This study is the result of extensive investigations of leeches on the Balkan Peninsula. Our aim was to detect actual and potential (modeled) distributions of common Erpobdellidae species, and to identify their ecological differentiation with respect to the altitudinal and waterbody type gradient. Although widespread, these species rarely live together. Intense competition is avoided by preferences for different types of habitats. This was confirmed by Pearson correlation analyses that yielded negative results. Differentiation of these species was clarified by the results of logistic Gaussian regression analyses. While Erpobdella octoculata and Dina lineata have a similar distribution along the altitudinal gradient, they prefer different waterbody types. Erpobdella vilnensis prefers higher altitudes than the other two species. Its preferred habitats are smaller rivers and streams located at altitudes from 400 to 1000 m a.s.l. Although present in all waterbody types, large lowland rivers and standing waterbodies are the preferred habitats of E. octoculata. Fast-flowing springs and streams are mostly inhabited by D. lineata. While the distribution of the species overlaps to a large degree, the ecological preferences of species differ significantly and thus they can be used as confident typological descriptors and indicators of ecological status.

The presence and distribution of Rhyacophila fasciata Hagen 1859 in Europe were revised, based on bibliographic study, collection specimens, and new material collected in different countries. The status of formerly synonymized species, Rhyacophila ferruginea (Scopoli 1763) and Rhyacophila septentrionis McLachlan 1865 was also assessed. The type of R. ferruginea is missing, the taxon is still unidentified, and thus we propose Rhyacophila ferruginea as a nomen dubium. Morphological features and genetic evidence revealed that R. septentrionis differs from R. fasciata, so we propose to change its status to status resurrectus. We therefore include new descriptions of the different stages (larva, pupa, male, and female) of R. fasciata and of R. septentrionis, together with a molecular analysis based on mitochondrial cytochrome oxidase I (mtCOI) and ecological notes. The species R. coppai Oláh 2020 NEW SYNONYM and R. soreda Coppa & Oláh 2020 NEW SYNONYM are synonyms of R. sociata Navás 1916; the species R. kopasa Oláh & Coppa 2020 NEW SYNONYM and R. rova Oláh & Coppa 2020 NEW SYNONYM are synonyms of R. denticulata McLachlan 1879; the species R. matrensis Oláh & Szczęsny 2020 is probably a synonym of R. fasciata, so more study of this species is needed in order to confirm or deny that it is a valid species.

Grossuana maceradica n. sp. from the Radika drainage, Macedonia is described. A lectotype is selected for Amnicola marginata Westerlund, 1881, and a neotype is designated for Paludina hohenackeri Küster, 1853. Photographs of type material of these and 2 other Balkan taxa, Amnicola filiola Westerlund, 1881 and Hydrobia haesitans Westerlund, 1881, are published here for the first time.

The morphology of all postembryonic stages (larva, pupa, male, and female) of Rhyacophila macedonica Karaouzas, Valladolid, & Ibrahimi n. sp. from Greece, Republic of North Macedonia, Kosovo and Serbia was examined. Morphological data were supplemented by a molecular analysis of the mitochondrial cytochrome C oxidase subunit I (COI) and compared with samples of the nominate species Rhyacophila fasciata Hagen 1859, as well as with other species and subspecies in this group: Rhyacophila septentrionis McLachlan 1865, Rhyacophila denticulata McLachlan 1879, Rhyacophila sociata Navás 1916, Rhyacophila kykladica Malicky & Sipahiler 1993, Rhyacophila fasciata delici Kucinic & Valladolid 2020, and Rhyacophila fasciata viteceki Valladolid & Kucinic 2020. Our results revealed morphological differences between the nominate species and R. macedonica, as well as genetic differences among the taxa in the Rhyacophila fasciata Group, so we propose R. macedonica as a new species of the Group. Based on the new molecular data, we also elevate R. f. delici to the status of a distinct species, R. delici (status promotus).

The Sharr Mountains are one of the most important hotspots of terrestrial and freshwater biodiversity in the Balkan Peninsula, with many endemic and rare species. The caddisfly studies in this area increased during the past years, although insufficiently investigated areas still remain. In this paper, we describe a new species, Potamophylax humoinsapiens sp. n. from the Sharr Mountains in the Republic of Kosovo, which is morphologically closest to Potamophylax idliri Ibrahimi, Bilalli & Kučinić, 2022 and Potamophylax juliani Kumanski, 1999. The males of the new species differ from all known species of the Potamophylax winneguthi Species Group by their uniquely-shaped parameres, which are long, bulbous in their basal half and thin in the remaining length, with a bunch of very thin and long, hair-like spines, grouped uniformly at the apex. The new species further differs from its most similar congeners by its very wide distance between the dorsal and ventral edges of the apical part of inferior appendages in lateral view. The new species was found at three localities from 1416 to 1505 m a.s.l. Similar to the other species of the Potamophylax winneguthi Species Group, which have very narrow distribution areas, we posit that Potamophylax humoinsapiens sp. n. is a microendemic of the Sharr Mountains. The new species is the second known caddisfly species occurring only in the Kosovan part of the Sharr Mountains.