Platystethus cornutus (Gravenhorst, 1802)
This species is among the most widespread of the genus, occurring throughout much of the Palaearctic and Oriental regions and being recorded from tropical Africa; it occurs on the Atlantic islands, across North Africa, the Middle East and Asia Minor, throughout Asia to China and Japan and south to Southern India, Vietnam, and North and South Korea. The species is locally common throughout Europe, including the Mediterranean islands, though more sporadic and less so in the north where it extends to the UK and some southern provinces of Sweden and Finland. It is generally common across Southern England and Wales, less so further north, very local and scarce in Ireland and probably present in southern Scotland. The species is typical of wetland habitats; it occurs in reedbeds and marshes, beside stagnant ponds and slow-moving sections of rivers, marshes, tidal pools, estuaries and reservoirs, sometimes in disturbed situations and on a wide variety of substrates from clay to chalk, sand and silty gravel, often with little organic matter. Adults occur year-round; they usually migrate to overwintering sites during the autumn where they pass the winter in tussocks or among leaf-litter but are often active during mild winter spells, they are active from early spring until the autumn and peak in abundance during May and June. Breeding occurs in spring and early summer and eggs, larvae and pupae develop in moist soil, the saproxylic larvae bore galleries through the substrate as they search for organic detritus, and their presence is often betrayed by small heaps of excavated soil left on the surface. Adults also burrow in search of prey but they are often active in numbers on the surface during warm weather, they fly well and sometimes swarm before dispersing, especially in spring and early summer. Specimens often occur in pitfall and flight-interception traps and they are frequent in extraction samples and among flood refuse, they may be sieved from red-litter etc. but the easiest way to find them is simply to examine the surface of damp or wet soil during sunny periods.
Platysytethus cornutus 1
Platysytethus cornutus 2
2.2-4.5 mm. Elongate and broad with the head broadly produced in front of the antennal insertions, body entirely black but for the elytra which have a variable but usually broad dull reddish or yellow marking from below the shoulders to the inner apical angle. Outside the UK the species is much more variable with entirely dark to substantially pale populations occurring, these have sometimes given rise to sub specific names or, in the past, to distinct species. Head, pronotum and elytra finely shagreened, abdomen very finely microsculptured, without obvious pubescence. Head broadest across small convex and protruding eyes, temples long and converging, surface moderately densely punctured, strongly so in males, less so in females, transversely impressed across the base and with a longitudinal furrow beside each eye extending towards the base. In males the head is more robust and the apical clypeal margin is produced into a pair of forward-pointing spines. Mandibles long, projecting and with several prominent internal teeth before the apex. Antennae long and slender, weakly expanded from the sixth segment. Pronotum transverse, almost semi-circular from slightly protruding anterior angles and distinctly recurved apically, lateral and basal margins bordered although this may be interrupted medially, surface moderately strongly punctured (weaker in females) and with a deep median longitudinal impression. Elytra transverse, dilated from rounded shoulders to separately-rounded apical margins, sutural and apical margins bordered, surface moderately strongly but not densely punctured throughout. Abdomen usually weakly curved or near parallel sided, tergites strongly bordered, variously impressed across the base and finely punctured throughout. Seventh sternite in males with two widely spaced apical teeth, between which the margin is shallowly emarginate, in females this margin is straight. Legs dark with paler tarsi. Femora unarmed, tibiae with strong external spines, front tibiae externally emarginate before the apex. Tarsi 3-segmented, basal segments short, terminal segment long and curved.
Very similar to P. degener Mulsant & Rey, 1878, with which it often occurs. The pale elytral colour is usually distinctive as in degener it appears to form a patch on the disc that does not extend obliquely towards the shoulder, but this is variable and sometimes obvious only when series are compared. Males can always be separated by the form of the apical margin of the seventh sternite; here it is widely and shallowly emarginate between distinct obtuse teeth while in degener it is straight between two tiny tubercles. Females are usually distinctively patterned but can be ambiguous, especially from a mixed population.