Odonteus armiger (Scopoli, 1772)
This is the only European member of its genus and, at 5-10mm, is also the smallest of the European geotrupids. It is widely distributed and sometimes locally abundant through Europe north to southern Scandinavia and east to the Urals and Caucasus although it is rare or absent from parts of the central Mediterranean region. In the U.K. it is considered to be very rare, although this may be a reflection on how difficult it is to find, and is restricted to southern and eastern parts of England with a few records from south Wales. The species has been recorded in a wide range of habitats from lowlands to low mountain altitudes, more especially so in Europe where it is often associated with woodland and wooded borders although the majority of records are from light trapping and may simply reflect the habits of lepidopterists, and open heathland and grassland on sandy or chalky soil. Adults occur from May to November, peaking during June and July, and may be conspicuous as they fly over pasture on warm evenings or, during cooler weather, in the afternoon, and on the continent they have been recorded swarming. Most U.K. records are from open situations, often on cattle or sheep pasture, and during June and July a large proportion are from light traps, sometimes from disturbed situations such as domestic gardens and scrubland. They have been found under or around cattle and sheep droppings, around rabbit burrows and on fungi, both on trees and underground, and while the adults are often associated with mammal burrows there seems to be no unique association although it seems the beetles can exploit this environment in some way. Both adults and larvae are known to feed on fungi; the adults on a range of species but especially on decomposing subterranean fruiting bodies and there may be a particular association with mycorrhizal fungi of the genera Endagone and Glomus, while the larvae appear to feed on mycelia and decaying fruiting bodies, especially on tree roots. The life cycle is only poorly understood; the adults excavate burrows down to about 70cm, which may be branched but are often not, and are thought to provision them with dung or compost but decaying fungi have often been found in the main tunnels and it may be that the species is primarily fungivorous. Captive adults have been recorded feeding on a range of fungi.
This species is rather atypical among the U.K. geotrupids and therefore distinctive; oval and very convex, and dark to pale brown with the appendages often lighter. Head finely and quite densely punctured, with large and prominent eyes divided by a wide canthus which is obvious from above. The mandibles are robust and produced forward and the antennal club is relatively very large. The male has a smooth vertex and a long and upright horn on the frons below the eyes which is absent in the female but here there are usually variously developed tubercles between the eyes. The pronotum is broadest at the base and narrowed to the front angles, the anterior and basal margins are widely sinuate and all the margins are strongly bordered. The surface is strongly though often sparsely punctured and, in the male, has 2 backwardly produced horns in front of widely depressed basal fovea. Scutellum large and cordate. The elytra have 7 or 8 deep and strongly punctured striae between the suture and the humeral prominence, and wide and convex interstices of which the sutural, or the first 2, are raised towards the apex. The tibiae are greatly developed; the pro-tibiae have 3 very large and several smaller teeth along the outer margin and, in the males, a greatly developed internal apical spine, the meso- and meta-tibiae have 2 strongly developed transverse ridges on the outer face. Tarsi 5-segmented and simple, without any dilated or lobed segments, the terminal segment is long and bears well-developed claws that are smooth and lack a basal tooth.
Odonteus Samouelle, 1819
This is a small and widespread genus of about 15 species; 10 are Nearctic, 2 occur in the eastern Palaearctic and a single species occurs in Europe. They are very distinctive; medium sized, 5-10mm, dark to pale brown earth boring beetles. Within the genus they can be very difficult to identify but with only a single U.K. species the identification is straightforward and certain. The appearance is typical of the Geotrupidae; oval and very convex with 11-segmented antennae bearing a well-defined club, prominent eyes divided by a canthus and, unusually among our U.K. fauna, strongly punctured elytral striae. Sexual dimorphism is very strong with the males having a long cephalic horn and variously modified pronotal sculpture. The adults dig burrows which they provision with food for the larvae, usually dung, decayed humus or fungi etc. The life cycle of some Nearctic species is well understood whereas the European species is only poorly known. In some species a single burrow has been found to contain larvae, pupae and adults together. It seems that most overwinter underground as adults so giving peaks of occurrence in spring/early summer and late summer/autumn. Many fly in the afternoon or at dusk and are attracted to light although under certain circumstances they also display photophobic behaviour, and some species have been recorded swarming.