Insects of Britain and Ireland: the families of Coleoptera
Minute Marsh-roving Beetles.
General appearance. 1.5–1.8 mm long. Body length/maximum body width 1.3–2.15. Elytral length/pronotal length 2.4–4.7. 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, or distinctly narrower than greatest elytral width. Beetles oval; dorsally strongly convex; having ventral body cavities into which the legs fold to conform with the general body surface; not necked; not waisted. Upper surfaces of body glabrous or subglabrous, or non-glabrous; not bristly; with neither scales nor scale-like setae.
Detailed morphology. Inclination of the head slight to very strong. Eyes strongly protuberant, or not strongly protuberant; without bristles. 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, or multidentate or multilobed. 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 cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae short; 7 segmented, or 11 segmented; weakly clubbed. Antennal clubs 1 segmented. Antennal insertions visible from above, or hidden from above.
Cervical sclerites present. Pronotal length/maximum pronotal width 0.3–0.65. 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 narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process complete; moderately or strongly overlapping the mesoventrite. Metaventrite with a transverse groove, or 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; strongly transverse; 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; posteriorly shaped to receive the retracted femur; with a steep transverse declivity against which the femur retracts. 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. Hind tarsi with as many segments as the mid-tarsi; 5-segmented.
Elytra present. Elytral length/maximum width across the elytra 1–1.65. Elytra exposing no more than part of the terminal tergite. Elytra apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae. Scutellary striole absent. Wings well developed, or absent or much reduced. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites 5; comprising both fused and moveable components, or all fused and immoveable. Basal abdominal sternites immovably joined 3, or 5. Abdominal segment 8 with apparently functional spiracles. The male external genitalia trilobate.
Adult habitat, ecology. Water-beetles, or land-dwellers (? - aquatic or sub-aquatic, living on the margins of water courses). 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 if geniunely aquatic, presumably replenishing air for respiration beneath the elytra indirectly from that acquired and held in the ventral plastron (?); not noticeably posing regularly at the water surface to replenish air; incorporating bubbles of oxygen directly into the plastron (?). Phytophagous (on algae or bryophytes?).
Larvae. Mature larvae minute (less than 3 mm long). The larvae campodeiform; elongate and more or less parallel-sided. Body circular in cross-section, or circular in cross-section to somewhat flattened. Vestiture restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized; ventrally only very lightly pigmented. The antennae 3 segmented; less than 0.15 x the width of the head. Stemmata present; on either side of the larval head 5, or 6. The larval fronto-clypeus not extended forwardly. The frontoclypeal suture between frons and clypeus distinct. The labrum and head capsule separated by a complete suture. Apices of the mandibles bilobed or bidentate, or trilobed or tridentate. The maxillary palps 4 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 entirely dorsal. The abdomen having functional spiracles on anterior segments, or with functional spiracles confined to the the eighth segment (?); without spiracular tubes. The larvae without abdominal gills. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The last abdominal segment without cerci.
Larvae aquatic; in decaying plant material (usually found near water, in damp soil or humus at the edges of ponds or streams, or in spray-wetted littoral sand); consuming decaying plant material.
Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea.
Representation in Britain and Ireland, and worldwide. About 270 species worldwide; genera about 35. 1 species in Britain (L. pygmaeus); genera in Britain 1; Limnichus.
General comments. The tarsi in non-British species sometimes pseudopentamerous or tetramerous, sometimes also with lobed segments. The beetles ventral surface exhibits cavities into which the legs can be folded.
Illustrations. • Limnichus pygmaeus (from Joy). Limnichus pygmaeus (Limnichidae). Eubria palustris (Psephenidae). • Limnichus pygmaeus (with Dermestidae, Byrrhidae, etc.): Fowler 3, 97 (1889). • Fowler 3, 97 (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. delta-intkey.com’.