ELMIDAE Curtis, 1830

Riffle Beetles

Although several are widespread and locally common these secretive beetles are generally difficult to find. Most species occur in waterside environments. 

Introduction

This is a cosmopolitan family of about 1500 species, although the group has never been popular and  many more remain to be named worldwide, of  150 genera and 2 subfamilies, which is most diverse in warmer regions e.g. more than 250 species of about 40 genera occur in the Neotropical region and the Australian fauna includes more than 100 species while 100 species of 26 genera are Nearctic; the European fauna includes about 45 species of which 25 occur in central Europe and 12 species of 7 genera extend to the UK. Larainae Boving & Craighead, 1930 is the smaller subfamily with 27 described genera; it is present in tropical areas worldwide and the greatest diversity is in the Neotropical and Afrotropical regions, the genera tend to be of restricted distribution with many endemic to various regions e.g. Africa, southeast Asia or Australia and temperate faunas tend to be very poor e.g. 3 species of 2 genera are Nearctic while only a single species, Potamophilus acuminatus (Fabricius, 1792) occurs in Europe but does not extend to the UK. The wider Palaearctic fauna includes only 6 species. Elminae Curtis, 1830 includes more than 120 genera in 3 tribes and is cosmopolitan. Ancyronychini Ganglbauer, 1904 includes the single genus Acyronyx Erichson, 1847 with about 35 species, mostly from the eastern Palaearctic and southeast Asian regions; it is absent from Australia and only a single species, A. variegata (Germar, 1824) occurs in North America. Macronychini Gistel, 1848 includes 22 genera and is mostly Old World tropical and subtropical in distribution; they are absent from South America and only 3 species of 2 genera occur in the Nearctic region; the tribe includes genera endemic to Japan and Malaysia as well as many more generally distributed through the eastern Palaearctic and southeast Asia. Despite being extensively distributed in e.g. the Philippines and other Old World islands, extending south to Sumatra, the tribe has not been recorded from Australia or the African region. Elmini Curtis, 1830 includes the majority of the species in about 100 genera, it is cosmopolitan in distribution and generally comprises the greater part of temperate faunas but even so diversity falls drastically with increasing latitude; the majority of the European fauna belongs to this tribe including all but one of the UK species, and more than 90% of the North American fauna are included; compared with the 100 or so American species only 33 occur in Canada. The higher-order classification of the Elminae has yet to be worked out but the  3 tribes mentioned are generally

POLYPHAGA Emery, 1886

BYRRHOIDEA Latreille, 1804

8

12

1.2-4mm

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accepted and the Elmini is usually further divided into 2 subtribes, Elmina Curtis, 1830 and Stenelmina Mulsant & Rey, 1872, both of which are represented in the UK fauna. An example of the tropical diversity of the tribe is provided by the genus Austrolimnius Carter & Zech, 1929 which includes more than 100 species distributed in the Australasian and Neotropical regions, conversely Lamalelmis Spangler, 1981 includes only 2 Neotropical species while the monotypic Aesobia Jách, 1982 occurs in Sri Lanka and Anommatelmis botosaneanui Spangler, 1981, also the only member of the genus, occurs in Haiti.

Description

The following general description will suffice for much of the family but some exotic forms are atypical e.g. some Oriental Macronychini have 8-segmented antennae with the terminal segment large and oval, forming a distinct club, and the dorsal surface of the head is covered with hydrophobe hairs, extending the ventral plastron. Nonetheless these species have the distinctive habitus of the family and a familiarity with the European or even the UK fauna will allow most species of the family to be recognized.

