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| Insects of Britain and Ireland: water beetles |
Minute Marsh-roving Beetles.
Adults. Beetles terrestrial, or sub-aquatic (? - living in or on mud on the margins of water courses); 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; perhaps replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron (?); perhaps incorporating bubbles of oxygen directly into the plastron (?); 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 non-glabrous. Beetles with very short, appressed pubescence throughout. Upper surfaces of body not bristly; with neither scales nor scale-like setae. Inclination of the head slight to very strong.
Eyes strongly protuberant, or not strongly protuberant; without bristles. 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.
Prothorax without notopleural sutures. Pronotal length/maximum pronotal width 0.3–0.65. The scutellum elevated. Metaventrite with a transverse groove, or without a transverse groove. The prosternal process complete; moderately or strongly overlapping the mesoventrite. The fore-leg coxal cavities broadly open; quite widely separated; strongly transverse; without lateral extensions; internally open. Hind coxae posteriorly shaped to receive the retracted femur; with a steep transverse declivity against which the femur retracts. Tarsal segmentation formula 5, 5, 5. 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; 5-segmented. Mid-leg tarsi 5-segmented; pentamerous. The claws of the mid-leg tarsi not appendaged. The claws of the mid-leg tarsi simple. Hind tarsi with at least 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. Scutellary striole absent. Elytra non-glabrous (with very short, fine recumbent pubescence). Wings well developed, or absent or much reduced; without an anal lobe; without a medial fleck. Exposed abdominal sternites 5; comprising both fused and movable components, or all fused and immovable; immovably joined 3, or 5. Abdominal segment 8 with apparently functional spiracles. The male external genitalia trilobate.
Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized; only very lightly pigmented. The antennae less than 0.15 x the with of the head. Stemmata 6, or fewer than 6 (then five). 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. The labial palps 4 segmented. The mesothoracic legs 5 segmented (including the pretarsus); with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal.
The larvae non-aquatic (but in wet places). The abdomen exhibiting functional abdominal spiracles. The larvae without gills. Abdominal tergum 8 without amature.
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.
Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea.
General comments. The tarsi in non-British species are sometimes pseudopentamerous or tetramerous, sometimes also with lobed segments. Stout-bodied, ovoid beetles with both dorsal and ventral surfaces convex; ventrally with cavities into which the legs can be folded; the mid coxae widely separated, the hind ones very colsely approximated.
Miscellaneous. • Limnichus pygmaeus (from Joy). Limnichus pygmaeus (Limnichidae). Eubria palustris (Psephenidae). • Limnichus pygmaeus (with Dermestidae, Byrrhidae, etc.): Fowler 3, 97 (1889). DERMESTIDAE. 1, Megatoma undata. 2, Ctesias serra. 3, Anthrenus scrophulariae; 3a-3c, clubs of subgenera Anthrenus, Florilinus and Helocerus respectively. 4, Anthrenus varius. 5, Trinodes hirtus. BYRRHIDAE. 6, Syncalypta setigera. 7, Byrrhus pilula; 8, Byrrhus pustulatus. 9, Cytilus sericeus. 10, Morychus aeneus. 11, Symplocaria semistriata. LIMNICHIDAE. 12, Limnichus pygmaeus. SPHINDIDAE. 13, Aspidiphorus orbiculatus; 13a, A. orbiculatus, antenna. From Fowler's plate, with the names from the original legend (q.v.) updated. • Fowler 3, 97 (1889): original legend..
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, 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. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2012 onwards. Insects of Britain and Ireland: water beetles. Version: 14th April 2022. delta-intkey.com’.
