Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz



General appearance. 1.5–3.3 mm long. Body length/maximum body width 1.9–3.42. Elytral length/pronotal length 1.04–4. Base of prothorax not or scarcely narrower than the combined elytral bases. Greatest prothoracic width not narrower or only slightly narrower than the greatest elytral width. Beetles elongate; not necked; not waisted; related to Elateridae. Upper surfaces of body glabrous or subglabrous, or non-glabrous; not bristly; with neither scales nor scale-like setae. Beetles not clicking and ‘jumping’.

Detailed morphology. Inclination of the head slight to strong. Eyes strongly protuberant, or not strongly protuberant; without bristles; finely facetted. The frons without a transverse ridge between the eyes. The labrum at least partly visible in antero-dorsal view; labrum mostly moderately to heavily sclerotized. Mandibles without a mola; with reduced prosthecae, or without prosthecae. The mandibular apices simple. The incisor edges of the mandibles simple. The maxillae with distinct galea and lacinia apically to the palp. The apical segment of the maxillary palps somewhat expanded and truncate to subtriangular. The apical segment of the labial palps more or less expanded apically. Antennae short; 8–9 segmented, or 11 segmented; clubbed. Antennal clubs 3 segmented (serrate). Antennal insertions visible from above, or hidden from above; ‘countersunk’ within saucer-like fossae, or not in fossae.

Cervical sclerites present. Pronotal length/maximum pronotal width 0.5–0.93. The pronotum with lateral keels (pronotal carinae); keels incomplete. Prothorax without notopleural sutures. Hind corners of the thorax extended rearwards into sharp points. Scutellum conspicuous; elevated above the mesoscutum in lateral view; anteriorly simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process complete; moderately or strongly overlapping the mesoventrite to concealing most or all 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; quite widely separated; circular to longer than wide; without lateral extensions; internally open. 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; contiguous, posteriorly shaped to receive the retracted femur (with transverse cavities). Tarsal segmentation formula 5, 5, 5. The tarsi without bilobed segments; without ‘hidden’ segments. Front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented; pentamerous; 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 (this with no more than two setae), or without an associated empodium. Hind tarsi with as many segments as the mid-tarsi; 5-segmented.

Elytral length/maximum width across the elytra 0.66–2.52. Elytra covering most to all of the abdomen; exposing no more than part of the terminal tergite to at least three complete abdominal tergites. Elytra with six or more longitudinal lines of punctures; the lines per elytron 9. Scutellary striole absent. Wings well developed, or absent or much reduced. Wings with an anal lobe. Wings without a medial fleck. Abdominal sternites 5; comprising both fused and moveable components (only the fifth being movable). Basal abdominal sternites immovably joined 3, or 4. Abdominal segment 8 with apparently functional spiracles, or apparently without functional spiracles. The male external genitalia trilobate.

Adult habitat, ecology. Land-dwellers; on living vegetation and associated with flowers.

Larvae. Mature larvae minute (less than 3 mm long), or 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 2 segmented, or 3 segmented. Stemmata absent. 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. The maxillary palps 3 segmented. The labium with a short ligula between the palps. 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); without spiracular tubes. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The last abdominal segment with cerci.

Larvae in decaying plant material, or in rotting wood, or in the soil; phytophagous and consuming decaying plant material, or mycetophagous (with Trixagus dermestoides reported as feeding on mycorrhyzal fungi).

The larvae soft-bodied and small-headed with very short legs, the mandibles biting outwards as in Eucnemidae.

Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Elateroidea.

Representation in Britain and Ireland, and worldwide. About 150 species worldwide. 5 species in Britain; genera in Britain 2; Aulonothroscus, Trixagus. E.g., T. obtusus (Minute Throscus).

General comments. Elateroid beetles, but without the clicking mechanism of Elateridae (the elongated prosternal extension here being broad and flat). As in the Elateridae but unlike the Eucnemidae, the adult labrum is visible from above..

Illustrations. • Trixagus obtusus Curtis (Minute Throscus: B. Ent. 163). • Trixagus obtusus: B. Ent. 163. • Trixagus obtusus: B. Ent. 163, legend+text. • Trixagus obtusus: B. Ent. 163, text cont.. • Trixacus dermestoides and T. obtusus (as Throscus, with Buprestidae, Eucnemidae, etc.) Fowler 4, 104 (1890). • Fowler 4, 104 (1890): 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.’.