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Specificity of partners in host-parasite system is one of its main characteristics. Unfortunately this term has different senses in scientific literature. In everyday practice one judges an extent of host specificity of a parasite mainly by indices of its occurrence and abundance on different host species. An occurrence of parasites in nature reflects general result of complex eco-physiological interrelationships between partners in hostparasite system. Specificity of parasites in a choice of hosts may depend on a belonging of the latter to certain taxa (phylogenetic specificity), or on biotic and abiotic factors (ecological specificity). In arthropods, the phylogentic specificity and coevolution are characteristic to a greater extent for permant hosts (lice, Mallophaga, cheyletoid and feather mites). A coevolutionary phylogenesis is disturbed by transfers of parasites onto new hosts, by differet rates of speciation in filial lines or by an extinction of several parasite taxa. In temporary parasites different forms of ecological specificity are prevalent. A host specificity is expressed to the lesser extent in mosquitoes, horseflies and in other blood-sucking Diptera. In temporary parasites with a long-term feeding (ticks) coevolutionary sequences are relatively rare, because this parasites had to adapt not only to a life on host, but also to a lesser stable environment. In some nest-burrow bloodsuckers (fleas, gamasid mites and argasid ticks) the ecological specificity is shown no by their relations with certain host species, but by an associations with habitats occupied by hosts (burrow, nests, caves). In relation with a high dynamics of host-parasite system, a specificity of its partners is comparative and it is kept up only under specific ecological conditions.

Histopathological changes of skin of Passeriformes birds in the places of attachment and feeding of the Ixodes persulcatus and I. lividus were investigated. The inflammation of bird skin is analogous to one of mammalian skin. The mouthparts of tick are surrounded by the fibrin cone and collagen capsula. They are formed by host. Ticks of these species do not form the cement. A proliferation stage of skin inflammation in birds is expressed. The thick collagen capsula develops in a result of proliferation. The feeding cavity is formed by the process of sucking a blood, inflammate cell infiltrate and products of tissue lysis. Neutrophils and eosinophils predominate in the inflammate cell infiltrate.

A revision of chigger mites species being closely related to Neotrombicula talmiensis (Schluger, 1955) has been performed. 2 new species are described: N. pontica sp. n. from Krasnodar Territory (Western Caucasus) and N. sympatrica sp. n. from Krasnodar Territory, Daghestan, Tuva, Armenia, Kirghizia and Turkey (Rize Province). N. pontica sp. n. is closely related to N. carpathica Schluger et Vysotzkaya, 1970 and differs from this species by the larger number of idiosomal setae (NDV = = 75—99 against 63—77), shorter legs (Ip = 782—847 against 844—920, TaIII = 67—74 against 70—80), lesser m-t (0.180 against 0.192), slightly lesser scutum and slightly longer setae. N. sympatrica sp. n. is closely related to N. carpathica and differs from this species by the longer scutal and idiosomal setae (PL = 67—78 against 57—69, H = 65—75 against 57—68, Dmin = 43—52 against 39—48, Dmax = 61—70 against 54 — 64), longer legs (Ip = 892—973 against 844—920, TaIII = = 77—86 against 70—80) and lesser m-t (0.168 against 0.192). N. carpathica is reported for the first time from Northern Caucasus (Karachai-Cherkess Republic, Kabardino-Balkaria, North Ossetia); N. talmiensis is reported for the first time from Khakasia. Data on joint occurrence of 3 species are reported. The 3 types of sympatric pairs of the species have been found in the Western Caucasus: 1) N. pontica sp. n. and N. sympatrica sp. n., 2) N. sympatrica sp. n., and N. carpathica, 3) N. pontica sp. n., and N. carpathica. Differences between species in that localities, where the joint occurrence was recorded, have been analysed. Sympatric relations between N. sympatrica sp. n. and N. carpathica are characterized by increased value of some general diagnostic characters of these species, such as the length of idiosomal setae and length of legs. Different characters play main distinguishing role in different sympatric localities. Besides that, some local features appear in certain species of sympatric pairs (narrow scutum in 3 samples of N. carpathica, numerous idiosomal setae in 1 sample of N. sympatrica), that differs the populations of the sympatric pair, and also discriminates such aberrant group from other material of the same species. Functions produced by the method of discriminant analysis are proposed for the purpose of diagnostics in talmiensis group. Tests for assumptions of discriminant analysis, such as normality and homogeneity of variances, has been performed beforehand. Verifications of the functions on test samples demonstrated the good quality of its working. Key to species using discriminant functions has been constructed.