Adults are small, between 1-9mm long, elongate, flattened and often parallel-sided beetles, generally well-sclerotized and dark coloured or drab although in a few e.g. some Stenelmis Dufour, 1835 with pale maculae, longitudinal or transverse stripes to the pronotum or elytra, and most have relatively long legs. The head is usually visible from above, with relatively large eyes, 11-segmented and usually filiform antennae (which will distinguish them from the closely related Dryopidae) and a distinct frontoclypeal suture. In Elminae the ventral surface is covered by a dense hydrofuge pubescence, this is augmented or replaced by coarse hairs in Larainae. The pronotum is very variable in shape and convexity; usually quadrate to elongate and only exceptionally transverse, often strongly sinuate across the base, smooth to strongly crenulate laterally and with protruding anterior angles or margin, sometimes smooth but often with surface structure well-developed and consisting of various longitudinal impressions or carinae and basal fovea. Prosternum long anterior to circular and open coxal cavities, the anterior margin often deflexed, process wide and rounded apically. Mesosternum short and usually recessed to receive the prosternal process and the anterior margin of the metasternum, coxal cavities generally circular but variable and usually widely separated. Pro- and mesocoxae projecting, sometimes e.g. in Stenelmis Dufour, 1835 strongly so, metacoxae strongly transverse and sometimes with a ventral lobe. Metasternum long and weakly convex, often with a median longitudinal impression, posteriorly incised to accommodate the basal ventrite, coxal cavities transverse and generally widely separated. The scutellum is usually visible and well-developed, often proportionally large, and variable from almost circular, distinctly angled laterally or simply triangular. Abdomen with ventrites 1-3 (at least) connate. Elytra elongate and continuously rounded to acuminate or produced into a spine apically leaving at most 2 abdominal tergites exposed, laterally gently curved to near parallel-sided and angled towards the apex, sometimes distinctly denticulate, with punctured striae which often consist of rows of strong punctures that are much wider than the interstices, often smoothly convex or with depressed striae and convex interstices but larger sculpture consisting of longitudinal ridges or tubercles is frequent. Legs long or very long; trocanters visible and often long, obliquely attached to and often in line with the ventral surface of the femora; femora without teeth or other sculpture, slender and parallel-sided to clavate and often covered in hydrofuge pubescence, tibiae long, slender and without obvious terminal spurs. Tarsi 5-segmented, the segments long and slender or widened apically but without lobes, the terminal segment very long, sometimes longer than the others combined e.g. in Stenelmis, claws usually large, curved and smooth, generally without or with only a weakly-developed basal tooth. Larvae are up to 15mm long when fully grown, elongate and variously shaped although in some exotic species e.g. the Australian genus Kingolus Carter & Zeck, 1929 they are broad and flattened, strongly sclerotized and often with granulate sculpture; many have the superficial habitus of wireworms. The antennae are short, at most as long as the head width, and the legs 5-segmented, short and placed ventrally. Head visible from above but usually much smaller and narrower then the prothorax, with a complete transverse suture dividing the labrum from the cranial capsule and mandibles lacking an internal mola. Thorax and abdomen about equal in width with all segments distinct, prothorax usually longer than the transverse meso- and metathoracic segments, abdominal segments quadrate to transverse, abdominal apex with a ventrally flexible and hooked operculum concealing tufts of extrusibe fine and hair-like anal gills which are retracted for protection or can be rhythmically extruded and retracted to increase water flow.

Ecology

The common name refers to the tendency of most species to live in running water habitats on rocky, gravelly or sandy substrates; they mostly occur where the water is clear and has high oxygen content and for this reason are sometimes used as indicators of water quality. A very few exotic species live in more unusual habitats; some are subterranean or live in thermal pools or hot springs and a few develop in waterlogged wood. In general species of Elminae are fully aquatic with adults and larvae living under water whereas those of Larainae are riparian, living among rocks and detritus in the splash zone beyond the water’s edge, they carry a film of water, breathe via a plastron and enter and leave the water with ease but cannot remain permanently submerged and they are good fliers. Those of Elminae are able to remain submerged using a plastron on the ventral and lateral surfaces of the body for respiration, they cannot swim and tend to move slowly, clinging to the substrate, they mate in the water and females lay single eggs or small groups of eggs in crevices or among the substrate Adults of both subfamilies feed on fine detritus, diatoms and algae etc. and some species of Elminae feed on fungi or algae among sunken or waterlogged wood. Larvae are exclusively aquatic and breathe using tracheal gills, they pass through 5 to 8 instars and the final instar develops a series of spiracles on the thorax and abdomen which allows it to emerge from the water to pupate, fully-grown larvae can survive for long periods in damp conditions out of the water as they search for a suitable pupation site among marginal substrate or debris, those of some species do not leave the water but simply wait for the water level to drop during the warmer months, and some e.g. Macronychus quadrituberculatus Muller, P.J.W., 1806 pupate among submerged wood in which they develop. The pupae breathe air and generally develop rapidly although they can survive for long periods, adults emerge and either return to the water immediately or, in many species, disperse by flight and this is usually the only period when adults live out of water, many are thought to disperse with drifting river currents or when rivers flood and carry debris after storms. They are long-lived; several species have been kept for several years, and many require a year or more to mature and reproduce; adults of most species eclose during the summer and reproduce the following spring or summer. Elmids are rarely encountered in standing water or silty rivers but in many clean-water rivers flowing over gravel or sediments they may very common and are often the most abundant of all the beetle fauna but because many species have rather narrow ecological and habitat requirements many species worldwide are threatened by pollution, habitat modification or water extraction, and this is occurring at a time when many more species are being discovered; of course this applies to aquatic life generally but the present group is acknowledged as very sensitive to changes in the environment and may suffer more than most.

