ELAPHRINAE Latreille, 1802
These metallic ground beetles can often be seen in waterside habitats. Elaphrus cupreus and E. riparius are common and widespread, while the other species are less frequently encountered.
Previously regarded as a tribe within the Carabinae Latreille, 1802 this small group includes about 50 species in 3 genera and is Holarctic in distribution. Diacheila Motschulsky, 1844 includes 3 cold-adapted species which occur mostly at high latitudes; two are Nearctic and one occurs in Siberia. Blethisa Bonelli, 1810 includes 10 species; 6 are Nearctic, 4 occur in Siberia and 2 are European of which one, B. multipunctata (Linnaeus, 1758) extends to the far north including the UK. Elaphrus Fabricius, 1775 is the largest genus of the group with about 40 species in 4 subgenera; 19 occur in the Nearctic region and 11 are European of which 4 extend to the UK.
Species of Blethisa are recognized by the deeply grooved frons and foveate elytral interstices. The lateral margins of the pronotum are more widely explanate than those of Elaphrus or Diacheila and the upper surface is variably metallic. The elytra are characteristic with irregular and finely punctured striae which tend to curve around the large fovea on the third and fifth interstices, the base has a strongly raised margin and an impression behind the shoulder and the lateral margins are explanate. Males of all species have some pro-tarsal segments dilated. All are associated with wetland marginal situations and generally occur to high latitudes and altitudes.
Species of Elaphrus are instantly recognized by the elytral sculpture; the striae are represented by alternating series of groups of setiferous punctures, which are often strikingly coloured, and raised and shiny ‘mirrors’. In most the body is metallic throughout. The huge convex eyes and protruding mandibles are also characteristic, the head and pronotum are more or less equal in width and combined are proportionally long, both punctured and coarsely sculptured and the lateral margin of the pronotum is narrow, sometimes almost obsolete. The wings are usually well-developed, only rarely reduced e.g. in the Holarctic, mostly sub-arctic, E. angusticollis Sahlberg, 1844, and most are strong flyers. Males of all species have some pro-tarsal segments dilated. All species are associated with wetland environments, they are diurnal predators with nocturnal predatory larvae, adults generally occur year-round and are active in the spring and summer.
The five UK species are readily separated:
Elytra with irregular and finely punctured striae. Frons with deeply impressed grooves delimiting four raised and flat areas.
Elytra without punctured striae; irregularly and characteristically sculptured. Frons punctured, without grooves although sometimes finely striate.
Tibiae dark metallic, head not wider than the pronotum.
Tibiae mostly pale, head slightly wider than the pronotum.
Depressions between elytral mirrors shallow and sparsely punctured, the punctures widely spaced. Elytra dull from dense and strong microsculpture. Colour variable, black, coppery, purple, blue or green. Prosternum pubescent. 8.5-10.0mm.
Depressions between elytral mirrors deep and distinct, the punctures close and clustered in the centre. Elytra shiny with weak microsculpture between the punctures. Colour usually coppery green with violet pits between the mirrors. Prosternum glabrous. 8.5-10.0mm.
Larger, 8.0-9.5mm. Tarsi blue. Elytra shiny, with weak microsculpture, dark bronze or greenish, with violet elytral fovea. Prosternum glabrous.
Smaller, 6.5-7.5mm. Tarsi green. Elytra dull from strong microsculpture, green, sometimes with a yellowish, blue or bronze lustre. Prosternum pubescent.