Platystethus degener Mulsant & Rey, 1878
This is a mostly southern and western Palaearctic species; it is locally common across southern Europe, Asia Minor and east at least as far as northern India, it is also present on the Atlantic islands although not in North Africa. Further north it is very local and generally scarce, extending as far as Germany and the UK, which is probably the northern limit of the distribution, but it is absent from the Baltic region. First recorded from North America in 1971, the species is now widespread across the eastern United States and has become established in eastern Canada. In the UK it is locally common across England and Wales as far north as Nottingham and it is known from a few sites in northern England and south east Scotland. Typical habitats are similar to those of P. cornutus, with which it often occurs i.e. margins of stagnant ponds, slow-moving stretches of rivers, bogs and reed beds etc., but often in more organic-rich environments. The biology is probably much the same, with reproduction in the spring and larvae developing among damp substrate in spring and summer. Adults occasionally occur in dung at cattle ponds and on woodland pathways etc., they have been sampled from drying sludge in sewage works and once, in Germany, from a Wood ant nest but these records are atypical and there seems to be no particular association with such habitats. They are present year-round, the overwintering sites are unknown but adults may be found in marginal situations through the colder months, they are sometimes seen on damp soil during milder winter spells but are otherwise active from early spring until the autumn, peaking in abundance during June and July. Searching among reed- or leaf-litter or pitfall trapping at any time may produce adults in numbers, they also occur in extraction samples, flood refuse and tussocks during the winter and occasionally swarm in flight in the spring. Specimens from mixed populations of cornutus and degener will need to be examined carefully as the difference in colour can be very subtle when viewed in the field, and populations should be viewed carefully as one or other species is likely to predominate.
Platysytethus degener 1
Platysytethus degener 2
2.2-4.5 mm. Elongate, broad and strongly narrowed between the pronotum and elytra. Forebody and abdomen shiny black, elytra black with a variable red mark; this is usually on the disc and not extending to the shoulder, as in cornutus, but is variable and may cover most of the elytra but for a dark base and lateral margins. Entire body with very fine cellular microsculpture, on the abdomen mostly transverse, and with scattered pale pubescence. Antennae black, legs red with darker femora. Head transverse, broadest across weakly convex eyes and with long converging temples, surface with moderately strong but not dense punctures and an elongate furrow from the inner margin of the eyes towards the base, vertex often impressed across the base and with a fine longitudinal median furrow. Anterior clypeal margin in males with two forwardly-projecting horns, these vary in size but should always be obvious. Antennae inserted laterally under raised tubercles in front of the eyes, long and narrow; only weakly broadened from the fifth segment. Pronotum transverse, broadest about the middle and continuously rounded from slightly projecting anterior angles, apical margin widely produced forward medially. Pronotal surface moderately strongly but not densely punctured, about the same as the vertex of the head, and with a median longitudinal furrow, lateral and basal margins bordered. Elytra transverse and usually only weakly dilated from rounded shoulders to separately-curved apical margins, sutural and apical margins distinctly bordered, surface without striae, sparsely and rather weakly punctured and usually with distinct microsculpture. Abdomen dilated from the base, tergites strongly bordered, variably impressed across the base and with scattered fine punctures and fine pale pubescence. Males may be distinguished from those of P. cornutus by the form of the apical margin of the seventh abdominal sternite; here it is almost straight between two tiny tubercles whereas in cornutus this margin is distinctly emarginate between two projecting ‘teeth’. Females of both species are very similar but in most cases the elytral pattern is distinctive.