British insects: the families of Coleoptera


L. Watson and M. J. Dallwitz


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. Beetles 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 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; 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.

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).

Worldwide and British representation. 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 on this taxon. 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. • Fowler 6, 14 (1913): original legend.. • Dryops auriculatus and Pomatinus, with Elmidae and Heteroceridae: Fowler 3, 98 (1889). • Fowler 3, 98 (1889): 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. British insects: water beetles. Version: 18th September 2012.’.