UK Species

It is in some way indicative of the lack of popularity of the group that it is not covered in the latest (2006) Coleopterists Handbook. The keys given in Joy’s handbook cover all the species with the exception of Stenelmis canaliculata (Gyllenhal, 1808) but the arrangement of species within genera is very out of date and the recent (2018) checklist will be needed to make sense of it. A complete key can be found here Mike’s insect keys and some very useful information on the group is given in Foster & Friday (2011).

All our species live in flowing water, typically fast flowing streams or wave-washed shores of lakes with high oxygen content, and it seems cooler temperature are preferred, perhaps because of the higher oxygen tension. Adults may be sampled by sweeping among vegetation or by sampling moss etc. at the water edge or by disturbing sediment and netting the resulting debris as it washes downstream. Our species are small, 1.3-4.75mm and will need to be searched for very carefully. Macronychus Müller, 1806 is a Holarctic genus of 11 species, one of which occurs in North America and the rest are Palaearctic. The most widespread, M. quadrituberculatus Müller, 1806 occurs throughout Europe and much of northern Asia and is very local and generally rare across the south of England. It occurs on decaying logs in rivers and is distinctive among our fauna in having an elongate pronotum with large basal tubercles. Stenelmis Dufour, 1835 is the largest genus of the family with about 170 species and an almost worldwide distribution; the single widespread European species S. canaliculata (Gyllenhal, 1808) is a very local and rare species of deep rivers and lake margins in southern England and Wales and there are a few records from the Lake District and the northern Scottish Highlands, it is distinguished among our fauna by the median longitudinal furrow on the elytra. Elmis Latreille, 1802 is a small Palaearctic genus of 12 species of which the very widespread E. aenea Muller, 1806 is common in running water throughout the UK. It is a small species, about 2mm, characterized by convergent ridges on the pronotum. Oulimnius Gozis, 1886 includes 17 species from the western Palaearctic and Nearctic regions, the 4 UK species will usually need to be dissected to be identified. O. major Rey, 1889 and O. rivularis (Rosenhauer, 1856) are rare species of running water and fen drains in southern England. O. troglodytes (Gyllenhal, 1827) is more widespread throughout mainland UK and Orkney though generally very local and uncommon while O. tuberculatus (Müller, 1806) is common in running water and lakes throughout the UK. Esolus Mulsant & Rey, 1872 includes 11 species from Europe and North Africa of which one, the very widespread E. parallelepipedus (Müller, 1806), is generally common throughout the UK but absent from much of southeast England, it is unusual in having the base of the seventh interstice raised into a ridge. Limnius Illiger, 1802 includes 14 species from North Africa and the wider Palaearctic region; the type species, L. voclkmari (Panzer, 1793) is common in running water throughout the UK. The western Palaearctic genus Riolus Mulsant & Rey, 1872 includes 10 species of which 3, all very widespread on the continent, extend to the UK. R. nitens (Müller, 1817), often included in the genus Normandia Pic, 1900, has a few records scattered across South Wales and England but is local and very rare. Both R. cupreus (Müller, 1806) and R. subviolaceus (Müller, 1817) occur throughout mainland UK and are locally common in base-rich running water and lakes.

Esolus parallelepipedus

Limnius volckmari

Macronychus quadrituberculatus

Normandia nitens

Oulimnius rivularis

Oulimnius troglodytes

Oulimnius tuberculatus

Riolus cupreus

Riolus subviolaceus

Oulimnius major

Stenelmis canaliculata

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