British insects: the families of Coleoptera
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 (? - at least in Australian Sphaerius, the beetle has no plastron but respires via air stored beneath the elytra). Beetles 0.6–0.8(–1.2) 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 (globular); not necked; not waisted. Upper surfaces of body glabrous or subglabrous; not bristly; with neither scales nor scale-like setae. Inclination of the head slight.
Eyes two; not strongly protuberant; without bristles; coarsely facetted. 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 maxillary palps not especially elongated. The apical segment of the maxillary palps aciculate. The apical segment of the labial palps not expanded apically. Antennae short; not strongly asymmetric; 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.
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 0.37–0.4. Metaventrite without a transverse groove. Mid-and hind-legs not as in Gyrinidae (q.v.). The fore-leg coxal cavities broadly open; medianly confluent; strongly transverse; without lateral extensions; internally closed by a slender bar. Hind coxae not extended laterally to meet the elytra; peculiarly posteriorly shaped to receive the retracted femur. The hind coxae triangular, almost contiguous, much enlarged and produced behind into large flat plates which partly conceal 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 3, 3, 3. 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; 3-segmented. Mid-leg tarsi 3-segmented; trimerous. 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 at least 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. Scutellary striole present. Elytra glabrous. Wings well developed, or absent or much reduced; with the apical part spirally rolled; fringed with long hairs on the hind margin. Exposed abdominal sternites 3 (first and third long); all articulated and movable. Abdominal segment 8 with apparently functional spiracles.
Under stones near water.
Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized, or only very lightly pigmented or sclerotized; only very lightly pigmented. The antennae 2 segmented; 0.15–0.5 x the head width. Stemmata fewer than 6 (four). 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. The labial palps 2 segmented. The mesothoracic legs 5 segmented (including the pretarsus); with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen extending to the underside.
The larvae aquatic. The abdomen exhibiting functional abdominal spiracles; having functional spiracles on anterior segments. The larvae with dorsal, balloon-like tracheal gills on abdominal segments 1–8. Abdominal tergum 8 without amature.
The larvae with stout, 5-segmented legs and prominent, 2-segmented antennae.
Worldwide and British representation. 1 species in Britain (S. acaroides). Recorded from South-west England and Wales and South-eastern England.
Classification. Suborder Myxophaga; Superfamily Sphaeroidea; Spaeriidae.
General comments on this taxon. Tiny beetles, black and globular.
Miscellaneous. • 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. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.