This paper presents numeric morphology-based evidence on the broadly overlapping distribution of two thief ant species Solenopsis fugax (Latreille, 1798) and S. juliae (Arakelian, 1991) in the East European Pontic-Caspian region. The paper integrates two autonomous data collections and independent analyses performed by different researchers, using different equipment, considering different character combinations, and evaluating partially different samples. Five type series, the neotype series of Solenopsis fugax (Latreille 1798) and the type series of S. flavidula (Nylander, 1849), S. (Diplorhoptrum) fugax var. furtiva Santschi, 1934, S. (Diplorhoptrum) fugax var. pontica Santschi, 1934, S. (Diplorhoptrum) fugax var. scytica Santschi, 1934 were nested in one cluster and we propose the junior synonymy of the latter four taxa names with S. fugax. The other cluster contained only one type specimen of Solenopsis nitida (Dlussky & Radchenko, 1994) measured from AntWeb images. The naming of this cluster was based on both verbal statements and measurements of gynes given in the original description of Solenopsis juliae (Arakelian, 1991), which represents the oldest available name for this cluster. Hence, S. nitida is proposed as junior synonym of S. juliae. Solenopsis cypridis Santschi, 1934 is raised to species rank based on investigation of worker and gyne type specimens.

Two ant species, Leptothorax gredleri Mayr, 1855 previously known from Western and Eastern Europe and Solenopsis juliae (Arakelian, l991) formerly known from the Transcaucasia (Armenia), were found in the Northern Caucasus (the republics of Kabardino-Balkaria and Karachay-Cherkessia) for the first time; both the species are new to the fauna of Russia. The distribution, ecology, and morphological characters of the species are considered.

Temnothorax unifasciatus (Latreille, 1798) is a widely distributed pan‐European species from the Iberian Peninsula to the Caucasus. This taxon's relatively high morphological variability prompts the taxonomists of earlier times and today to mention the morphologically different elements at specific or subspecific ranks. This paper aims to understand the population structure and genetic diversity within this lineage via integrative taxonomy, incorporating molecular phylogenetics, species delimitation analyses and multivariate analyses of continuous morphometric data from across the geographic range of the T. unifasciatus complex. Phylogenetic analyses yielded incongruent trees. The genealogical diversity index (gdi) and the confirmatory analyses on morphological data found only weak, ambiguous delimitations within the unifasciatus complex. The most highly supported scenario splits T. brackoi from the remaining unifasciatus complex with ambiguous support (gdi = 0.56). This scenario is supported by multivariate morphometry with 100% accuracy in classification success. Instead, our results suggest complex morphological and genetic population structuring within the broad range of T. unifasciatus. Therefore, we confirm the validity of two species, T. brackoi Salata & Borowiec, 2019 and T. unifasciatus (Latreille, 1798), and propose five new junior synonymies, T. cordieri (Bondroit, 1918) syn. nov., T. tauricus (Ruzsky, 1902) syn. nov., T. berlandi (Bondroit, 1918) syn. nov., T. unifasciatus staegeri (Bondroit, 1918) syn. nov., T. tuberum ciscaucasicus (Arnol'di, 1977) syn. nov. with the latter. To achieve maximal taxonomic stability, we designated a lectotype for Temnothorax unifasciatus (Latreille, 1798).

Data on the location of the ant Pyramica argiola (Emery 1869) in Russia, in the territory of the Northern Caucasus (Nalchik), are presented for the first time. The species was previously known from Transcaucasia. The species chorology and ecology and morphological peculiarities of individuals from the regional population are considered.