Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz


= Sphaeriusidae; ~Hydrophilidae, Microsporidae.

Minute Bog-beetles.

General appearance. 0.6–0.8 mm long. Body length/maximum body width 1.34–1.37. Elytral length/pronotal length 3.25–3.5. Base of prothorax not or scarcely narrower than the combined elytral bases. Greatest prothoracic width distinctly narrower than greatest elytral width. Beetles round; not necked; not waisted. Upper surfaces of body glabrous or subglabrous; not bristly; with neither scales nor scale-like setae.

Detailed morphology. Inclination of the head slight. Eyes not strongly protuberant; without bristles; coarsely facetted. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. Mandibles with a well developed mola; with well developed prosthecae. The mandibular apices bidentate or bilobed. The incisor edges of the mandibles simple, or with a single tooth. The maxillae with a single apical structure additional to the palp; with stylet-like lobes, or without stylet-like lobes. The apical segment of the maxillary palps aciculate. The apical segment of the labial palps not expanded apically. Antennae short; 11 segmented. Antennal scape and pedicel swollen. Antennae clubbed (the scape and pedicel swollen, the third segment long and slender, the fourth and fifth sub-globular, 6th to 8th short and transverse, and the terminal three forming the compact club). Antennal clubs 3 segmented. Antennal insertions visible from above to hidden from above; inserted under lateral edges of frons.

Cervical sclerites absent. Prothorax shorter than wide. Pronotal length/maximum pronotal width 0.37–0.4. 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 highly reduced; not elevated; anteriorly simple; posteriorly narrowly rounded or acute. The prosternal process present; incomplete; falling short of the mesoventrite. Metaventrite without a transverse groove. The fore-leg coxae countersunk in ‘procoxal cavities’. The fore-leg coxal cavities open behind externally; broadly open; medianly confluent; strongly transverse; without lateral extensions; internally closed by a slender bar. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by more than the shortest diameter of the cavity. The mid-leg coxal cavities moderately to widely separated; not or scarcely oblique; open laterally. Hind-leg coxae contiguous or narrowly separated; extending laterally to meet the elytra; peculiarly posteriorly shaped to receive the retracted femur; triangular, almost contiguous, produced behind into flat plates which conceal the hind femora when these are retracted. Tarsal segmentation formula 3, 3, 3. The tarsi without bilobed segments; without ‘hidden’ segments. Front tarsi with as many segments as the mid-tarsi; 3-segmented. Mid-leg tarsi 3-segmented; trimerous; 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; with an empodium between them (with 2 or 3 long setae between the claws, according to Britton), or without an associated empodium (Lawrence et al.). Hind tarsi with as many segments as the mid-tarsi; 3-segmented.

Elytral length/maximum width across the elytra 1.03–1.07. Elytra exposing no more than part of the terminal tergite. The pygidium entirely concealed by the ends of the elytra even when viewed from behind. Elytra glossy; glabrous. Elytra apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae. Scutellary striole present. Wings well developed, or absent or much reduced; with the apical part spirally rolled; fringed with long hairs on the hind margin. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites 3 (first and third long); all articulated and moveable. Abdominal segment 8 with apparently functional spiracles. The male external genitalia not classified.

Adult habitat, ecology. Water-beetles (in wet mud, gravel and roots, and under stones, at the margins of fresh water). 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 (? - at least in Australian Sphaerius, the beetle has no plastron but respires via air stored beneath the elytra); collecting air at the water surface by exserting the tip of the abdomen through the surface film (?). Phytophagous (supposedly feeding on algae).

Larvae. Mature larvae minute (less than 3 mm long). The larvae campodeiform; elongate and more or less parallel-sided. Body somewhat flattened. Vestiture restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized, or dorsally only very lightly pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 2 segmented; 0.15–0.5 x the head width. Stemmata present; on either side of the larval head 4. The larval fronto-clypeus not extended forwardly. The frontoclypeal suture between frons and clypeus indistinct or absent. The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate. The maxillary palps 2 segmented. The labium with a short ligula between the palps, or with a ligula at least as long as the palps. Labial palps present and segmented; 1 segmented. Mesothoracic legs present and segmented; 5 segmented; with 1 moveable claw. Visible abdominal segments 10. Tergum 9 of the abdomen extending to the underside. The abdomen having functional spiracles on anterior segments (these annular); with anterior and 8th-segmental spiracles borne at the ends of a series of spiracular tubes. The larvae with abdominal gills; with dorsal, balloon-like tracheal gills on abdominal segments 1–8. The abdominal apex without a respiratory chamber. Abdominal tergum 8 bearing a single median process without apical spiracles. The last abdominal segment without cerci.

Larvae aquatic; inhabiting wet sand and gravel at the edges of streams and rivers; seemingly phytophagous (on algae).

The larvae have stout, 5-segmented legs, spiracular gills and prominent, 2-segmented antennae.

Classification. Suborder Myxophaga; Superfamily Sphaeroidea.

Representation in Britain and Ireland, and worldwide. 23 species worldwide; genera 1 (Sphaerius, represented more or less worldwide). 1 species in Britain (S. acaroides); genera in Britain 1; Spaerius.

Illustrations. • Sphaerius acaroides. • Sphaerius acaroides, with Corylophidae and Ptiliidae: Fowler 3, 80 (1889). • Fowler 3, 80 (1889): original legend..

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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.’.