A new species of the genus Nigrobaetis Novikova & Kluge, 1987 is described based on larvae from the Caucasus Mountains. Nigrobaetis (Takobia) katerynae Martynov, sp. nov., is easily distinguishable from other Takobia species by the following combination of characters: the shapes of frontal suture, mandibles, labial palp, paraproctal prolongation, gills; the number of setal rows along the external margins of femora; the absence of gills on tergum I; and the colour pattern of body.

A new species of the subgenus Rhodobaetis Jacob, 2003, namely Baetis (Rhodobaetis) vadimi sp. nov., is described based on immature larvae from several localities in Georgia and Turkey. It is the ninth species of the subgenus for Transcaucasia, following six species formally recorded from Turkey and two endemic species described from the Central Caucasus. Larval characters are investigated and illustrated by means of scanning electron microscopy, and discussed in details. The differential diagnosis of this species is provided with regard to other representatives of the subgenus considering the feature set common for Rhodobaetis. The new species can be distinguished from its relatives (species which lack stout setae on the gills’ margins), mainly by some characters of mouthparts (particularly labium), the shape and arrangement of stout setae and scales on the body surface, as well as the lack of subapical tiny setae on tarsal claw, and presence of numerous marginal spines and semilunar scale bases on the paraproct plate.

As part of our detailed study of the Caucasian mayfly fauna, we describe Epeorus (Caucasiron) abditussp. nov., a new species of the genus Epeorus Eaton, 1881, subgenus Caucasiron Kluge, 1997, based on larvae collected in Türkiye, Georgia, and Russia. We use several methodological approaches to delimit the new species by analysing COI sequence data and larval morphology. We provide a comparison with related taxa and diagnostic characters allowing determination of the larvae. We also update the identification key for the Caucasian species of E. (Caucasiron) with E. (C.) abditussp. nov. and two recently described species, E. (C.) hyrcanicus Hrivniak & Sroka, 2021 and E. (C.) tripertitus Hrivniak & Sroka, 2022.

The mayflies of the genus Epeorus Eaton, 1881 subgenus Caucasiron Kluge, 1997 are distributed from the eastern Mediterranean to the mountains of south-west China. In contrast to the Caucasus, the Mediterranean and Irano-Anatolian regions, where E. (Caucasiron) represents one of the most extensively studied mayfly taxa, the species diversity in the more eastern mountains of Asia has been studied only sporadically. In this study, the species diversity of E. (Caucasiron) from the mountains of Central Asia (Pamir, Tian Shan) and the western part of the Himalayas was analysed using DNA barcoding and the morphology of larvae and adults. The distance- and phylogenetic tree-based molecular species delimitation analyses revealed five E. (Caucasiron) species occurring in the study area. Three of them did not correspond morphologically to any known species of the genus Epeorus. These species were described herein as E. (C.) himalayensis Hrivniak & Sroka, sp. nov., E. (C.) lanceolatus Hrivniak & Sroka, sp. nov. and E. (C.) lineatus Hrivniak & Sroka, sp. nov. All new species were compared with other representatives of the subgenus and other related species of the genus Epeorus, and appropriate morphological diagnostic characters were provided. Morphological revision, main diagnostic characters, and information on the distribution of E. (C.) guttatus Braasch & Soldán, 1979 and two other potentially related Epeorus species from the area, E. psi Eaton, 1885 and E. suspicatus (Braasch, 2006), are also given.

The genus Centroptilum Eaton, 1869 is widely distributed in the West Palaearctic region, but poorly diversified as it previously only encompassed two valid species, C. luteolum (Müller, 1776) and C. pirinense Ikonomov, 1962. The discovery of a new congener, C. volodymyri Martynov, Godunko and Palatov, sp. nov., is therefore remarkable; the new species is described based on larvae collected in Georgia, Turkey and Iran in 2012–2019. This new species can be distinguished by the structure of the cuticle with flattened, stretched shallow, triangular and semilunar-shaped corrugations; scales and their bases densely scattered on body surface; features of the mouth parts, including the presence of an expanded laterally labrum, in combination with segment III of maxillary palp and superlinguae of hypopharynx rounded apically, without any pointed projections at the tip; numerous small teeth scattered proximally in two rows along inner margin of the pretarsal claw; the shape of denticulation along the posterior margin of the abdominal terga, and the number of prominent posterolateral spines on tergum VII; and the shape of gills II–VII, with a distinctly concave posterior margin. Together with a morphological description, data on the distribution and mitochondrial cytochrome c oxidase subunit I gene (COI) barcode sequences of the species are provided. The species delimitation of C. volodymyri Martynov, Godunko and Palatov, sp. nov. and C. luteolum is tested using COI barcode sequences. The data obtained show that the new species can be distinctly recognised as a separate lineage. The habitat preferences associated with the different parts of the area of C. volodymyri Martynov, Godunko and Palatov, sp. nov. are described and compared. Distribution of C. volodymyri Martynov, Godunko and Palatov, sp. nov. confirms the connections between Caucasian and Asian fauna of Ephemeroptera, as already demonstrated in various families and genera. Short notes on the distribution of mayfly species common to the Caucasus, north-western Iran and Central Asia are presented. http://www.zoobank.org/urn:lsid:zoobank.org:act:1D2B7557-0DEC-4ACD-8CA3-A6B50FCC4A51.

Nigrobaetis rhithralis (Soldán & Thomas, 1983) is completely redescribed at larval and imaginal stages of both sexes based on the type material. This species and the closely related Palaearctic species Nigrobaetis digitatus (Bengtsson, 1912) were compared to clarify their relationships. Several new distinguishing characters useful for separation of these two species are proposed and discussed; they may be further used to distinguish cryptic species within the genus Nigrobaetis Novikova & Kluge, 1987.