Thymalus limbatus (Fabricius, 1787)
POLYPHAGA Emery, 1886
CLEROIDEA Latreille, 1802
PELTINAE Latreille, 1806
THYMALINI Léveillé, 188
THYMALUS Latreille, 1802
Thymalus is a Holarctic genus extending south to Thailand, northern India and North Africa. Nine species are recognized of which three occur in Europe and one of these, T. limbatus (Fabricius, 1787), extends into the U.K. A single species occurs in the Nearctic region; T. marginellus Chevrolat, 1842 is widespread across Canada and the far north of the United States. All are saproxylic and associated with fungi in which the larvae are thought to develop; they occur in a range of fungi on both deciduous and coniferous trees. The adults occur under bark or among decaying wood during the day and are generally active on the surface at night. All species are of a very characteristic general appearance, superficially resembling dark and pubescent Cassida; the south Russian T. oblongus Reitter, 1889 even appears as a metallic green form. They are readily separated by the clubbed antennae and lack of bilobed tarsal segments in the present genus. All are medium sized insects, 4.3-7.5mm, the head is small and narrow in relation to the pronotum, variously punctured and with large, convex eyes and prominent bifid mandibles which are toothed on the inner margin. The palps are sub-clavate and at least to some extent truncate. Antennae moderately long with the basal segment very large, broad and curved, segments 3 and 4 are elongate, sub-equal and gradually thickened towards the apex and segments 8 to 10 are transverse; segments 8 to 11 form a distinct and symmetrical club. The entire dorsal surface is pubescent and in some species this is long and erect. The pronotum is transverse, weakly emarginate along the anterior margin which usually covers the base of the head, and explanate and bordered laterally. The disc is moderately finely punctured, occasionally densely so. Pro-coxal cavities open laterally, the meso-coxal cavities open. Scutellum visible and curved in outline. Elytra convex, often strongly so e.g. in the two species referred to above, broadly oval and variously explanate with strongly, if sometimes irregularly, punctured striae which are about as broad as the interstices. Entire surface finely punctured. Epipleura broad throughout and sometimes widened towards the apex. The underside and legs are finely punctured and pubescent; the femora broad and extending to the sides of the thorax and abdomen and so not visible from above. Tibiae relatively long and weakly curved, sometimes with small spines along the outer margin, apical spur, if present, very small. All the tarsi are robust and without bilobed segments; all segments with dense, pale pubescence below, the terminal segment long and expanded towards the apex. Claws curved, sharp and obtusely angled at the base.
The type species, T. limbatus, is widely distributed throughout Europe and Asia, extending north into Scandinavia and the U.K. and south to North Africa. In the U.K. it is widespread but local and sporadic; central southern England, the West Country, Wales and the midlands, Cumbria and there are a few records from the Scottish Highlands. Here it is associated with a range of deciduous trees while on the continent it also occurs on fir trees. They generally occur in long-established deciduous woodland where adults live under bark near to the larval host material and larvae live among or under bark or within fruiting bodies of various polypore basidiocarps where they consume mycelia and spores. Adults are nocturnal and may be found by searching trunks by torchlight. The species may have a preference for oak.
As the only member of this very distinctive genus to occur in the U.K. it should not be confused with any other species; the clubbed antennae and the form of the tarsi will distinguish it from anything superficially similar.