DRYOPIDAE Billberg, 1820
Long-toed Water Beetles
These beetles occur in most wetland habitats. Although some species are common, they can be cryptic and difficult to find. Identification can be very difficult.
POLYPHAGA Emery, 1886
BYRRHOIDEA Latreille, 1804
Dryops luridus larvae
This small family includes about 300 species in 33 genera and occurs worldwide with the exception of Australia, most larger islands have their own fauna and many endemics are known. The greatest diversity occurs in warmer Oriental and Neotropical regions and in general temperate faunas are small; 13 species of 5 genera occur in the United States whereas more than 70 species of 12 genera have been recorded in the Neotropical zone. The European fauna includes 21 species of which 14 occur in central Europe and 9 extend to the UK. Although distinct they are very closely related to elmids and for many years the two groups were included in the single family Parnidae Leach, 1817. The common name most appropriately applies to members of the speciose and cosmopolitan genus Helichus Erichson, 1847 (and similar species), and the alternative name of ‘riffle beetles’ is now more generally applied to elmids. The majority of species, especially in temperate regions, are of a characteristic elongate and parallel-sided shape and densely pubescent, they are immediately recognizable from the very short and modified antennae; pectinate with the inner margin finely comb-like, but in life these are usually retracted into cavities in front of or under the eyes, and when mounting specimens some considerable dexterity is often needed to extract them. In temperate regions most species are associated with wetland habitats and this is general with about 75% of all known species recorded from wetlands but in warmer areas many occur among leaf-litter and debris in permanently damp grassland and woodland environments. A single species is known to be subterranean; Stygnoparnus comalensis (Barr & Spangler, 1992) occurs in an aquifer at a single location in Texas, the larvae are thought to develop in air-pockets and the pale translucent brown adults have vestigial eyes and very fine pubescence, they are otherwise quite typical of the family. Most species occur within a short distance of rivers or lakes etc. e.g. all members of the largest genus, Dryops Olivier, 1791, are riparian occurring beside water throughout most of the world and often in great abundance in Neotropical areas. They occur among foliage and roots in fine-grained substrates and may be very active, taking flight in warm weather or burrowing into waterlogged soil and, like those of many elmid genera, employ plastron respiration for an independent aquatic mode of life; some species of Helichus have been observed to use their antennae
to replenish air in the plastron. Adults do not swim but crawl among aquatic vegetation and substrates, those that develop in riparian situations tend not to disperse very far; recently eclosed adults may take flight in large numbers and enter nearby water bodies where they will feed and mate, and at such times large numbers have been recorded at light. So far as is known both larvae and adults feed on organic remains or microscopic algae etc. In general oviposition occurs in the spring when eggs are deposited among marginal substrates or on the stems of nearby vegetation, all known larvae are terrestrial and those of riparian species develop within metres of the water, generally within the soil, under debris or among roots etc. and pupation occurs in a cell within the substrate. In temperate regions most species are thought to be univoltine but some Nearctic species of Helichus are known to spend several years in the larval stage. Adults occur throughout the year; they are active from spring to autumn and spend the winter among leaf-litter etc. near the water although some have been found hundreds of metres from water and some are active during mild spells. Adults are readily swept from marginal vegetation or may be found on the soil or among submerged leaves and stems in shallow margins, sometimes they appear in numbers after the soil has been flooded and often they will appear in the pond net while sweeping for water beetles. In warmer climates they occur among sieved sample of forest or grassland litter etc. and large numbers may occur in light or flight-interception traps during dispersal in the spring.
