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Batrisus formicarius is a European species of myrmecophile beetle located within the Pselaphinae subfamily of the Staphylinidae family.

Biology and distribution
B. formicarius is found inside nests of the ant Lasius brunneus as well as, occasionally, Lasius emarginatus. The beetle acts as a commensal organism, feeding on the mites which are often found inhabiting the ants' nests. As such, it is not only tolerated, but also cared for by the ants. Similar relationships are found in other related beetles such as Batrisodes, however these are only more or less facultative myrmecophiles, as opposed to the truly exclusive myrmecophilic biology of the Batrisus genus. The main host of B. formicarius, Lasius brunneus, is a widespread eurocaucasian species found from Iberia to the Caucasus and from Scandinavia to Turkey. It mostly inhabits forested areas were it nests in rotting logs and under bark. The beetle species likely shares the same habitat preferences and much of its range, except for Northern Europe, Anatolia and the Caucasus. It is considered to be a Southern and Central European species, having been found in Spain, France, Italy, Switzerland, Luxembourg, Netherlands, Germany, Czechia, Slovakia, Austria, Hungary, Slovenia, Bosnia, Serbia, Montenegro, Macedonia, Greece, Bulgaria, Poland, Ukraine and Russia.

Diagnosis
B. formicarius is one of the only two species of the Batrisus genus to be known from Europe alongside Batrisus ormayi, a species known from the Carpathian mountains of Transylvania, Romania. The two species can be differentiated through several external characters:

1. The antennae of B. formicarius are more slender than those of B. ormayi (which has very thick antennae), especially in the central and lower segments (antennomeres). As a result, the last 3 antennomeres appear to be more enlarged when compared to the rest. The central antennomeres are only slightly wider than long, as opposed to B. ormayi in which they are much wider.

2. The whole body surface of B. ormayi is covered in a much more dense pubescence, giving it a more matt appearance than B. formicarius, which is mostly shiny. Furthermore, the elytra of B. ormayi specimens have longer hairs than those of B. formicarius.

3. The two species are thought to live alongside different ant species. As previously mentioned, B. formicarius is mostly found in Lasius brunneus nests, with Lasius emarginatus being a rare secondary host. B. ormayi, on the other hand, has been collected in the nests of Lasius flavus.

A third species, Batrisus sibiricus, has been collected in Siberia. While its range may include parts of Europe, this is considered to be quite unlikely. Regardless, it can be easily differentiated from the other two by the presence of a pair of small spines behind the eyes. it also has longitudinal notches spanning across the whole length of the first exposed dorsal segment of the abdomen, rather than only halfway.