Insects of Britain and Ireland: water beetles

L. Watson and M.J. Dallwitz

Dryopidae

Parnidae; including Chiloeidae

Long-toed Water Beetles.

Adults. Beetles terrestrial to sub-aquatic (found under stones or wood); 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; replenishing air for respiration beneath the elytra indirectly from that acquired and held in a ventral plastron; incorporating bubbles of oxygen directly into the plastron; 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 non-glabrous; exhibiting stiff, erect, dark bristles, or not bristly; with neither scales nor scale-like setae. Inclination of the head slight to strong.

Eyes strongly protuberant, or not strongly protuberant; bristly, or without bristles. 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; without stylet-like lobes. 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; 3–6 segmented, or 8–11 segmented, or 13 segmented; clubbed and pectinate (of unusual form, with a pectinate club preceded by an enlarged segment). Antennal clubs 6–10 segmented (‘six or more’). Antennal insertions visible from above, or hidden from above; not in fossae.

Prothorax without notopleural sutures. Pronotal length/maximum pronotal width 0.4–1.05. The scutellum elevated. Metaventrite with a transverse groove, or without a transverse groove. The prosternal process complete; moderately or strongly overlapping the mesoventrite to concealing most or all of the mesoventrite. The fore-leg coxal cavities broadly open; quite widely separated; strongly transverse; without lateral extensions; internally open. Hind coxae 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; without an associated empodium. Hind tarsi with at least as many segments as the mid-tarsi; 5-segmented. Hind tarsi without ‘swimming’ hairs.

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

Adults not predacious; close to fresh water.

Larvae. The larvae campodeiform; elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; dorsally heavily pigmented or sclerotized; more or less heavily pigmented. The antennae 3 segmented; less than 0.15 x the with of the head, or 0.15–0.5 x the head width. Stemmata 6. 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. 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 aquatic to non-aquatic. The abdomen having functional spiracles on anterior segments. The larvae without gills. Abdominal tergum 8 without amature. The larvae not predacious; consuming decaying plant material and consuming rotting wood (in wet soil and leaf litter, and waterlogged wood).

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

Classification. Suborder Polyphaga; Infraorder Elateriformia; Superfamily Dryopoidea. 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.).

Miscellaneous. • 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. SILVANIDAE. 1, Oryzaephilus mercator. CRYPTOPHAGIDAE. 2, Micrambe bimaculatus; 3, Micrambe abietis. 4, Henoticus serratus. DERMESTIDAE. 5, Globicornis nigripes. DRYOPIDAE. 6–12, Dryops spp. 6, D. luridus; 7, D. ernesti; 8, D. striatellus; 9, D. anglicanus; 10, D. auriculatus; 11, D. griseus; 12, D. nitidulus. SCARABAEIDAE. 13, Aphodius nemoralis. From Fowler's plate, with the names from the original legend (q.v.) updated. • Fowler 6, 14 (1913): original legend.. • Dryops auriculatus and Pomatinus, with Elmidae and Heteroceridae: Fowler 3, 98 (1889). HYDROPHILIDAE. 1, Georyssus crenulatus. ELMIDAE. 2, Elmis aenea. 3, Limnius volkmari. 4, Esolus parallelepipedus. 5, Oulimnius tuberculatus. 6, Macronychus quadrituberculatus. DRYOPIDAE. 7, Potaminus substriatus; 7a, Potaminus substriatus, antenna. 8 and 9, Dryops auriculatus. HETEROCERIDAE. 10–13, Heterocerus. 10, H. flexuosus; 11, H. fenestratus; 12, H. fusculus; 13, H. maritimus. From Fowler's plate, with the names from the original legend (q.v.) updated. • Fowler 3, 98 (1889): original legend..