Recent studies based on morphologic and molecular genetic data have revealed quite a serious variety in the trans-Palearctic species, which brought about taxonomic status changes in 14 of 18 Russian Far Eastern bat species. Far Eastern bat status revisions resulted in species growth whose chromosome characteristics have been described either under other names or have not been studied at all. This paper has inventoried bat chromosome research in the Russian Far East and neighboring regions and has improved the accuracy of chromosome characteristics for 17 of 18 valid species today. For the first time, the karyotypes and their variation type for the valid bat species in the Russian Far East have been described.

The DNA-barcoding and chromosomal study of the eastern water bat, Myotis petax Hollister, 1912, from the earlier unexplored localities in the Russian Far East are carried out. The COI barcoding obtained for 18 from a total of 19 individuals captured in five localities in the Russian Far East showed the low nucleotide variability with the prevalence of the central, the most abundant haplotype. The chromosomal characteristics of eight M. petax specimens (2n = 44, NFa = 52) in the Russian Far East are clarified. The number and localization of NOR in karyotype of M. petax is described at the first time and differ from distributional patterns of NOR in the sibling species M. daubentonii Kuhl, 1819 that can be used as diagnostic feature. The considerable intraspecific variability in the distribution of heterochromatin material revealed is not typical of the genus Myotis, but it has been found in other species of the family Vespertilionidae.

The DNA-barcoding and chromosomal study of the eastern water bat, Myotis petax Hollister, 1912, from the earlier unexplored localities in the Russian Far East are carried out. The COI barcoding obtained for 18 from a total of 19 individuals captured in five localities in the Russian Far East showed the low nucleotide variability with the prevalence of the central, the most abundant haplotype. The chromosomal characteristics of eight M. petax specimens (2n = 44, NFa = 52) in the Russian Far East are clarified. The number and localization of NOR in karyotype of M. petax is described at the first time and differ from distributional patterns of NOR in the sibling species M. daubentonii Kuhl, 1819 that can be used as diagnostic feature. The considerable intraspecific variability in the distribution of heterochromatin material revealed is not typical of the genus Myotis, but it has been found in other species of the family Vespertilionidae.

Larvae of the chigger mite Leptotrombidium album (Kamo, Kawashima & Nishimura, 1957) (Acariformes, Trombiculidae) are found in Kunashir island and recognized as parasites of bats Barbastella pacifica (Kruskop, Kawai & Tiunov, 2019) for the first time. Larvae were measured and photographed using SEM and DIC approaches. A stylostome is described in bats for the first time by histological with additional CLSM, DIC and polarized light methods. The stylostome belongs to the mesenchymal type and penetrates deep into the host dermis perpendicular to the skin surface. The stylostome is a straight relatively wide (80 µm) tube and may reach 250 µm long in the maximum development. The axial stylostome canal varies from 7.5 to 20 µm wide and distally is opened to the subjacent host tissue. A particular eosinophil cone of a gel consistency to which the larval cheliceral blades are tightly cemented represents the proximal stylostome portion. The main stylostome portion usually remains unstained and also shows a gel-like consistency. The distal stylostome portion—supposedly a fresh saliva portion—is typically stained by Azur in blue and seems to be a transitional condition between sol to gel. No birefringent components are found in the stylostome substance. The stylostome may immediately contact host tissues or may be surrounded by a mixed eosinophil substance composed of destroyed inflammatory cells. If present, this substance is tightly associated with the stylostome walls and may be considered as an analog of scab but distributed not on the surface but inside the skin. The stylostome formation is accompanied by a generalized inflammatory reaction with the predominance of neutrophil leukocytes. The activity of basophils, lymphocytes, macrophages and mast cells is relatively weak. A particular tissue sinus filled with inflammatory cells, an analog of the feeding cavity, situates underneath the stylostome. The disclosed stylostome organization confirms its species-specific character.

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