Steninae | uk beetles

STENINAE Macleay, 1825

Instantly recognizable, this group includes several common species. They occur in almost all terrestrial habitats, particularly marginal wetland environments.

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POLYPHAGA Emery, 1886

STAPHYLINOIDEA Latreille, 1802

STAPHYLINIDAE Latreille, 1802

Approx. 75

1.7-8mm

Introduction

This very speciose subfamily is undoubtedly the most recognizable group within the Staphylinidae; all members are of a similar and very characteristic appearance which is distinct from other staphs. Despite the large number of species only 2 genera are described (although a third from Australia remains undesribed); the Holarctic Dianous Leach, 1819 includes about 200 species with the greatest diversity in Asia, about 100 have been recorded from China while only 2 occur in the United States and only a single species is recorded from the Western Palaearctic including the U.K., Stenus Latreille, 1796 is a cosmopolitan group of about 3000 described species and subspecies including some fossil taxa, about 400 are recorded from China and about 850 occur in the Palaearctic of which more than 260 have been recorded from Europe, of these about 75 occur in the U.K., and about 170 have been recorded from the United States. Northern Hemisphere Stenus have traditionally been divided into 6 subgenera based on differences in abdominal and leg structure, but when the world fauna is considered these form a continuum and the system has not been maintained; numerous recently described species have not been assigned, and a new species-group concept may be more appropriate. The subgeneric system remains useful for more restricted faunas e.g. in Europe, and is likely to be retained for ease of identifying species. The uniformity of the group is reflected in the tiny amount of synonymy associated with various regional faunas, on the other hand many species are closely similar in appearance and need to be dissected as male genitalia generally afford straightforward identification characters. The colouration varies but most are black or dark brown and shiny, many are metallic and some very strongly so; a common pattern is a pale spot on each elytron although this is present in only a few U.K. species, Dianous are generally metallic and on the whole more colourful than Stenus with many blue or blue-green species. The appendages vary from entirely black to entirely pale. The dorsal surface is often uneven, usually quite strongly and densely punctured, and variously pubescent. All species have a quadrate or transverse head with very large and convex eyes which occupy most of the lateral margin, the temples are short and strongly constricted although generally longer in Dianous, and the vertex is often depressed, sometimes strongly so and sometimes furrowed beside

the eyes and raised medially. The antennae are long and slender with a variously developed 2- or 3-segmented club, and inserted on the upper surface of the head near the front margin of the eyes, and the palps are usually long and slender with a thickened terminal segment. The pronotum is cylindrical, generally widest near the middle and usually narrower than both the elytra and the head when measured across the eyes; the lateral margins lack a raised border and the angles are usually obtuse or rounded. A fossil species, Eocenostenus fossilis Clarke, Huang & Nel, 2014 is atypical; it has a strongly transverse pronotum with lateral projections and anteriorly placed antennal insertions. The elytra are generally cylindrical and vary from quadrate to elongate, the shoulders are usually well-developed and the lateral margin is not bordered. In general they are rather weakly curved laterally but vary from parallel-sided to quite strongly rounded as in e.g. S. fornicatus Stephens, 1833. The abdomen varies from long and parallel-sided to short and conical, the side margins vary from weak and only present on the basal segments to strongly developed throughout, and the terminal segments tend to be elongate and strongly tapering, the basal segments are usually transversely impressed and often have small ridges along the basal margin. The legs vary from long and slender to robust and compact, and the tarsi may include bilobed segments; generally the third and/or fourth but sometimes others as well, and this is often an adaptation to climbing marginal vegetation. Stenus can be generalized broadly into those with a compact body and short legs which like to live among litter and dense vegetation, and those with a larger, more articulated body and legs which live in more open habitats e.g. sandy or gravelly and sparsely vegetated marginal situations. Adults are distinct in having a specialized and extendible hypopharynx and labrum which is used for capturing fast-moving prey such as springtails, this structure is extended rapidly using fluid pressure, and it ends in a pair of sticky pubescent plates, or paraglossa, between which are pores that exude a sticky substance which adheres to the prey. This structure is absent in Dianous. Many species inhabit marginal environments and are adapted to moving rapidly on the water surface; some can swim or walk on the surface much like pond skaters, and some can secrete a hydrophobic substance-hence the old vernacular of camphor beetles-from pygidial glands which propels the beetle rapidly forward by reducing the surface tension. When specimens of both genera fall into the water they will often be seen to skate rapidly back to the margin or to emergent vegetation.

Ecology

All species are diurnal and predatory and the majority, certainly the majority of U.K. species, live in marginal environments. Some have particular preferences such as fine sand or gravelly substrate, or open rather than vegetated habitats, and some will be found on coastal sand dunes or even the seashore. Some species will be found by sweeping grassland and vegetation generally, often away from water, and some occur commonly in dry, sun-exposed situations such as hillside calcareous grassland. They rarely occur in woodland habitats although vegetated margins of streams in wooded areas can be productive. Many of the U.K. species are widespread and common and some will soon be encountered by sweeping or searching substrates in marginal areas, and reed beds will soon yield a few species. Some are restricted to a particular region or area and will need to be researched and looked for very carefully e.g. the Highland S. glacialis Heer, 1839, and some are known only from historical records e.g. S. ludyi Fauvel, 1885 which is included on our list from a single Scottish record circa 1920. Species of Dianous inhabit vegetation and substrate around cascading water, or may be found in marginal situations beside rivers and other flowing water.