Calathus melanocephalus (Linnaeus, 1758)
This widespread and generally common species occurs from lowland to mountain altitudes throughout Europe except for the extreme southwest, extending to the far north of Fennoscandia and east through the Caucasus and Russia to Siberia. Here it is common and often abundant across southern and central England and Wales, becoming more local and sporadic in the West Country and further north; it is present on Anglesey and the Western Scottish islands but apparently absent from Man and the Isle of Wight. Adults occur year-round and are active from April until the first frosts of autumn, the typical habitat is sparsely-vegetated open and rather dry soils in parkland, wasteland, coastal dunes and shingle, but in dry periods they will move to dense vegetation on grassland or agricultural borders and during the warmest part of the summer they enter the soil to aestivate. They are nocturnal and should soon be seen on pathways or around the base of trees in open situations, they generally occur in numbers and often in company with other carabids, they are predatory but will also feed on carrion, especially insects and worms that have been flattened on pathways, they disperse by flight in the spring and autumn, moving between summer and overwintering quarters, and so may turn up in any suitable situation. Breeding occurs in late summer and autumn; new-generation adults appear in July and begin maturation feeding, they may enter diapause or remain active depending on the season, and mating occurs during August and September. Eggs are laid in the soil or among leaf-litter etc. and the predatory larvae develop through the autumn, winter and following spring, pupation occurs from June to produce July adults. Some of the autumn-breeding adults will go on to overwinter but mortality can be high, up to 70%, and survival depends on building up fat reserves before the cold weather begins. Overwintered adults may go on to for a second time; on the continent they breed much earlier than those from overwintered larvae, from July or August but the fecundity is much lower. Adults may be sampled through the summer by nocturnal searching by torchlight; they are active on warm and humid evenings and become more common in late summer when aggregations may be found on pathways, among accumulated leaf-litter or around logs and trunks in open situations.
The bicoloured adults are easily recognized and the only confusion might be with the superficially similar Calathus cinctus Motschulsky, 1850, but here the head and elytra are brown rather than black and there are differences in the aedeagi; in cinctus it does not have the subapical constriction or the distinct apical tooth seen in melanocephalus. C. cinctus has only recently been recognized as occurring in the UK (Anderson & Luff, 1994), it is mostly coastal, occurring on shingle and dunes where it overlaps with the present species, but it also occurs inland, e.g. we have found both in a local Watford park.
6.0-8.5mm. Head shiny black, sometimes with a metallic lustre, with weakly convex eyes, shallow, often obliterated, frontal furrows and paired supra-orbital setiferous punctures. Antennae and palps entirely pale, mandibles variable, pale to black. Pronotum widely transverse, broadest about the middle and narrowed to protruding anterior angles, basal lateral margin almost parallel to blunt, almost perpendicular posterior angles. Surface smooth and shiny, with weekly-impressed basal fovea. Elytra elongate and with a tiny humeral tooth, broadest near the middle and evenly curved, striae punctured and finely impressed to the apex, interstices flat; the third with several large punctures which generally join the third stria but these may be missing on one or both sides, eighth striae with large punctures throughout. Epipleura not crossed before the apex. Legs entirely pale. Claws serrate. Male basal pro-tarsal segments dilated.