Four new species of the nasal mite family Rhinonyssidae collected in different regions of the former USSR are described: Neonyssus (Otocorinyssus) alaudae sp. n. from Alauda arvensis L. (Alaudidae, Passeriformes) from Turkmenistan; Rhinonyssus clangulae sp. n. from Clangula hyemalis (L.) (Anatidae, Anseriformes) from Yakutia; R. marilae sp. n. from Aythya marilae L. (Anatidae, Anseriformes) from the Russian Far East; Locustellonyssus sibiricus sp. n. from Locustella certhiola (Pall.) (Sylviidae, Passeriformes) from Siberia.

The body-wall musculature of invasive cercariae and metacercariae of Diplostomum chromatophorum at different intervals after the penetration into the experimental intermediate host Cyprinus carpio (1, 3, 5, 6, 7, 10, 12, 20, 34, 40 days) has been investigated with the help of ТЕМ technique. During the first 10 days after the invasion (in conditions of our experiment), the cercarial subtegumental muscle fibres degenerate. These muscles are replaced by newly formed ones. Mass differentiation of myoblasts beneath the tegument was observed in 7—10-day-old metacercariae. Obtained data indicate the metamorphosis of body-wall musculature during the morphogenesis in Diplostomum chromatophorum metacercariae.

Distribution of Bunodera luciopercae marites in the intestine of the fuff, sande and perch were studied in the Rybinsk Reservoir. For the fuff and perch fry, except three summer months, the more proportion of helminths was observed in the posterior region of the intestine. Because of a continuous passage of B. luciopercae in predatory fishes (sanders and adult perches), more proportion of trematodes was always observed in anterior region of the intestine. In all groups of hosts, 20—30% of total parasite number was registered in the middle intestine.

The haptors Dactylogyrus spp., Anacanthorus sp., Trianchoratus sp. and Schilbetrematoides pseudodactylogyrus are investigated. On the base of the morphology, transfer of the domus, etc. a homology of the hooks in dactylogyrids (s. s.) and the pin-like structures (4 "А") in the haptor of Anacanthorinae and Dactylogyridae sensu Bychowsky et Nagibina, 1978 is demonstrated. The vestiges of the anchors in the haptor of dactylogyrids (s. 1.), according to morphological data, correspond to the point of anchor formed during the ontogenesis. In S. pseudodactylogyrus the pins are redescribed as typical vestiges of the anchors.

The paper provides the data on the invasion of unionids in the Zhitomir Polesye by parasitic worms Aspidogaster conchicola. The aspidogasters have been found in 9 species of molluscs (Unio — 5, Colletopterum — 1, Pseudonodonta — 1, Batavusiana — 2). An extensity and intensity of invasion of unionids by A. conchicola are unequal in different host species. The highest values of them were recorded for Unio conus borysthenicus (87% and 12 specimens), U. tumidus falcatulus (75%; 5 specimens), and Batavusiana nana (59.8%; 20 specimens). The invasion of molluscs by aspidogastrids from the stagnant waters are higher, than from the fluvial waters. The aspidogasters were found in the pericardium and kidneys of unionids. More often the parasites were met in the pericardium exclusively, rarely in the pericardium and kidneys simultaneously, and most rarely in the kidneys only. Within the pericardium, A. conchicola prefer the furthest sites of the contractile ventricle of the molluscan heart.

Summary is absent.