Family ELMIDAE

Museums

Museum list

Regional Maps

Family ELMIDAE

Compiler and date details

31 December 1999 - Andrew A. Calder, CSIRO Entomology, Canberra, Australian Capital Territory, Australia

Introduction

Elmidae, commonly called riffle beetles, occur worldwide and are well represented in most Zoogeographical Regions except for the high Arctic and Antarctic. Worldwide the family is represented by about 145 genera and 1,308 species arranged in two subfamilies: Larainae and Elminae, and is the largest group of byrrhoid beetles (Brown 1981b; recent literature). The Australian fauna is comprised of nine genera and 104 described species, the largest genus being Austrolimnius Carter & Zeck with 53 species, a staggering 51% of the described Australian fauna. Most of the Australian genera are endemic except for Austrolimnius (Elminae) which is shared with New Guinea, the Moluccas (Boukal 1997) and South America (Hinton 1971a) and Hydora Broun with New Zealand and southern South America (Spangler & Brown 1981). The subfamily Larainae contains three genera and four described Australian species. This subfamily has been reviewed by Calder and will appear in print shortly. The subfamily Elminae contains a majority of the Australian species and is represented by six genera and 100 described species.

Several family-group names have been applied to the riffle beetles over the last two hundred years. Stephens (1828) was the first to place the elmids in a family of their own under the name Limniidae. However, Madge & Pope (1980) noted that this earlier name is unavailable since the name was based on Limnius Illiger which Stephens regarded as a junior synonym of Elmis Latreille. Curtis (1830) was the first to propose the family-group name Elmidae based on Elmis, although Madge & Pope (1980) argued that Elmis must be regarded as a modern Latin word and thus the correct family name would be Elmididae. However, Elmidae was the preferred spelling for most of the 19th century. In recent years the names Helmidae, Elmididae, Helminthidae, or Elminthidae have been incorrectly employed although the correct derivation was given by Steyskal (1975) and the correct form Elmidae, had been employed by Hinton as early as 1939. The ICZN (1995) has ruled that the correct original spelling of the family-group name is Elmidae based on the type genus Elmis Latreille to avoid any further confusion.

The family-group name for the subfamily of riffle beetles that was originally known as Larinae, has been emended to Larainae, based on Lara LeConte (stem Lara-), to avoid confusion with the gull family-group name Laridae based on Larus Linnaeus (stem Lar-) (ICZN 1988).

In earlier classifications elmids were often lumped with Dryopidae. Westwood (1838) first included the Elmides as a subfamily of Parnidae and this usage continued for most of the 19th century and was the situation current when King (1865) described the first Australian elmids. The Parnidae later became known as Dryopidae when Grouvelle (1896) used it as the correct family-group name without comment. Carter published all of his elmid work under the family-group name Dryopidae. As late as 1935 Hinton still considered the Dryopidae to comprise the three subfamilies: Larinae (=Larainae), Elminae and Dryopinae. However, Hinton (1939) later showed convincingly on the basis of characters derived from the larvae, pupae, internal anatomy and adults that the dryopids and elmids formed two separate families and further, Larini was more closely related to Elminae than to Dryopinae and that only tribal rank for the group was justified. Recent authors regard the Larainae as a subfamily although a thorough phylogenetic analysis of the constituent genera may reveal that the group is paraphyletic.

King (1865) described the first Australian elmids from specimens collected mainly from submerged branches and under stones in the Parramatta River while another two species were described from the Murray River and a stream on Mount Kembla in the Illawarra district, New South Wales. Blackburn (1894) described a single Notriolus species from Tasmania, Lea (1895) a single species of Coxelmis from Tamworth and Grouvelle (1896) described another Notriolus species also from Tasmania. Carter was the first author to take a major interest in the Australian elmid fauna and over the next 10 years either solely or coauthoring with Zeck described a total of 57 new species. A major milestone was Carter & Zeck's (1929) monograph of the Australian Elmidae which was then known as Dryopidae. Zeck (1948) added another two species including the first species (Austrolimnius isdellensis) to be described from Western Australia. In 1965 Hinton revised the Australian Austrolimnius species concentrating mainly on the eastern Australian forms, describing another 42 species and totally neglecting Zeck's Western Australian species. Currently, work is in progress by Calder revising the Australian Elmidae.

