Anisotoma Panzer, 1797
POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
LEIODINAE Fleming, 1821
AGATHIDIINI Westwood, 1838
This small genus of very distinctive beetles included about 50 species, it is thought to have originated in North America from a common ancestor of Anisotoma/Agathidium and spread to Central America and the Palaearctic region. The European fauna includes 5 species, all of which are widespread and all with the exception of A. axillaris Gyllenhal, 1810 extend north to the UK. They are sometimes referred to as Round Fungus Beetles for their ability to curl into a compact ball when alarmed. Typical habitats are established woodland and wooded parkland with plenty of old trees and fallen timber in various staged of decay, often on heavy soils in damp and humid situations. All species are associated with slime moulds and while adults will often be found on a range of sporocarps or among mycelia under bark etc. it is uncertain whether any of these are larval hosts. Among the most frequented slime moulds are (on a world basis) Comatrichia Preuss, 1851 (Amaurochaetaceae), Fuligo Haller (1768) (Physaraceae), Metatrichia Coquillett, 1900 (Scenopinidae), Stemonitis Gled (Stemonitidaceae), Trichia Haller, 1786 (Trichiidae) and Tubifera Gmel. G.F., 1792 (Tubiferaceae), while in Europe the most frequented species is very probably Enteridium lycoperdon (Bull.) M.L. Farr, 1976 (Reticulariaceae). Most species are associated with several host species, a single mould may host many adults and larvae and several species may be present on a single host. Adults typically occur year round, overwintering within host tissue, under bark or among decaying leaf-litter, they are active from late spring until late summer and often occur in numbers. They are nocturnal and easy to sample by torchlight as they walk on exposed decaying wood, usually in the vicinity of fungal infections, and they disperse by flight during the evening and at night. The specialized distal antennomeres include internal vesicles which are characteristic of the family and probably include chemoreceptors sensitive to slime-mould volatiles. The life cycle is usually univoltine with mating occurring in the spring and larvae developing and pupating within host tissue to produce new-generation adults during late summer. The early stages of some species have been described HERE.
Anisotoma humeralis 1
Anisotoma humeralis 2
Among the European fauna they are distinguished by the broadly-convex body form and 5-segmented antennal club with segment eight being diminutive, in our other genera of the Agathidiini the club is either 3-segmented (Agathidium Panzer, 1797) or 4-segmented (Amphicyllis Erichson, 1845). Adults are otherwise rather typical, broadly-oval and continuous in outline, dark coloured (although many non-European species are entirely pale brown) or with various red or pale markings, pale legs and pale or bicoloured antennae. Head usually rather flat between convex and protruding eyes, temples converging, palps short and inconspicuous and the labrum either truncate or weakly emarginate anteriorly. Pronotum transverse, evenly and usually quite strongly convex and finely punctured throughout (in some Nearctic species virtually impunctate), without impressions or sculpture. Elytra broadest at or near the base and smoothly narrowed to a continuous apical margin, surface finely to moderately strongly punctured, usually randomly so but in some these from partial or complete longitudinal striae and in some Nearctic species there are up to 9 complete striae, all have a partial impressed sutural striae from the apex to at least well into the apical half. Most species are glabrous but there are exceptions e.g. the European A. humeralis (Fabricius, 1792). The legs are short and slender with slightly expanded femora that hardly reach the lateral outline. Front and middle coxae closely approximated, hind coxae widely separated. Tibiae only weakly broadened from the base, straight or weakly curved, sometimes with weak longitudinal ridges or rows of setae, often with larger stiff setae along the external margin towards the apex and always with well-developed terminal spurs. Tarsi simple and often sexually dimorphic; commonly 5-5-4 in males and 4-segmented in females but in many the females are 5-4-4.
Our UK species are readily keyed out as follows (tarsal formulae in brackets):
-Black with pale humeral markings, elytra finely pubescent. (Male 5-5-4, female 5-4-4).
-Body black to dark brown, elytra glabrous.
-Elytra with regular and distinct punctured striae, apex smoothly rounded. (Male 5-5-4, female 5-4-4).
-Elytra for the most part randomly punctured although there may be partial longitudinal rows on the disc, apex acuminate.
-Larger, 3.0-4.2 mm. Antennomeres 7, 9 & 10 serrate, second antennomere shorter than the third. Basal pronotal margin weakly and evenly curved. (Male 5-5-4, female 4-segmented).
-Smaller, 2.2-3.0 mm. Antennomeres 7, 9 & 10 almost symmetrical, second and third antennomeres about equal in length. Basal pronotal margin unevenly curved; obliquely angled towards the lateral margin. (Male 5-5-4, female 4-segmented).