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The structure of the Haller's organ, the main sensory organ of hard ticks, studied by means of scanning electron microscopy, was analyzed in 40 species of the subfamily Amblyomminae (Metastriata), representatives of the genera Amblyomma Koch, 1844 (10 species, including 3 species earlier separated as the genus Aponomma), Anomalohimalaya Filippova, 1994 (1 species), Dermacentor Koch, 1844 (6 species), Haemaphysalis Koch, 1844 (6 species), Hyalomma Koch, 1844 (6 species), Margaropus Pomerantzev, 1947 (1 species), and Rhipicephalus Koch, 1844 (10 species, including 2 species earlier distinguished as a separate genus Boophilus). The main taxonomic characters (number and topography of anterior pit sensilla and shape of capsular orifice) were distinguished. It was demonstrated that the structure of the organ is specific for separate genera, whereas subgeneric and species differences within the genus are virtually insignificant. On the basis of these results, some modern revisions of the subfamily based exclusively on genetic and molecular data seem doubtful. The obtained data are compared with existing views on taxonomy of metastriate ticks.

Infection with trematodes of the littoral snails Littorina obtusata, that have different shell colour genotypes, was studied. It was found that activity of genes that are responsible for the formation of the purple and orange background shell coloration does not affect infection level of trematodes in periwinkles. Prevalence of L. obtusata with the yellow-purple background shell coloration and with the pattern of white pigment spots on the shell by Microphallus piriformes and M. pygmaeus partenits was lower than the theoretically expected one. Prevalence of mollusks with purple, orange or white bands on the shell did not differ from that of bandless individuals. The differences found are apparently associated with a genetically determined susceptibility to infection. Possible consequences of different infection level of trematodes in periwinkles with different shell colour genotypes for stability of parasitic systems and changes in the phenotypic structure of the host population are discussed.

A retrospective analysis of the infection of the giant grenadier Albatrossia pectoralis by four species of parasites — Gonocerca sp., Anisakis sp., Nybelinia sp. and Kudoa sp. in the Far East is represented. Historical material includes data collected on board of fishing and research vessels in 1968-2018 (selectively). A total of 2,091 individuals of A. pectoralis were dissected. The greatest prevalence and intensity of infection among helminths were recorded for Gonocerca sp. In the Bering Sea the infection with this trematode was the highest. In the Sea of Okhotsk and in the open waters of the northwestern part of the Pacific Ocean it was slightly lower.

A method for measuring catalase activity, which was proposed by Peizer and Widelock in 1955 and then, in simplified form, by Korolyuk and coauthors in 1988, was adapted for measuring this activity in homogenates of mosquitoes. In this method, the decrease in hydrogen peroxide concentration is measured by color reaction with ammonium molybdate. In present modification, the method includes the use of the buffer based on MOPS (pH 7.4). Absorbance of yellow product at 410 nm is increased almost linearly with increase of hydrogen peroxide concentration until approximately 8 mM, and linearity disappears with further increase of hydrogen peroxide concentration. The initial concentration of hydrogen peroxide in a reaction mixture, which was chosen for the method, was 8.2 mM, the duration of incubation was 4 min, the temperature of incubation was 25°С. At the activity value less or equal to 0.72 mM H₂O₂ per min, the rate of decomposition of H₂O₂ under action of catalase is almost constant during 4 min; the consumption of H₂O₂ is proportional to concentration of protein of mosquito homogenate in reaction mixture. The results of measuring specific catalase activity in mosquitoes by this method are in close correlation with the results obtained by a direct measurement of hydrogen peroxide concentration decrease in an ultraviolet range of wavelengths. The method is convenient for serial measurement of catalase activity (up to 8 samples per series before the reaction termination by adding ammonium molybdate). By applying this method, the differences in catalase activity between some taxa in fam. Culicidae were revealed.

The analysis of the literature data showed that the present day flea fauna of mammals in Vietnam is represented by 50 flea species. In 2019-2020, we surveyed seven northern provinces of Vietnam, including Bien Biên, Lai Châu, Lao Cai, Ha Giang, Lang Son, Cao Bang, and Quang Ninh, adjacent to the People's Republic of China. During the study period, 569 specimens of small mammals, belonging to 28 species were captured. 375 specimens of fleas, belonging to 11 species were collected, of those, one species (Peromyscopsylla himalaica (Rothschild, 1915)) was discovered in Vietnam for the first time.

The paper highlights new data on the species composition of gamasid mites and provides a comparative analysis of species inhabiting nests of the sand martin (Riparia riparia (Linnaeus, 1758) in the Saratov region. A total of 16 species belonging to 12 genera and 8 families were recordered. 8 species of gamasid mites that were not previously recorded in the region were identified: Geholaspis mandibularis (Berlese, 1904), Pachylaelaps perlucidus Masan, 2007, Ameroseius delicatus Berlese, 1918, Aceoseius muricatus (Koch, 1839), Protogamasellus mica (Athias-Henriot, 1961), Dermanyssus hirundinis (Hermann, 1804), Haemogamasus liponyssoides Ewing, 1925, and Hypoaspis (Geolaelaps) expolitus Berlese, 1905.