Species are typically elongate and, in most temperate species, rather parallel-sided and either continuous in outline or constricted between the pronotum and elytra, most appear silky or silvery due to fine and dense pubescence over the entire body and in some there is a double pubescence with long and rather erect setae over the dorsal surface. Most are drab, black to brown or grey and some are bicoloured with the forebody darker, and some tropical species, mostly associated with wood or terrestrial habitats, are vividly metallic. Head strongly deflexed and retracted into the prothorax to the posterior margin of the eyes, surface punctured and variously rugose, the vertex sometimes longitudinally impressed. Eyes convex, entire and moderately large, labrum narrow and curved, labial palpi 3-segmented, maxillary palpi 4-segmented; each with a large and fusiform terminal segment. Mandibles small, flat and curved apically sharp, bi- or multi-dentate. Antennae 8-11 segmented and short; serrate to pectinate with the second segment often expanded and covering some of the others, third segment small and the remainder produced internally to form a loose lamellate or pectinate club, often inserted within a wide cavity anterior to the eyes. Pronotum quadrate or nearly so, weakly convex and smooth; without deep basal fovea but sometimes with median or sub-lateral furrows or sub-lateral carinae, surface variously punctured and microsculptured. Lateral margins straight and parallel or tapering from the base, to evenly rounded, smooth to finely denticulate, generally not explanate or only so towards the base and usually with a distinct lateral bead. Prosternum long in front of coxae, without grooves for the appendages, process variable; narrow to broad between variously transverse coxal cavities which are broadly open posteriorly, and usually inserted into a basal mesosternal groove. Mesosternum short with round to weakly transverse coxal cavities which are open laterally and separated by at least the diameter of a coxa. Metasternum long and wide, sometimes with distinct post-coxal lines or a median impression but otherwise smooth, coxae variable from weakly transverse to widely so and reaching the elytral epipleurae, and contiguous to widely separated. Scutellum usually large and obvious. Hind wings usually well-developed sometimes reduced or, rarely, absent. Elytra entire, covering the abdomen and usually continuously rounded apically, surface smooth to randomly punctured or with punctured striae, lateral margins generally straight or smoothly rounded, sometimes sinuate or constricted about the middle, epipleura complete and continuous to the apex, often only narrowly so. In some ground-living species the elytra are fused. Abdomen with 5 ventrites; 2 basal ventrites connate, the basal ventrite sometimes with distinct post-coxal lines. Legs long and slender; pro- and meso-coxae small and usually separated, meta-coxae variously transverse and separated, trocanters triangular and usually visible in side view. Femora slender and unarmed, tibiae with tiny spurs on the inner apical angle. Tarsi 5-5-5 without expanded segments, the terminal segment much longer than the others, claws smooth.
Familiarity with the UK species will allow much of the family to be recognized but some tropical species are brightly coloured and metallic and lack the dense pubescence typical of most. In most species the sexes are very similar and so far as is known there is only a single example of dimorphism; Geoparnus Besuchet, 1978 includes 3 tropical species that occur among rain forest leaf-litter, and in one of these, G. rhinoceros Kodada, Jach, Ciampor & Ciamporova-Zatovicova, 2007, the male possesses a clypeal horn. All three species are atypical; compact and rounded with very large elytral punctures and odd-looking, apically branched setae over the body, femora and tibiae
Larvae are elongate, cylindrical and tapering towards the apex, they lack external gills and most are tapered apically, the ninth abdominal segment encloses the tenth which in some species possess tracheal gills. 5-12mm. Head usually hidden from above; with a distinct frontoclypeal suture, short mandibles and 3-segmented antennae. Most species have a single pair of simple eyes although these may be absent and represented by spots of darker pigment. Legs 4- or 5-segmented with apical claw-like tarsunguli. Abdomen 9-segmented, 1-5 usually with a ventral plica, the eighth with an operculum through which the ninth extends. Spiracles are usually present on the mesothorax and abdominal segments 1-8, rarely only on the eighth.
Two genera occur in the UK; our single species of Pomatinus Sturm, 1853, P. substriatus (Muller, P.W.J., 1806) is a widespread though generally rare species distinguished by absence of longitudinal lateral furrows on the pronotum, a feature present in all our remaining species. Of our 8 species of Dryops Olivier, 1791 only 2 are generally common and widespread; D. ernesti des Gozis, 1886 and D. luridus (Erichson, 1847). Our other species tend to be widespread but local and generally scarce, though much more common on the continent; D. anglicanus Edwards, J., 1909 is more or less restricted to fenland in Norfolk while most records of D. griseus (Erichson, 1847) are also from Norfolk but there are a few widely spread records from elsewhere in England. D. nitidulus (Heer, 1841) has a sporadic and scattered distribution around England and Wales and also occurs in southern Scotland. Adults of Pomatinus live below the water, usually among substrate etc. near the margins of deep brooks and streams, adults of Dryops are generally riparian but the common and widespread D. ernesti will often occur on permanently wet grassland away from rivers etc. All larvae are terrestrial and generally occur in the same habitats as the adults; among wet soil and organic litter or in waterlogged logs and fallen trees, they are wireworm-like and superficially resemble elaterid larvae but are distinct in lacking the pygopodium seen on the tenth abdominal segment of that family.
Our species of Dryops can be very difficult to identify, most males will need to be dissected, and many females can only be assigned through association. A key to our species can be found HERE.