Integrative taxonomy suggests species status for Philonthus sideropterus Kolenati, 1846 sp. propr., the former color form of Philonthus splendens (Fabricius, 1793) (Coleoptera: Staphylinidae)

Philonthus splendens sideropterus Kolenati, 1846 (Coleoptera: Staphylinidae) was considered a Caucasian intraspecific form of the widespread West Palearctic Philonthus splendens (Fabricius, 1793) with an unclear diagnosis and volatile status. Mostly, it has been considered a variety of P. splendens or, recently, its subspecies, without firm justification. Using material from across the range of P. splendens and methods of integrative taxonomy, we conclude that Philonthus sideropterus, restricted to the forests of the Caucasus in the broad sense and northeastern Asia Minor, and Philonthus splendens, confined to a broad range of habitats in the Euro-Siberian forest belt, represent two allopatric sister species. Their clear divergence in the COI barcode (4.3%), distinct morphological difference in elytral coloration, lack of transitional forms, and allopatric distribution prompted us to raise Philonthus sideropterus sp. propr. to species level. The origin of both sister species from their common ancestor can possibly be explained by the paleoclimatic fluctuations that fragmented the initially or periodically more widespread forest biota into the Eurasian forest belt and the montane forests of the Caucasian region. A lectotype is designated for Philonthus splendens (Fabricius, 1793).