Pterostichus madidus (Fabricius, 1775)
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ADEPHAGA Clairville, 1806
PTEROSTICHINI Bonelli, 1810
Steropus Stephens, 1828
A generally common species with a distribution restricted to temperate regions of Europe; recorded from the following countries: Spain, France, Switzerland, Holland, Poland, Czech Republic, Austria, Germany, Denmark, Belgium, Estonia, The Netherlands and across Scandinavia. In the U.K. it is the most frequently recorded carabid, occurring throughout Great Britain and Northern Ireland with the exception of Orkney and Shetland. It occurs in a very wide range of both open and shaded habitats e.g. under logs and bark or among leaf litter etc. in all types of woodland, under debris or among vegetation in meadows, arable land, parks, gardens and it is often common among grass etc. on dry moorland. In North Somerset we found then to be common on peat cuttings along with Pterostichus anthracinus (Panzer, 1795). In Europe they are more strictly woodland insects in the southern parts of their range and become more general in northern parts. They often occur alongside other common carabids e.g. P. melanarius (Illiger, 1798), Calathus fuscipes (Goeze, 1777) and Harpalus affinis (Schrank, 1781). This species is active both by day and by night; during the day they are active and easily found in woodland and other shaded habitats that are not too wet, but it is nocturnal searching that can be very rewarding when looking for madidus; during the warmer months they may be seen running on pathways in gardens and parks etc. and will often be seen feeding on the remains of insects, slugs or worms. Along with C. fuscipes this is probably the most common large carabids in our South Herts. area. The adults are flightless but nonetheless quick to colonize new areas. They are scavengers as well as predators and at some times of the year vegetable matter may constitute an important part of their diet but more generally the adults feed on small insects, larvae, worms and small slugs; larger slugs are avoided as madidus cannot deal with their defence mucus. Damaged or crushed insects, worms and slugs are readily consumed. The larvae, which have three instars, feed on small, soft-bodied organisms. The species breeds in the autumn and to a lesser extent in the spring and adults overwinter before they begin mating. Larvae emerge during the summer and overwinter; the life-cycle is usually annual but at higher altitudes it may be biennial.
P. madidus (Fabricius, 1775) has two subspecies: P. m. validus Dejean, 1828, which has black femora, and P. m. concinnus (Sturm, 1818), which has red femora. The ratio of leg colour is thought to depend on ecological factors. Tibial colour is related to body thermal regulation and it seems, from this study of the species in north-eastern France, that concinnus is active at lower temperatures and prefers wetter climatic conditions. Subspecies validus (which is sometimes considered as the valid species) may prefer open situations and is more diurnal while concinnus may prefer woodland and is more nocturnal.
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Among the U.K. Pterostichini, P. madidus should be easy to identify. The large size, 13-18mm, dark colouration and smoothly rounded pronotal hind angles characterize three of our species. P. atterimus Hb. Is immediately obvious by the series of 3 or 4 strongly foveate punctures on the third elytral interstice. The remaining two represent the U.K. species of the subgenus Steropus Stephens, 1828. P. aethiops (Panzer, 1796) is generally the smaller species and has three punctures on the third interstice. P. madidus generally has a single puncture but there may, rarely, be an extra one or, exceptionally, two on one or both sides. The pronotal fovea in madidus are bordered externally by a sinuate carina, which is absent or only vaguely present in aethiops, and the basal segment of the hind tarsus lacks the deep external furrow seen in aethiops.
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Both sexes are shiny black with pale palps and apical antennal segments. The large head and separately rounded pronotum and elytra are distinctive, even in the field. The legs are either black or red and both morphs of the species are usually found in the same area. The basal pronotal foveas are deep, wide and keeled externally. The elytra are widest at the middle and bear well impressed and punctured striae. The interstices are weakly convex and the puncture in the third is small and not foveate. The last segment of the tarsi bear setae underneath, a character also present in aethiops but not in aterrimus. In the male the first three pro-tarsal segments are dilated and the apical sternite bears a strong tooth.