Examination of the syntypes of Helmis pallidipes Carter showed that they had important diagnostic characters of Graphelmis Delève. These included a silky fringe of setae on the interior face of the fore tibiae, elytral striae 3 and 4 were short and fused together posteriorly and the aedeagus resembled that of the type species of Graphelmis rather than that of Stenelmis Dufour. Helmis pallidipes Carter is hereby transferred to Graphelmis.

Adult Elminae are totally aquatic and obtain their oxygen by diffusion through a plastron that is confined to the ventral surface and legs and can appear as a silvery layer in live beetles. The plastron is held in place by either highly modified, scale-like hydrofuge microtrichia or a very fine dense hair-like pile (Hinton 1971b). Larainae have no such area of modified setae but are densely covered with a hydrofuge pubescence that is used to trap a film of air for breathing when under water. Larvae are totally aquatic, breathing by means of tracheal gills situated in the ninth abdominal segment which can be retracted through an operculum. Functional spiracles only develop in the last instar when they leave the water to pupate in damp areas near the water's edge. Northern Hemisphere elmids have short flight periods before re-entering the water as adults, totally unlike the Australian elmids except for Ovolara Brown which are possibly riparian. Elmids feed on waterlogged wood, decaying vegetation, algae, moss or fine detritus. Adults and larvae occur together in various running water habitats such as riffles, on substrate pebbles and stones in slower moving sections of rivers, in sandy substrates of shallow streams or moss covered rocks in the splash zone of waterfalls in upland streams.

Elmids are gaining increasing recognition as indicators of water quality in streams. Young (1961) stated that elmids were extremely sensitive to even mild pollution and quickly disappear from streams which receive even moderate amounts of industrial wastes. In general, the presence of elmids has been considered diagnostic of clean, well-aerated water conditions (Gaufin & Tarzwell, 1956; Williams 1980; Brown 1981b). Biological information on elmids can be obtained from the following publications: Davis (1942) discussed the respiration of Coxelmis novemnotata (King), Glaister (1985) discussed the laboratory rearing of Australian elmid larvae, Chessman (1986) looked at the diet of elmids inhabiting two Victorian streams, Brown (1987) has reviewed the biology of riffle beetles generally and McKie & Cranston (1998) discussed the ecological role of xylophagous elmids, in particular Notriolus Carter & Zeck.

Hinton (1968) provided a key to the eleven subgenera of Austrolimnius, while Brown (1981a) gives a key to the genera of Larainae worldwide. Glaister (1999) has provided a profusely illustrated guide and key to both the genera, as well as many species of Australian elmid larvae.

Diagnosis

Adult elmids are small to minute, elongate to ovate, dark coloured beetles ranging from less than 1 mm to 6 mm in length. The head is deflexed and usually retracted into the pronotum often concealing the mouthparts and the frontoclypeal suture is present. The antennae are 11-segmented and filiform (Elminae) or with the apical antennal segments widened and forming a loose club (Larainae). The maxillary and labial palps can be greatly enlarged, especially so in Larainae. The pronotum usually has complete lateral edges, its disc with or without sublateral carinae, with or without impressions and fovea. The procoxae are transverse (Larainae) or globular (Elminae) with the trochantins exposed or concealed. The procoxal cavity is open behind both internally and externally. The prosternum is well developed in front of the procoxae and the intercoxal process moderately wide. The mesocoxae are widely separated and mesocoxal cavities are partly closed by the mesepisternum. The metacoxae are usually close together or sometimes widely separated. The hindwing has an open or closed radial cell and the anal (wedge) cell is either present or absent. The tarsi are 5-segmented, simple and the terminal claw segment is greatly enlarged and often as long as all the others combined. There are five visible abdominal segments the first three being connate (Lawrence 1982; Lawrence & Britton 1994).

Larval elmids are elongate, heavily sclerotised, flattened to cylindrical, with a movable operculum enclosing retractile gills and paired anal hooks on segment 9. The head is narrower than the thorax and the abdomen tapers posteriorly. The head is prognathous and protracted with five closely clustered stemmata each side. The mandibles have a prostheca and the maxillary articulating area is reduced. The mouthparts and antennae are shorter than the head. Distinct pleurites are visible on at least the first five abdominal segments. Sternite 9 has an operculum and a pair of hooks. Three tufts of anal gills are present. The spiracles are biforous and are functional only in the last instar (Brown 1991; Lawrence & Britton 1994; Glaister 1999). Elmid larvae cannot swim and are found crawling about on the substrate of streams or clinging to submerged tree branches, under submerged rocks or clinging to moss covered rocks in the splash zone of waterfalls.