Carabus problematicus Herbst, 1786
This is a very widespread species occurring commonly throughout Europe and extending east to northwest and northern Russia and north to Faroe and the east coast of Iceland, at least 13 subspecies have been described many of which are sympatric on the continent but several are restricted to more northern latitudes e.g. ssp. faroensis Vacher de Lapouge, 1910. Our U.K. subspecies, harcyniae Sturm, 1815, which also occurs in central and northern Europe, is a local insect of woodland, scrub grassland and moorland to the far north of Scotland including Anglesey and Man but not, apparently, Orkney or Shetland, and while there has been a decline over recent decades it remains common in many areas, especially in northern and eastern forests and moorland. Both adults and larvae are predatory and nocturnally active, feeding on other insects, worms and molluscs etc. on the ground among leaf-litter and under logs, in the south larvae develop quickly through the late spring and summer and produce adults in the autumn. Adults are flightless and generally terrestrial but nocturnally they will often be found ascending tree trunks etc., they occur year-round; overwintering under logs, in the soil under accumulated leaf-litter or under matted Calluna on open moorland, and become active from March or April. Mating occurs mostly in the spring and early summer following a period of feeding and eggs are laid in the soil from late spring. These overwintered adults survive through the summer, aestivating during the warmest periods, and into the autumn, joining the new generation adults and so at this time the species may be locally abundant. New generation adults generally overwinter and breed in the spring but a few mate and oviposit in the autumn producing larvae that develop through the winter and produce new adults in the spring. The life-cycle is thus flexible; some adults will survive over several seasons and autumn generation males have been observed mating with previous-season females. Over most of the U.K. the life cycle is annual but in northern areas and at higher altitudes it takes two years.
A large carabid 20-28mm, entirely black, generally with a metallic blue or violet lustre which tends to be most obvious on the pronotum and the elytral margins. Head elongate with convex and prominent eyes, robust mandibles and prominent palps which are truncate apically. Vertex extensively wrinkled although this is finer than that on the pronotum, clypeus mostly smooth. Pronotum weakly transverse (in some continental subspecies very strongly so), broadest in front of the middle and with well-developed lateral and basal borders, posterior angles strongly produced backwards and raised above the level of the weakly convex disc. Surface strongly and densely wrinkled throughout. Elytra smoothly rounded and narrowly explanate laterally, surface sculpture usually strongly impressed and consisting of many fine longitudinal ridges, several of which are more prominent and regularly interrupted throughout their length. Legs robust and long, the pro-tibiae smooth along the internal margin i.e. without an antenna-cleaning notch. Male pro-tarsi dilated. Very similar to the widespread and generally more common C. violaceus, in the field the elytral sculpture soon becomes obvious but in some specimens it is weak and there is a superficial overlap, with experience the slight pronotal convexity will identify the species.
Fig. a) Pronotal convexity in C. problematicus (left) and C. violaceus (right).