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The population diversity of the cestode Proteocephalus longicollis population in whitefish Coregonus lavaretus from Lake Kamennoye (Kostomukshsky State Nature Reserve, northern Karelia) was studied. Variability of groupings identified in the P. longicollis population according to discrete parameters of attachment and trophic-reproductive features is characterized. The diversity, similarity, and distinctions of the morphometric parameters and the contributions of specific groupings to the phenotypic diversity are assessed. The P. longicollis population is represented mostly by the phenotype with a clavate scolex, which is easily differentiated from the nucleiform and the lanceolate phenotypes by larger scolex dimensions and lower variability. We conclude that an essential factor behind the phenotypic diversity and structure of the helminth population is the ecological type of host's behavior, which determines the niche occupied by the population. The adaptive response of the P. longicollis population to change of the host species is manifested in a modified frequency distribution of phenotypes with different attachment features.

The helminth fauna of dabbling and diving ducks (Anatinae) was studied in the territory of Northwestern Russia (Leningrad, Kaliningrad, and Pskov Provinces; Loukhi District of Karelia) and Estonia (Kallaste). In total, 43 helminth species belonging to Trematoda, Cestoda, Acanthocephala, and Nematoda were found. The revealed differences in the species composition of duck helminths in different regions and different seasons are explained mainly by differences in the availability of trophic objects - invertebrates and fish, which serve as intermediate hosts for helminths. Most of the helminths recorded in ducks by their life cycles are associated with fresh waters. Only in the White Sea coast the helminth fauna of the mallard duck consists exclusively of "marine" species that can be explained by feeding on the intertidal zone.

This study is aimed to comparatively estimate host abandonment times for different taxonomic groups of small mammal ectoparasites after host capture in standard snap traps. The input for the analysis was the data obtained in experiments with specially designed trap line with automatic logging of animal capture instant. Overall, our results support the basic rule that ectoparasites abandon the dead host after some time. The most tangible and rapid reduction in infection rates after host's death was detected for Ixodes persulcatus. Fleas and parasitic gamasid mites abandon the host during the first 2-4 hours; lice - 9-13 hours after snap-trapping. Ixodes trianguliceps displayed no sharp decline in abundance for a prolonged period of time.

Results of the study of fleas, parasites of five insectivore and rodent species, dwelling in the territory of northern Karelia, are represented. Fleas were collected in August-September 2001, 2003-2004, 2006-2009, 2011, 2015, and 2019 in environs of the White Sea biological station "Kartesh" of the Zoological Institute Russian academy of sciences. Literary data on peculiarities of distribution, host-parasite relations, and seasonal dynamics of eight flea species distributed in the territory of northwestern European Russia are summarized.

The types of exoskeletal abnormalities and their frequency are described in 293 females and 243 males of Ixodes pavlovskyi occidentalis Filippova et Panova, 1998, collected by flagging from vegetation in the suburbs of Novosibirsk, Berdsk, and Krasnoyarsk in 2017-2021. Six types of exoskeletal abnormalities were detected in 44 females of I. p. occidentalis (15.0±2.09%). The most frequent damage of the scutum surface is "shagreen skin" (77.3±6.32% of the number of tick individuals with exoskeletal disorders). Four females (1.4±0.68%) were registered with two body abnormalities at the same time. Four types of abnormalities were detected in eight individuals of male I. p. occidentalis (3.3±1.15%), which is significantly less than in females. Most males of I. p. occidentalis had paired depressions on the conscutum, so this trait was considered the norm of structure in this species, whereas in Ixodes persulcatus Schulze, 1930, this phenotype is the most common type of exoskeletal disorders. No males of I. p. occidentalis with two anomalies were registered. Similarity of types of exoskeleton abnormalities in I. pavlovskyi, I. persulcatus and Ixodes ricinus (Linnaeus, 1758) was registered. The frequency of exoskeletal disorders in females and males of I. p. occidentalis is less than in populations of I. persulcatus from the Asian part of Russia, but exceeds than in I. p. pavlovskyi. Thus, the proportion of individuals with anomalies in I. persulcatus and I. pavlovskyi in populations from higher latitudes with a continental climate (Siberia) is higher than in ticks living to the South (Primorsky Territory, Russian Island).

The article describes the geographical distribution and parasitizing of the louse Polyplax hannswrangeli in Western Siberia. Parasitizing of P. hannswrangeli has been noted on 8 small mammal species. The list of species most strongly associated with this louse was determined. For five species of small mammals (the common shrew, Siberian chipmunk, Eurasian water vole, East European vole, and root vole) inhabiting the forest zone of Western Siberia, the feeding of P. hannswrangeli was detected for the first time for this region. In Western Siberia, P. hannswrangeli is found only in the forest-tundra and forest zones. It was found that the average population density of P. hannswrangeli in the northern taiga of Western Siberia was 2 times higher than in the forest-tundra and middle taiga zones.