Atholus bimaculatus (Linnaeus, 1758)
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POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
HISTERINAE Gyllenhal, 1808
Atholus Thomson, C.G., 1859
This species is native to the entire Palaearctic region; it is locally common though sporadic throughout Europe north to the UK and some central provinces of Fennoscandia, it occurs on most of the Mediterranean islands, the Azores and the Canaries and is widespread across North Africa, to the east it extends through Asia Minor, Russia and southern Siberia to China and Japan. It was first recorded in North America in 1825 from specimens imported from Europe and is now widespread and established across the United States and southern Canada, and it has more recently been recorded from introductions in Brazil, Argentina, India, Thailand and Chad. In the UK it is very local though not uncommon across southern and central England north to the Humber and much more local and rare in the West Country, Wales and further north into southern Scotland, there are very few records from Ireland and these are mostly older and coastal. Adults occur year-round; they overwinter among decayed vegetation and are active over a long season from early spring until October or November, peaking in abundance during May and again in September. Typical habitats are herbivore dung and compost; adults generally occur in small numbers in cattle, horse and sheep dung and may be common in undisturbed dung/straw mixtures, bale refuse and neglected heaps of grass cuttings, especially where these are decaying and producing heat. Compost heaps may also host the species, and where they are old, established and producing heat the beetles may be active year-round. Both adults and larvae are predatory upon larvae and so are likely to occur where diptera (especially) have infested the host material, although the beetle is not associated with carrion, and, typical of the family it is likely that larvae develop rapidly in early summer, pupate in situ, and that new-generation adults occur later in the summer. Adults disperse by flight and often do so on warm summer days when they may also be seen basking on low vegetation. Sieving likely material or taking samples for extraction are the best way to find the species but adults will occasionally appear when sweeping vegetation near suitable host material.
3.5-5.0 mm. Broadly-oval and discontinuous in outline, glabrous and shiny, body black with characteristic red markings to the elytra, antennae black with the funiculus partly red and the club appearing pale due to dense pubescence, legs dark brown with the tibial and tarsal spurs pale. Head with weakly transverse eyes and a complete frontal carina which is sinuate anteriorly, surface smoothly convex and finely punctured, labrum short and transverse, terminal maxillary palpomere long and slender. Antennae inserted outside the base of the mandibles near the lower margin of the eyes, 11-segmented with the scape long and angled downwards towards the apex, segments 4-7 short and transverse, club smoothly rounded. Pronotum transverse, broadest across perpendicular posterior angles and narrowed to projecting anterior angles, with a single sub marginal stria which does not reach the base, surface extremely finely microsculptured and with a distinct dimple inside each anterior angle. Elytra transverse, evenly curved laterally to separately rounded apices that leave the pygidium and pro-pygidium exposed, each with an abbreviated sutural striae (usually more developed in the apical half), five long striae which almost reach the basal and apical margins and without subhumeral striae. Front tibiae with a strong internal spur at the apex and usually four external teeth (there may be an extra very small tooth towards the base), middle and hind tibiae strongly dilated and with strong spurs along the external margins. In very rare cases the red elytral markings are missing but here the form of the pronotum and elytra are distinctive.