Anisotoma humeralis (Fabricius, 1792)
POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
LEIODINAE Fleming, 1821
AGATHIDIINI Westwood, 1838
ANISOTOMA Panzer, 1797
A widespread and locally common native of broadleaved and mixed woodland throughout Europe north to the Arctic Circle in Scandinavia and east through Asia Minor to the west of Russia although as with many saproxylic species it has been in general decline throughout much of its range in recent decades, and following introductions from Europe it is now established in several areas of Canada and the United States. In the UK it is locally common throughout England and Wales, becoming less frequent further north and known only from a few central and northern records in Scotland. Adults occur year-round and are generally associated with fungi in wooded situations; deciduous and mixed woodland with plenty of decaying wood, parkland, orchards and gardens etc. They are active from March until October; they remain under bark or among decaying wood during the day and become active at night when they may be observed on trunks or among fallen and decaying timber; in late spring they often occur in numbers at fungus and disperse by flight on warm evenings. Larvae develop among slime moulds or, less commonly, in fruiting bodies of bracket fungi on decaying wood, adults also consume slime moulds, usually in the mature powdery stage, and have also been observed feeding upon various brackets e.g. Mucidula mucida (porcelain fungus) and Fomes fomentarius (tinder fungus), and at sap runs on Fagus and Quercus. The species is sometimes quoted as having a preference for Quercus but it will be found on a wide range of trees e.g. on several occasions we have recorded them from Alnus and Salix bark in marginal situations, populations peak in early summer and adults present through the summer will go on to overwinter under loose bark on stumps and fallen timber.
Although superficially similar to members of several other families e.g. Tritoma or various Coccinellids, the present species is readily distinguished as a leiodid by the form of the antennae, and among our UK leiodid fauna it is quite distinctive.
2.5-4.7mm. Head black with pale mouthparts and sometimes a pale median line on the vertex, eyes large and convex anterior to short and curved temples, anterior clypeal margin straight or incurved and laterally concealing the antennal insertions, antennae pale with segments 7-10 and the base of the terminal segment dark, club 5-segmented (which distinguishes Anisotoma from other genera) with the second segment diminutive. Labrum curved anteriorly and not, or only very weakly, emarginate. Pronotum entirely shiny black or narrowly pale around the margins, and finely punctured, broadest at near-perpendicular posterior angles and evenly rounded to the anterior margin, basal margin sinuate laterally. Elytra broadest behind weakly convex shoulders and evenly curved to a continuously rounded apical margin; shiny black with ill-defined humeral areas, in ab. orca Mahony, they are completely dark, surface pubescent (which will distinguish the present species among our other Anisotoma), finely and randomly punctured and with several longitudinal series of larger punctures, each consisting of 2 or 3 confused rows. Legs short and stout; tibiae with dense spines and longitudinally grooved, tarsi 5-5-4 in the male and 5-4-4 in the female. Basal segments of pro- and meso-tarsi strongly dilated in the male.