British insects: the families of Coleoptera

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L. Watson and M. J. Dallwitz

Hydrophilidae

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

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

Adults. Beetles terrestrial, or sub-aquatic, or aquatic (British representation including the Giant Water-beetle, Hydrophilus piceus); walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; when aquatic, 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 (this often conspicuous as a bubble on the underside of the submerged insect); when aquatic, 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 collecting air at the water surface by exserting one antenna to achieve a continuum of air with the ventral plastron and incorporating bubbles of oxygen directly into the plastron. Beetles 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.

Eyes not strongly protuberant; without bristles. 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. The maxillary palps conspicuously elongated, sometimes longer than the antennae (apparently having taken over their sensory functions). The 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. Antennal clubs 3 segmented (the club hairy); preceded by a cupule, or without a cupule. 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; without notopleural sutures. Pronotal length/maximum pronotal width 0.25–1.08. The pronotum with five full length, longitudinal furrows (Helophorus), or without five full length longitudinal furrows. Metaventrite with a transverse groove, or without a transverse groove. The fore-leg coxal cavities broadly open, or broadly closed; medianly confluent, or narrowly separated; strongly transverse, or slightly transverse; with narrow lateral extensions, or without lateral extensions; internally closed by a slender bar, or broadly closed internally. Tarsal segmentation formula 5, 5, 5 (nearly always), or 4, 4, 4 (Georissus, Hydrochus, Spercheus). None of the tarsi with conspicuously bilobed segments. Some of the tarsi with a tiny basal segment that is hard to detect (rarely), or none of the tarsi with ‘hidden’ segments. The front tarsi with as many segments as the mid-tarsi (usually), or with one segment fewer than the mid-tarsi; 4-segmented, or 5-segmented (usually). Mid-leg tarsi 4-segmented (rarely), or 5-segmented; pentamerous (usually), or pseudotetramerous (then the first segment reduced), or tetramerous (rarely). 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 with at least as many segments as the mid-tarsi; 4-segmented, or 5-segmented; flattened oar-like for swimming (then the mid- and post-tibiae with fine, long ‘swimming hairs’, and also strongly compressed in Hydrophilus), or not flattened oar-like. Hind tarsi equipped with ‘swimming hairs’, or without ‘swimming’ hairs.

Elytral length/maximum width across the elytra 0.93–1.95. Elytra exposing no more than part of the terminal tergite; smooth. Scutellary striole present, or absent. Wings well developed, or absent or much reduced. Exposed abdominal sternites 4–6; all articulated and movable, or comprising both fused and movable components; immovably joined when present, 2. Abdominal segment 8 with apparently functional spiracles, or apparently without functional spiracles.

Adults not predacious; phytophagous, or consuming decaying plant material, or coprophagous; when non-aquatic, in decaying plant material, or associated with dung.

Larvae. The larvae campodeiform (mostly), or apodous; elongate and more or less parallel-sided (mostly), or oblong to ovate (Spercheinae only); vestiture restricted to fine hairs or setae; only very lightly pigmented or sclerotized; only very lightly pigmented. The antennae 3 segmented; 0.15–0.5 x the head width, or more than 0.5 x the width of the head. Stemmata 6 (mostly), or fewer than 6 (then one or three). 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 (mostly), or bilobed or bidentate (Spercheinae). The maxillary palps 4 segmented. The labium without ligula between the palps, or with a short ligula between the palps (Spercheinae). The labial palps 4 segmented. The mesothoracic legs 0 segmented, or 3 segmented, or 4 segmented, or 5 segmented; with 1 movable claw. Visible abdominal segments 8, or 9, or 10. Tergum 9 of the abdomen entirely dorsal.

The larvae aquatic, or non-aquatic. The abdomen nearly always exhibiting functional abdominal spiracles; having functional spiracles on anterior segments, 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. The larvae with long and narrow lateral gills on abdominal segments 1–7 (Berosus only), or with anal gill tufts (the rest). The abdominal apex without a respiratory chamber, or with a respiratory chamber, formed from the 8th and 9th terga and enclosing a pair of enlarged spiracles (mostly). Abdominal tergum 8 bearing a single median process without apical spiracles (Spercheinae), or without amature (mostly). The larvae usually predacious; when non-aquatic, in decaying plant material, or in dung.

Larvae of diverse form.

Worldwide and British representation. About 2800 species worldwide; genera about 160. 89 species in Britain; genera in Britain 20; Berosus, Helophorus, Hydrobius, Hydrochus, Hydrochara, Hydrophilus, Spercheus, Sphaeridium, etc.

Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea. 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 on this taxon. 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.

Miscellaneous. • 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..


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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’.

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