A new species of <i>Messor</i> from Bulgaria and redescription of the gyne of <i>M. oertzeni</i> Forel, 1910 (Hymenoptera, Formicidae)

A new ant species, <i>Messor odrysarum</i> <b>sp. nov</b>., of the <i>Messor structor</i> species group, is described based on worker, gyne and male castes collected in Eastern Rhodopes, Bulgaria. The description is supported by the morphology of all castes and COI barcoding. Both the morphological traits and COI sequence data indicate that the new species is closely related to <i>Messor oertzeni</i> Forel, 1910, a species restricted to the eastern Balkans and Anatolia. Furthermore, the little-known gyne of <i>M. oertzeni</i> was redescribed based on a specimen collected in Bulgaria. The new data increases the number of <i>Messor</i> species known from Bulgaria to nine, for which an updated key is provided.