Elaphrus riparius (Linnaeus, 1758)
This widespread and generally common Palaearctic species occurs throughout Europe extending from the northern Mediterranean to Scandinavia and the UK, and through the Near-East to Siberia. Here it is locally common throughout England and Wales, although less so in the West Country, becoming more scattered and sparsely distributed through Scotland except for the far north. The typical habitats are marginal wetland situations beside both moving and still water; lake and river banks, reed bed margins and fens etc. although it is usually absent from peaty or upland areas, on a wide range of fine to coarse substrates. Compared with the equally common E. cupreus the present species prefers more open and sparsely vegetated habitats more exposed to the sun but this is general and the two often occur together. Adults occur year-round, becoming active in early spring when they may be seen running on bare soil etc. and they soon become common, remaining so through the spring and summer. They generally occur in colonies extending as far as the soil remains permanently damp, adults are diurnal predators and each will patrol a specific area and display territorial behaviour towards conspecifics as well as other small carabids, and each will retire to a ‘den’ when threatened, usually running but sometimes taking flight, and under such circumstances can escape by running rapidly across the water-surface. Breeding occurs in the spring when mating pairs may be seen in the open, oviposition occurs soon afterwards where eggs are laid directly into wet soil or among damp moss or litter depending upon conditions. Larvae emerge after a few days and hunt other insects and worms etc. on the soil surface by night, they develop rapidly and pupation occurs in the soil or under matted vegetation near the water. The entire cycle from egg to adult takes about 30 days and new generation adults appear from July, they do not aestivate but remain active, feeding through the summer and becoming mature within a month or two and during September they burrow five to ten cm below the soil surface to overwinter.
Adults may be recognized by their small size <8mm and green colouration although there is often a weakly contrasting lustre. Head, measured across the eyes, a little wider than the pronotum, and produced forward with prominent labrum and mandibles. Antennae metallic green, usually with a metallic red lustre. Pronotum quadrate, broadest at the middle and strongly sculptured. Each elytron with a shiny elevated field anterior to the middle and adjacent to the suture and smaller ones arranged in longitudinal rows although these may be obscured by punctures and sculpture, between these are rounded and punctured depressions which appear violet or edged with silver in strong light. Femora and tarsi metallic green, tibiae mostly pale but variously dark metallic towards the apex. Prosternum pubescent. Male with three dilated pro-tarsal segments.