of a series of spiracular tubes (Georissinae only), or without spiracular tubes. The larvae with abdominal gills; with long and narrow lateral gills on abdominal segments 1–7, or with anal gill tufts. The abdominal apex with a respiratory chamber, formed from the 8th and 9th terga and enclosing a pair of enlarged spiracles (mostly), or without a respiratory chamber (Georissinae, Helophorinae, Berosini). Abdominal tergum 8 bearing a single median process without apical spiracles (Spercheinae), or without amature (mostly). The last abdominal segment with cerci, or without cerci (some Hydrophilinae).

Larvae aquatic, or non-aquatic; predacious (mostly, by contrast with the adults), or not predacious (e.g., a few phytophagous Helophorinae); when non-aquatic, in decaying plant material, or in dung.

‘The larvae of diverse form’.

Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea.

Representation in Britain and Ireland, and worldwide. About 2800 species worldwide; genera about 160. 89 species in Britain; genera in Britain 20; Berosus, Helophorus (Helophoridae), Hydrobius, Hydrochus (Hydrochidae), Hydrochara, Hydrophilus, Spercheus, Sphaeridium, etc. E.g., Berosus signaticollis (Berosus Water-beetle); Helophorus rufipes (Broad Elophorus); Hydrobius fuscipes (Salt-water Hydrobius); Hydrochus elongatus (Elongated Hydrochus); Hydrochara caraboides (Globose Pond-beetle); Hydrophilus piceus (Great Water-beetle); Spercheus emarginatus (Notch-headed Hydrophilus); Sphaeridium scarabaeoides (4-spotted Dung-beetle).

General comments. Drab beetles, with maxillary palps longer than the antennae, and some stridulating when alarmed. It is unclear from the literature seen whether the family as currently circumscribed includes non-aquatic forms. Typically, the posterior pair of abdominal spiracles of the submerged insect are kept in contact with air stored under the elytra, which communicates with that which is collected and held in the hydrofuge hairy covering on the ventral body surface; the larger species replenishing the air supply head first at the water surface by exserting one or both of the specialized antennae, but smaller forms can renew it via bubbles of oxygen produced by the algae on which they feed.

Illustrations. • Berosus signaticollis (Berosus Water-beetle: B. Ent. 240). • Berosus signaticollis: B. Ent. 240, legend+text. • Berosus signaticollis: B. Ent. 240, text cont.. • Georyssus crenulatus, with Dryopidae, Elmidae and Heteroceridae: Fowler 3, 98 (1889). • Fowler 3, 98 (1889): original legend.. • Helophorus rufipes (Broad Elophorus: B. Ent. 466). • Helophorus rufipes: B. Ent. 466, legend+text. • Helophorus rufipes: B. Ent. 466, text cont.. • Hydrobius fuscipes (Salt-water Hydrobius: B. Ent. 243). • Hydrobius fuscipes: B. Ent. 243, legend+text. • Hydrobius fuscipes: B. Ent. 243, text cont.. • Hydrochus elongatus (Elongated Hydrochus: B. Ent. 359). • Hydrochus elongatus: B. Ent. 359, legend+text. • Hydrochus elongatus: B. Ent. 359, text cont.. • Hydrochara carabiodes (Globose Pond-beetle: B. Ent. 159). • Hydrochara caraboides (details, B. Ent. 159). • Hydrochara caraboides: B. Ent. 159, legend+text. • Hydrochara caraboides: B. Ent. 159, text cont.. • Hydrophilus piceus (Great Water-beetle: B. Ent. 239). • Hydrophilus piceus (B. Ent. 239, legend+text). • Hydrophilus piceus (B. Ent. 239, text cont.). • Spercheus emarginatus (Notch-headed Hydrophilus: B. Ent. 394). • Spercheus emarginatus: B. Ent. 394, legend+text. • Spercheus emarginatus: B. Ent. 394, legend+text. • Sphaeridium scarabaeoides (B. Ent. 518). • Sphaeridium scarabaeoides: B. Ent. 518, legend+text. • Sphaeridium scarabaeoides: B. Ent. 518, text cont.. • Laccobius sinuatus, L. striatulus and Paracymus aenus: Fowler Suppl. 2, 1913. • Fowler Suppl. 2, 1913: original legend. • Cercyon, Helophorus, Hydrochus, Laccobius and Sphaeridium (with Hydraenidae and Staphylinidae): Fowler Suppl. 3, 1913. • Fowler Suppl. 3, 1913: original legend..


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. delta-intkey.com’.

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