Onthophagus similis (Scriba, 1790)
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POLYPHAGA Emery, 1886
SCARABAEOIDEA Latreille, 1802
ONTHOPHAGINI Burmeister, 1846
ONTHOPHAGUS Latreille, 1802
This is a widespread and locally common species throughout Europe from the northern Mediterranean to the UK and the south of Sweden and Finland and extending east it into Ukraine and western Russia. Historically there has been confusion with the similar O. fracticornis (Preyssler, 1790), both sometimes appear together in the same environments but the present species is generally rarer, at least in central and northern Europe, they are sometimes considered as forms of the same species and because of this the relative distributions are not well understood. Here O. fracticornis is a very rare species while O. similis occurs throughout Wales, including Anglesey, and remains locally common across two areas of England; south of London west to Dorset, and East Anglia, beyond this there are a few scattered records in the south. On the continent it occurs mostly in cattle and sheep dung in dry lowland areas exposed to the sun while here it is usually associated with horse and sheep dung on light sandy and chalky soils, although in one New Forest site we found them burrowing into waterlogged heavy clay soil, often near the coast and often in company with O. joannae Goljan, 1953. Adults are present year round, they become active in late spring and may be abundant during June and July-we have found them repeatedly in large numbers in the New Forest during early June-they mate during June (at least, and probably earlier) and females burrow directly beneath the host material, they lay a single egg into several buried brood masses and larvae develop through the summer, they pupate in situ and new generation adults appear from mid-summer. These may emerge and feed for a while or remain underground to overwinter. During the hottest days adults may be seen on host material or in flight but otherwise they will need to be searched for among dung, and by careful searching the burrows may be seen; we once observed several females entering burrows after the dung had been removed.
A bicoloured species with a dark and weakly metallic forebody and mottled pale brown elytra, the lateral pronotal margin is sinuate behind the anterior angles and the elytral epipleura are dark and so it is very similar to O. fracticornis, it is often quoted as being consistently smaller, at 4.5-7.0mm against 7.0-9.5mm, but on the continent there is considerable overlap. The common and widespread O. coenobita (Herbst, 1783) has a more strongly metallic forebody and less strongly mottled elytra and can be distinguished by the elytral epipleura which are pale, in the present species they are black, or at least the delimiting carina is black. O. similis and O. fracticornis may be distinguished by the form of the secondary sexual characters. In males the base of the cephalic horn is narrow, about half the head width, and rounded laterally in front of the eyes, in fracticornis it is broader across the base, about 2/3 as wide as the head, and distinctly angled externally. In side view the horn of fracticornis is characteristically angled when compared with that of similis. In female similis the posterior transverse cephalic ridge is relatively narrow compared with the anterior ridge so that lines constructed between their lateral margins will converge posteriorly, in fracticornis the ridges are more or less equal so that lines constructed between the lateral margins are parallel. Diminutive male fracticornis can be very similar to male similis and here they will need to be dissected as the parameres are very different; in the present species they are parallel and widely expanded apically while in fracticornis they are narrow to the apex and converge.