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Rigorous species delimitation is a challenge in biology and systematics in particular. In insects, male genitalia traditionally, and the barcoding region of the CO1 gene recently, are the main markers to identify species, even though a standalone use of CO1 for that is often criticized. In our systematic revision of the mycophagous and in other ways peculiar oxyporine rove beetles of Russia, the legacy alpha-taxonomy could not be improved by traditional investigation of genitalia as they are unusually character-poor in this group. Using phylogenetic inference and ancestral state reconstruction, we demonstrate that CO1 and endophallus are useful markers for species delimitation in Oxyporus. We also show that many morphological traits previously used for species delimitation in Oxyporus are in fact highly variable and thus inconsistent. We hypothesize that in Oxyporus diversification of the endophallic structures is driven by intense intra- and inter-species interactions of multiple individuals co-occurring in narrow spaces inside fungal bodies during mating. Our results encourage broader use of both markers, especially easy-to-generate dna barcodes, for the desired alpha-taxonomical work in Oxyporinae globally. The revision revealed 10 species of Oxyporus in the fauna of the Russian Federation; eliminated erroneous species records; established two new synonyms, Oxyporus (Oxyporus) basicornis Cameron, 1930 = O. (O.) aequicollis Bernhauer, 1935, syn.nov. = O. (O.) parvus Lee et al., 2020, syn. nov.; and raised the hypothesis that O. (O.) aokii Dvořák, 1956, O. (O.) basiventris Jarrige, 1948 and O. (O.) kobayashii Hayashi, 2015 are conspecific with O. (O.) maxillosus Fabricius, 1793. Lectotypes are designated for O. (O.) basicornis Cameron, 1930, O. (O.) germanus Sharp, 1889, O. (O.) niger Sharp, 1889, and O. (O.) triangulus Sharp, 1889. Comprehensive taxonomic treatment and an identification key are provided for all species. [ABSTRACT FROM AUTHOR]