Insects of Britain and Ireland: the families of Coleoptera

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L. Watson and M. J. Dallwitz


Parnidae; including Chiloeidae.

Long-toed Water Beetles.

General appearance. 3.5–5.5 mm long. Body length/maximum body width 1.7–3.2. Elytral length/pronotal length 2–4.15. 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 without ventral body cavities for reception of the legs; not necked. Upper surfaces of body glabrous or subglabrous, or non-glabrous; exhibiting stiff, erect, dark bristles, or not bristly; with neither scales nor scale-like setae. The underside exhibiting a plastron of hydrofuge hairs, detectable as a bubble when the insect is submerged, or without a plastron of hydrofuge hairs.

Detailed morphology. Inclination of the head slight to strong. 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, or multidentate or multilobed. The incisor edges of the mandibles simple, or with a single tooth, or with two or more teeth. 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; strongly asymmetric; 3–6 segmented, or 8–11 segmented, or 13 segmented; clubbed and pectinate (of unusual form, with a pectinate club). Antennal clubs 6–10 segmented (‘six or more’). Antennal insertions visible from above, or hidden from above; not in fossae.

Cervical sclerites present. Pronotal length/maximum pronotal width 0.4–1.05. The pronotum with lateral keels (pronotal carinae); keels complete. Posterior edge of the pronotum distinctly crenulate, or not crenulate. Prothorax without notopleural sutures. Scutellum conspicuous, or absent; elevated above the mesoscutum in lateral view; anteriorly when present, simple; posteriorly narrowly rounded or acute, or broadly rounded or obtusely angulate. The prosternal process present; complete; moderately or strongly overlapping the mesoventrite to concealing most or all of 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 less than the shortest diameter of the cavity, or 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; not markedly extended laterally; not shaped posteriorly to receive the femur; with a steep transverse declivity against which the femur retracts. Tarsal segmentation formula 5, 5, 5 (in British representatives). 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; without an associated empodium. Hind tarsi with as many segments as the mid-tarsi; 5-segmented.

Elytral length/maximum width across the elytra 1.1–2.3. Elytra exposing no more than part of the terminal tergite; hard; non-glabrous (covered with fine hairs, which are short and recumbent in Pomatinus but more or less raised in Dryops). Elytra with six or more longitudinal lines of punctures, or apunctate, irregularly punctate, or each with fewer than 6 longitudinal lines of punctures or impressed striae; the lines per elytron when more than five, 9. Scutellary striole absent. Wings well developed, or absent or much reduced; fringed with long hairs on the hind margin, or not fringed. Wings without an anal lobe. Wings without a medial fleck. Abdominal sternites 5; comprising both fused and moveable components. Basal abdominal sternites immovably joined 2. Abdominal segment 8 with apparently functional spiracles. The male external genitalia trilobate.

Adult habitat, ecology. Water-beetles to land-dwellers (aquatic or sub-aquatic, often found under stones or wood). 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 replenishing air for respiration beneath the elytra indirectly from that acquired and held in the ventral plastron; incorporating bubbles of oxygen directly into the plastron.

Larvae. Mature larvae minute (less than 3 mm long), or small to medium-sized. The larvae campodeiform; elongate and more or less parallel-sided; C-shaped in lateral view. Body circular in cross-section. Vestiture restricted to fine hairs or setae. The larvae dorsally heavily pigmented or sclerotized; ventrally more or less heavily pigmented. The antennae 3 segmented; less than 0.15 x the width of the head to 0.15–0.5 x the head width. Stemmata present; on either side of the larval head 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 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 (these biforous or bilbiate); without spiracular tubes. The larvae without abdominal gills. The abdominal apex without a respiratory chamber. Abdominal tergum 8 without amature. The posterior segments without swimming hairs. The last abdominal segment without cerci.

Larvae non-aquatic (not truly aquatic, despite living in wet habitats); in decaying plant material, in rotting wood, and in the soil (especially in wet soil and leaf litter, and waterlogged wood); consuming decaying plant material and consuming rotting wood.

Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea.

Representation in Britain and Ireland, and worldwide. About 200 species worldwide. 8 species in Britain; genera in Britain 3; Dryops, Pomatinus (Helichus). E.g., Dryops luridus.

General comments. The thorax of Pomatinus exhibits an impressed line on each side which is lacking in Dryops, and in the latter the front of the thorax is more contracted; this additional to the different arrangement of the elytral hairs (q.v.).

Illustrations. • Dryops luridus (Impressed Parnus: B. Ent. 080). • Dryops luridus (details, B. Ent. 080). • Dryops luridus: B. Ent. 080, legend+text. • Dryops luridus: B. Ent. 080, text cont.. • Dryops (7 spp.), with unrelated taxa: Fowler Suppl. 14, 1913. • Fowler 6, 14 (1913): original legend.. • Dryops auriculatus and Pomatinus, with Elmidae and Heteroceridae: Fowler 3, 98 (1889). • Fowler 3, 98 (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.’.