Platystethus Mannerheim, 1830
This is a relatively small Holarctic genus of about 60 species; the greatest diversity is in the Palaearctic region while only three species are known from North America (one of which is adventive). At present the genus is divided into two subgenera; Craetopycrus Tottenham, 1939 and Platystethus s.str. although species tend to fall into several natural subgroups and so this may change. The European fauna is modest, with twelve species of Craetopycrus, four species of Platystethus, and the uncertain P. rattus Gistel, 1857, which is native to Germany. The UK fauna includes seven species, all of which, with the exception of P. arenarius (Fourcroy, 1785), are included in Craetopycrus, and all of which are very widespread across the Palaearctic region. The other species represented in Europe tend to have much more restricted, primarily southern distributions e.g. P. (P.) muelleri Scheerpeltz, 1955 occurs across the Balkans while P. (C.) rufospinus Hochhuth, 1851 and P. (C.) brevipennis Baudi, 1857 are more widespread in the south east, and P. (C.) volgensis Csiki, 1901 occurs in southern parts of European Russia. P. (C.) luzei Bernhauer, 1899 is endemic to Austria, P. (C.) burlei Brisout de Barneville, 1861 occurs in parts of France, Spain and Italy, and P. (P.) oxytelinus Fauvel, 1875 is known from North Africa and Spain. The more widespread P. (P.) laevis Markel & Kiesenwetter, 1848 occurs sporadically in parts of central Europe and extends south into Asia Minor.
Most species occur in marginal wetland habitats; among our UK fauna only P. arenarius occurs in dung although it is sometimes quoted as occurring in carrion and other kinds of decaying organic matter as well. Both adults and larvae burrow through damp substrates, feeding on organic matter, and some may have preferences regarding soil type and organic content etc. They breed in the spring, larvae develop and pupate in the summer and the resulting adults overwinter, sometimes away from their summer habitats. Adults are often active on the surface during warm periods and they occasionally fly in swarms over their breeding habitats. Pitfall trapping is an effective way of sampling adults although they are easily found on bare substrate when active, they usually appear in numbers and sometimes several species are present, often among other riparian staphs and carabids etc. They may sometimes be detected by the small piles of substrate they eject from their burrows, and here both adults and larvae may be collected by local flooding. Adults may be found at any time among flood-refuse or in extraction samples.
The species are very typical of the subfamily and strongly suggestive of several other genera e.g. Aploderus Stephens, 1833, Oxytelus Gravenhorst, 1802 or Anotylus Thomson, C.G., 1859, but are readily identified by the single median longitudinal furrow on the pronotum coupled with small eyes and long, rounded temples. Some species of Bledius Leach, 1819 are superficially similar but here the front tibiae have two rows of spines along the outer margin while in the present genus there is only a single row. The subgenera are distinguished by the form of the head; in Craetopycrus there is a longitudinal furrow beside each eye that extends towards the base while in Platystethus s.str. this is absent. The species may be characterized as follows. Small, 2.2-4.8 mm, and rather flattened, head proportionally large and often sexually dimorphic; in males more robust and sometimes with spines or teeth on the anterior clypeal margin, pronotum short and transverse, abdomen strongly bordered. Entirely black or with the elytra patterned (sometimes almost entirely) red or yellow, antennae dark or with the base partly pale, legs variable in colour. Forebody and elytra variously microsculptured and pubescent, abdomen usually with transverse cellular microsculpture and scant pale pubescence. Head weakly transverse with small and slightly convex eyes, temples long and curved, vertex often impressed across the base and longitudinally in the basal half, clypeus produced forward between tubercles above the antennal insertions. Antennae usually gradually widened from the fourth or fifth segment, apical segments quadrate to transverse but not forming a distinct club. Labrum truncate or slightly emarginate anteriorly, second and third maxillary palpomeres equal and longer than the terminal segment which is narrow and appears diminutive by comparison. Pronotum transverse, broadest near projecting anterior margins and continuously curved and bordered around the lateral and basal margins. Pronotal surface randomly punctured and with a longitudinal impression but without other structure. Body strongly ‘waisted’ between the pronotum and elytral shoulders, scutellum triangular or constricted towards the base, the surface with distinct cordate or symmetrical reniform impressions. Elytra usually dilated from rounded shoulders to separately curved apical margins, without striae, epipleural ridge present, sutural margin overlapping from the base and diverging apically, sutural and apical margins distinctly bordered. Abdomen moderately long and dilated, tergites usually weakly impressed across the base, otherwise smoothly convex and lacking latero-basal ridges or impressions, eighth tergite concave across the apex, revealing the ninth. In males the eighth sternite (and sometimes the seventh as well) is often modified with tubercles, teeth or emarginations across the apical margin. Legs short and slender, femora unmodified, front tibiae narrowed before the apex and with a single row of spines along the external margin, middle tibiae with external spines, hind tibiae without or with much smaller spines. Tarsi 3-segmented; two basal segments small and together shorter than the terminal segment. Our UK species are generally distinctive enough (with the exception of certain females of some species) to identify without needing to examine the genitalia. Our UK species are keyed HERE and much useful information is given by Hammond (1971).