Revision of the bactrianus subgroup of the genus Eumerus Meigen (Diptera: Syrphidae) in Europe, inferred from morphological and molecular data with descriptions of three new species

With more than 300 described species, the genus Eumerus is one of the largest genera of hoverflies (Diptera: Syrphidae). In this paper, we define the Eumerus bactrianus subgroup and revise the subgroup in Europe for the first time, providing an illustrated key for its European species. We describe three new species: E. banaticus Nedeljkovie, Grkovie & Vujic n.sp. from Serbia and Romania; E. bicornis Grkovie, Vujic & Hayat n.sp. from Greece and Turkey; and E. bifurcatus van Steenis & Hauser n.sp. from Spain. Species of this subgroup exhibit a distinctive apomorphic morphological feature, i.e. the bifurcated posterior surstylus. We discuss the distributions of seven Palaearctic species of this subgroup: E. bactrianus Stackelberg, 1952, E. pannonicus Ricarte, Vujic & Radenkovic, 2016, E. turanicola Stackelberg, 1952, E. turanicus Stackelberg, 1952, and three species described here. Molecular species delimitation, which strongly supports our morphological results, is based on maximum parsimony analysis of mitochondrial COI sequences.