Siagonium quadricorne Kirby, 1815
Siagonium quadricorne is a widespread but very local species occurring throughout Wales and southern and central England although generally absent from the West Country and sporadic and very rare further north than Leeds. The typical habitat is damp woodland where they occur under bark on fallen and recently dead trees or under the margins of damaged areas of bark on living trees, but they also occur on recently fallen trunks in wetland situations and they have been recorded from a wide range of trees including oak, beech, poplar, willow, hazel, maple, elm and false acacia in various damp situations. On the continent they also occur under pine bark in mixed forests and have occurred in various scolytid galleries including Tomicus piniperda (Linnaeus, 1758) and Ips typographus (Linnaeus, 1758). They usually occur under close-fitting, damp bark and where the beetles are living is often visibly wet, they tend to be gregarious and may be very common in a small area that contains plenty of fallen timber, usually a single male will occur among several females and there are often larvae present although whether they are conspecific is not known. We have observed them at a local wetland site from late summer through the autumn and winter under close-fitting bark on felled poplar trunks with several small larvae present, and the following spring, when the bark had become loose and accumulated debris, only adults were present. The species is exclusively saproxylic, occurring as mentioned above, but little is known of its biology, it has been variously quoted as fungivorous, as a predator of other subcortical insects or as feeding within the galleries of other bark beetles, gut content analyses of both adults and larvae have revealed mostly fungal hyphae, and they may also feed and develop on sap from phloem vessels, hence the need to breed under damaged bark in humid situations. Adults spend most of their time under bark; we have not recorded them from light or seen them active nocturnally on material we know them to be occupying in numbers but Fowler quotes them ‘Under bark; often on walls and palings’. In our experience they are commonest, or at least most easily observed, during the winter and early spring.
4.0-5.5mm. In the field strongly suggestive of some Oxytelinae or Omaliinae but distinguished by the very flat and parallel-sided form, the modified head in the male and the unsculptured pronotum in the female, from all species of Omaliinae they differ in lacking simple eyes. Typically black to very dark brown with the pronotum, elytra, apices of the abdominal segments and legs variously lighter. Elongate and parallel, appearing broader anteriorly in the field, depressed or only weakly convex; head transverse with small convex eyes, short temples and prominent curved mandibles, in the male with anterior angles produced forward into a horn, which may be weakly developed in small specimens, and mandibles produced into a dorsal horn. Vertex diffusely and quite strongly punctured; in the female simply convex, in the male widely concave. Antennae inserted laterally; the insertions not visible from above, all segments elongate. Pronotum transverse and broadest in front of the middle, anterior angles distinct and lateral margin sinuate before perpendicular posterior angles. Lateral and basal margins finely bordered, surface flat, shiny and punctured as the head. Elytra long and parallel-sided, with abrupt epipleural folds and several irregular rows of larger punctures, the sutural row generally strongly impressed. Abdomen shiny and quite sparsely punctured; segments 1-5 distinctly but not strongly bordered and all segments with short pubescence as well as longer setae. Tergites 2-5 each with an oblique line beside the lateral margin, this feature will distinguish Siagonium from most of the UK staphylinids. Legs slender and relatively long; femora broad and unarmed, anterior tibia with a single row of spines externally, mid-tibia with 2 rows. Tarsi 5-segmented, the terminal segment longer than the others combined.
Siagonium Kirby, 1815 includes more than 20 species and has a Holarctic and Neotropical distribution; several species from Australia and New Zealand formerly included have been found to be wrongly assigned and have been included in other genera within the subfamily (Steel, W.O., 1950 Transactions and Proceedings of the Royal Society of New Zealand, Vol.78). As such the genus is very distinctive with all species exhibiting the sexually dimorphic head and mandibles. The majority of species occur in the Far East; 16 are known from Japan and the surrounding area, 3 are native to North America and 2 occur in Europe. Of the European species, S. humerale Germar, 1836 is a very rare species of south-eastern Central Europe while S. quadricorne is a more widespread, mostly western and central species reaching the Pyrenees, northern Italy and Transylvania and extending north to the UK and Denmark but otherwise absent from Scandinavia. The range may be increasing as it has recently (2007) been recorded from northeast Spain (Navarre, Bértiz).
PIESTINAE Erichson, 1839
Includes seven genera and more than a hundred species of mostly saproxylic, mycophagous species, some of which have preferences for particular types of wood. The Holarctic and Oriental genus Siagonium Kirby & Spence, 1815 includes >20 species of which one, S. quadricorne Kirby, 1815, occurs in the U.K. More than half the species are included in the Neotropical genus Piestus Gravenhorst, 1806, many of which develop in decaying cacti and succulents. The species are parallel-sided and flattened, often living beneath closely-fitting bark, with 11-segmented antennae inserted laterally in front of the eyes under an expanded margin of the frons. The pronotum and elytra lack ridges although the elytra may have deep and unpunctured longitudinal striae which give an obscure ridged effect. The head is transverse and often deeply sculptured and some species display pronounced sexual dimorphism with the head and/or mandibles greatly developed in males. The anterior coxae are characteristically small and globular, and the abdomen long with six visible sterna and a single pair of paratergites on segments three to seven. In species of Siagonium tergites two to five have an oblique line from the base to the lateral margin. Several extinct fossil species are known from early Cretaceous deposits in China.