Pterostichus Bonelli, 1810
This very large group is Holarctic in distribution and extends sporadically into the Oriental and Neotropical regions; the Palaearctic fauna includes about 2000 species and by far the greatest diversity is in Asia, about 200 occur in Europe and slightly fewer in North America. The morphological diversity is very diverse and the group includes e.g. eyeless cavernicoles and cold-adapted wingless forms in the far north, many subgenera are endemic to certain regions and this applies especially to areas such as China and the Himalayas where new species are regularly described and new subgenera are still being erected, this taxonomic diversity is also apparent even within the limited European fauna which is classified into 35 sub-genera (the UK fauna includes 18 species in 11 sub-genera) or the Nearctic fauna which includes 23 sub-genera. On the other hand the group is also being divided into distinct genera as is apparent from our own fauna where Poecilus Bonelli, 1810 and Pedius Motschulsky, 1850 (both included in the distinct sub-tribe Poecilina Bonelli, 1810) were formerly included. The group is therefore in a state of flux and very likely to change considerably in the future, especially as the Asian and, especially, the Chinese faunas are being extensively researched, furthermore there have been some modern molecular genetic analyses and these are producing evidence of a wider generic diversity. But so far as the UK fauna is concerned, with the possible annoying exception of Pedius longicollis (Duftschmid, 1812), such considerations are irrelevant because the group is sufficiently distinct to be treated as a genus with fairly well defined limits.
Although there are exceptions in every case, the genus is generally defined by the following combination of characters. Small to medium sized carabids (in the UK 5.5-21.0 mm.), elongate, rather flattened and with robust and moderately long legs, most are black but many brown and metallic species exist, even among the limited European fauna. Head with large convex eyes and paired supra-orbital setiferous punctures, frons with variously developed frontal furrows but vertex smooth and evenly convex, mentum tooth emarginate apically; the internal lobe of the maxilla extended into a heavily-sclerotized internal hook-like tooth, mandibles long, sharp and without an external setiferous puncture. Terminal segment of all palps well-developed and cylindrical or fusiform, penultimate labial palpomere with at most two internal setae, antennae densely pubescent from the fourth segment. Pronotum very variable but usually only slightly narrower than the elytra; rounded to cordate but usually with a single or doubled basal fovea and two lateral setae, scutellum in line with the elytral base. Elytra with distinct striae and usually a scutellary striole with a foveate puncture at the base, third interstice with at least a single setiferous puncture, epipleura ‘crossed’ before a continuously rounded or sub-apically constricted apical margin. Front tibia widely expanded towards the apex and with a distinct internal antenna-cleaning notch. Tarsal segments simple although the front basal and sometimes middle basal segments are dilated in the male, and in both sexes the first segment of the hind tarsi is usually furrowed externally. Claws smooth. Wing development varies between and sometimes within species. Exceptions among our UK fauna include P. cristatus (Dufour, 1820), in which the elytral epipleura are not ‘crossed’, and P. vernalis (Panzer, 1796) which lacks a distinct scutellary striole. With this general description in mind, some members of the Platyninae Bonelli, 1810 may be taken for the present genus but here the front tibiae are either sub-parallel or only weakly expanded towards the apex.
Most species occur in open and moist environments such as grassland and scrub, and only a few UK species occur exclusively in woodland or wetland habitats. All are primarily nocturnal and all are predatory as both adults and larvae although many also consume at least some plant matter and most will feed on dead insects etc. Among our UK fauna this is a very good group for the beginner as it includes many common or abundant species which can be collected at any time of the year and at least some that can be found by simply searching parkland pathways or gardens at night, they are relatively straightforward to identify using several very good and up to date publications and they will soon become familiar in the field. Adults may be found by searching among tussocks and litter, under logs and stones etc. and among dense vegetation on most soil types although they are generally infrequent in sandy situations, agricultural headlands and hedgerows can be very productive, especially in early spring before they move into more open habitats, and damp peatland may produce more unusual species. Pitfall trapping will usually produce numbers of several species but this should be avoided as much as possible as it can be very destructive.
The widespread and very common P. madidus (Fabricius, 1775) is probably our most eurytopic member of the genus and it is very likely to be among the first ground beetles identified by the beginner, it is also very distinctive and so will soon be recognized, ad nauseam, in the field. P. niger (Schaller, 1783) and P. melanarius (Illiger, 1798) are the largest of our common and widespread species, both occur on damp grassland, moorland and woodland and both are very distinctive although both are regularly confused with Abax parallelepipedus (Piller & Mitterpacher, 1783). A little smaller but no less robust, the sibling species P. nigrita (Paykull, 1790) and P. rhaeticus Heer, 1837 are widespread and common but will need to be dissected for certain identification. Several smaller species are also widespread and abundant; P. vernalis (Panzer, 1796) and P. minor (Gyllenhal, 1827) usually occur in damp and shaded situations while P. strenuus (Panzer, 1796) and P. diligens (Sturm, 1824) are more eurytopic. Among the more specialized species are P. oblongopunctatus (Fabricius, 1787), which is a very local species of open and dry woodland, and the very local and generally scarce P. anthracinus (Illiger, 1798) and P. gracilis (Dejean, 1828), both of which are wetland species. Some locally common species have restricted distributions; P. macer (Marsham, 1802) occurs through most of England except for the West Country and the far north but is absent from Scotland and known from only a few coastal records in Wales, while P. aethiops (Panzer, 1796) is almost exactly opposite, being widespread in the West Country, the north and across Scotland but otherwise absent from England. P. adstrictus Eschscholtz, 1823 has a similar distribution to P. aethiops but does not extend as far south, being absent from the West Country. P. aterrimus (Herbst, 1784) is a very local and scarce species with a disjunct distribution; it is widespread in Ireland but otherwise known from only a few sites in southern and eastern England. P. cristatus is common across the very north of England but otherwise very rare with records scattered across England and southern Scotland. P. quadrifoveolatus Letzner, 1852, a recent addition to our fauna, having arrived in the early 20th century, is now widespread though very local in pine woodland and on heathland across England, and southern Scotland.