Agathidium Panzer, 1797
POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
LEIODINAE Fleming, 1821
AGATHIDIINI Westwood, 1838
With more than 815 described species Agathidium is the largest genus within the Leiodinae; it occurs throughout the Northern Hemisphere and more than 720 are listed from the Palaearctic and Oriental regions with the Oriental region being by far the most diverse. Many species are endemic to certain countries e.g. of the 52 species known from Taiwan 50 are endemic, but many are widespread and several are Holarctic. About 100 species are described from North and Central America, and only a very few from South America although new species are regularly described from all regions. The European fauna includes 72 species and subspecies in four subgenera and of these 12 species of 3 subgenera extend to the UK. The species are all small, rounded and very convex and most are able to curl up into a spherical shape when disturbed, in this state the antennae lie in ventral grooves, the legs are folded beneath the body and the pronotal margins fold over the sides of the elytra so that the body is virtually impenetrable to predators. The European species are more or less typically of this form but more generally they are much more diverse, ranging in size between 1 and 6 mm and in shape from convex and fully contractile to elongate-oval and only partially contractile. Most are drab black to brown or reddish and many have contrasting forebody and elytra but a few are entirely pale yellowish e.g. the Northern Palaearctic A. pallidum (Gyllenhal, 1827), or have strikingly-patterned elytra e.g. the Nearctic A. maculosum Brown, 1928. Many are shiny and almost impunctate but the elytra may have larger punctures which may be random or form regular striae. Most species are sexually dimorphic and this often manifests in males as differing tarsal formulae, enlarged front and middle tarsomeres, setose fovea on the metasternum, toothed metafemora or an erect tooth on the left mandible which is used to fight rival males, this last feature being sometimes greatly enlarged e.g. in the Nearctic A. pulchrum LeConte, 1853. Species are either fully-winged and capable of flight or macropterous or apterous. Our UK species are fully winged. In general our species are small, 2.0-4.7 mm, with a large head, transverse pronotum and rounded elytra. Head transverse with large convex eyes that follow the outline, bordered and strongly converging cheeks and long weakly-converging temples, immediately behind each eye is
a short ridge, the length of which relative to the eyes is often used to separate some of the subgenera. The pronotum is shiny and very finely punctured, in side view widely-rounded or with very obtuse posterior angles and projecting anterior angles. The mesosternum may be ridged and the metasternum may have distinct femoral lines. The elytra are shiny, at most weakly microsculptured and finely and randomly punctured, they lack striae except, in most species, for a partial sutural stria in the apical half or third, in side view the humeri may be distinctly obtusely-angled or very obtuse and almost rounded. Legs short and slender, tarsal formula 5-5-4 in males, 5-4-4 or 4-4-4 in females. Among our UK fauna the genus may be distinguished by the form of the antennae; 11-segmented with a loose three-segmented club. Some species can be very difficult to identify and often need to be dissected. A good key to much of the European fauna, including our UK species, may be found here.
Our species are associated with slime moulds, both in their sporocarp or plasmodium stages, and adults may also be found on a range of fungal sporocarps, primarily Basidiomycetes. They therefore mostly occur in wooded habitats on standing or fallen timber but the nocturnal adults often occur on individual trees or timber in a wide range of habitats. Adults of most species occur year-round. Sampling usually involves searching under bark or among decaying wood but adults are active on the surface at night and are easily seen, they usually occur as single specimens or in pairs and sometimes two or more species may occur in a small area.