Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz



General appearance. 5–7 mm long. Elytral length/pronotal length 2.6–3. Base of prothorax distinctly narrower than the combined elytral bases. Greatest prothoracic width distinctly narrower than greatest elytral width. Beetles round to oval; terrapin-like in shape, with flattened and clear margins to thorax and elytra; dorsally flattened; not necked; conspicuously waisted. Upper surfaces of body glabrous or subglabrous, or non-glabrous; exhibiting stiff, erect, dark bristles, or not bristly (?); exhibiting scales or scale-like setae, or with neither scales nor scale-like setae (?).

Detailed morphology. Eyes strongly protuberant, or not strongly protuberant; bristly, or without bristles. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. Mandibles with a well developed mola. The mandibular apices bidentate or bilobed. The incisor edges of the mandibles simple (Thymalus), 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 to short; conspicuously elbowed (on the somewhat elongate scape); 11 segmented; with the scape much-elongated. Antennal scape somewhat swollen. Antennae clubbed. Antennal clubs loosely 3 segmented. Antennal insertions visible from above, or hidden from above (?).

Cervical sclerites present. Pronotal length/maximum pronotal width 0.5–0.7. The pronotum with lateral keels (pronotal carinae); keels complete. Prothorax without notopleural sutures. Scutellum conspicuous; elevated above the mesoscutum in lateral view; anteriorly simple; posteriorly broadly rounded or obtusely angulate, or truncate. The prosternal process present; complete; slightly overlapping the mesoventrite, or moderately or strongly overlapping the mesoventrite (at least in Thymalus). Metaventrite without a transverse groove. The fore-leg coxae countersunk in ‘procoxal cavities’. The fore-leg coxal cavities open behind externally; broadly open; narrowly separated, or quite widely separated; strongly transverse; without lateral extensions; internally open. The mid-leg coxae countersunk in ‘mesocoxal cavities’; separated by less 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. Tarsal segmentation formula 5, 5, 5, or 4, 4, 4. The tarsi exhibiting bilobed segments, or 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; 4-segmented, or 5-segmented. Mid-leg tarsi 4-segmented, or 5-segmented; pentamerous, or tetramerous. The claws of the mid-leg tarsi not appendaged. Hind tarsi with as many segments as the mid-tarsi; 4-segmented, or 5-segmented.

Elytra apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae. Scutellary striole absent. Wings well developed. Wings with an anal lobe. Wings without a medial fleck. Abdominal sternites 5, or 6 (?). Abdominal segment 8 apparently without functional spiracles (?). The male external genitalia cucujiform.

Adult habitat, ecology. Presumably not predacious; mycetophagous (associated with bracket fungi).

Larvae. Mature larvae small to medium-sized. The larvae elongate and more or less parallel-sided. Body somewhat flattened. 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. Stemmata present, or absent; on either side of the larval head when present, 3, or 5. The frontoclypeal suture between frons and clypeus indistinct or absent, or distinct (Thymalus). The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate. The maxillary palps 3 segmented, or 4 segmented. The labium without ligula between the palps, or with a short ligula between the palps (Thymalus). Labial palps present and segmented; 2 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-biforous); without spiracular tubes. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The last abdominal segment with cerci.

Larvae associated with fungi; mycetophagous (found in bracket fungi).

Classification. Suborder Polyphaga; Infraorder Cucujiformia; Superfamily Cleroidea.

Representation in Britain and Ireland, and worldwide. About 50 species worldwide; genera about 20. 2 species in Britain; genera in Britain 2; Ostoma, Thymalus. E.g., Thymalus limbatus (Rust-coloured Thymalus).

Illustrations. • Thymalus limbatus (Rust-coloured Thymalus: B. Ent. 039). • Thymalus limbatus (details, B. Ent. 039). • Thymalus limbatus: B. Ent. 039, legend+text. • Thymalus limbatus: B. Ent. 039, text cont..

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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.’.