Chlaenius nigricornis (Fabricius, 1787)
Chlaeniellus is sometimes given full generic status and so care should be taken when searching European literature. This widespread Palaearctic species occurs continually from France to eastern Siberia and is generally common in southern lowland areas throughout this range, it occurs throughout Europe except for the far north it is absent from North Africa but is known from many of the Mediterranean islands and is recorded from the Faroe Islands, it remains generally common across southern and central Europe but has declined over recent decades in the north. In the UK it remains locally common across England and Wales, including the islands, as far north as the Humber, there are scattered records further north to southwest Scotland, it was formerly widespread in Ireland but following a recent decline is now known mostly from a few sites in the north and west. Adults are diurnal predators of other insects and their early stages, they usually occur in small numbers and most often in wetland habitats such as lake and river margins, fens and marshes, sedge and reed beds and coastal and moorland pools, they occur year-round and peak in abundance from May until July. Breeding occurs in early spring and the predatory larvae develop during May and June in much the same habitats as the adults. Adults can be found among dense marginal vegetation or may be seen running in bright sun during spring and early summer but they tend to remain hidden in cracked substrate or under debris or matted algae on ground that completely dries out during the warmest parts of the summer, they become active again towards the end of summer and migrate to overwintering sites away from wetland situations during early autumn, these are typically damp and well-vegetated grassland above floodplain margins or under debris on wooded borders etc. Populations return to wetland habitats in March or April and at this time individuals may be very active as they search for prey in among vegetation or in the open, they often occur among populations of other wetland carabids but they tend to avoid water margins. Adults can be very difficult to find; they may appear in pitfall traps after extensive searching has failed to reveal them, and individuals or small groups of our local South Hertfordshire population appear under desiccated mats of algae after a large and deep spring has dried up in early summer, often with e.g. Carabus granulatus L. or Agonum marginatum L., but they are otherwise very elusive.
Chlaenius nigricornis 1
Chlaenius nigricornis 2
Chlaenius nigricornis 3
10-12 mm. Very distinctive due to the extensively punctured and pubescent dorsal surface and bright metallic colour which is variable but typically with the head green or coppery, the pronotum green with reddish or coppery reflections and the elytra brilliant green although these can be dark or almost black. Head with large convex eyes and short, almost parallel temples, surface extensively flattened, and with fine transverse wrinkles and punctures, inner margin of each eye with a single setiferous puncture, antennae densely pubescent from the fourth segment, black with the basal segment pale, at least underneath. Mandibles symmetrical, sharp and without a pore on the outer margin. Pronotum transverse, evenly curved from protruding anterior angles to obtuse and rounded posterior angles and more or less straight across the base, surface longitudinally impressed medially and with narrow longitudinal basal fovea, closely and rather strongly punctured throughout and often confluently towards the lateral margins and finely bordered laterally. Elytra almost parallel-sided from rounded shoulders to a weak subapical constriction, striae narrow and finely punctured, interstices densely punctured and pubescent. Legs vary in colour, in most UK specimens they are extensively dark but occasionally the femora and tibiae are pale reddish.