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Attempt to give a definition of the parasitism phenomenon from the symbiotological point of view is proposed. Parasites are the organisms, existence of which is possible only in the parasitic systems of any degree of complexity. Outside the parasitic system the parasite is not capable to realize its main vital function including feeding, dispersion of reproduction. The parasitism as the biological phenomenon could be defined as the existence of organisms in the structure of parasitic systems.

Behavioural actions of the malariae mosquito Anopheles messeae larvae and their correlations with the susceptibility to the entomopathogenic bacterium Bacillus thuringiensis israelensis (Bti) were studied. The larvae were differentiated according to their feeding mode ("filter-feeders" and "scraper-feeders") and to their avoidance reaction on a concussion of water surface by water drops ("divers" and "undivers"). The "scraper-feeders" and "divers" showed a tendency of decreased susceptibility to Bti, while the "filter-feeders" and "undivers" did not. The presence of the white pigment (striped genotypes) on a dorsum of the larvae correlated with the reaction of avoidance and fillotaxis but not with the susceptibility to Bti.

The paper continues the discussion of published materials on the dynamic of Ixodes persulcatus number in mountain forests of the central part of the Krasnoyarsk region during 1958—1990 (Korotkov e. a., 1992). An unstationarity and polycyclicity of examined processus is confirmed. It is shown, that long-term and middle-term quasi-periodical fluctuations of the tick number are determined by successive changings in forest biocenoses going under the influence of respective fluctuations of the climate. The synchronization of climatic and biocenotic processes is complicately organized in time and is not indicated phenomenologically. Certain quasi-periodical components in their indices of the tick number and climate are quite different by their amplitude, value of phase displacement, and longevity of terms. However, these structural differences are natural and determined by peculiarities of the response reaction of I. persulcatus to different intesivity of acting factor. This reaction under extremal environment conditions, both minimal and maximal ones, leads to the same result, the decreasing of the vital activity and the tick number.

Based on a statistic data on cases of different clinical forms of the tick-born encephalites in the Russian Far East during 35 years passed it is stated that ecological relations in natural foci of this infection have clear and stable spatio-temporal specific features and hierarchic structure. The epidemic potential of the tick-born encephalite foci is determined in a great degree by conditions of a cold period of year.

In a result of revision of the genera Heterobrephosceles and Alloptellus two new genera are erected. Phylogenetic relationships of new genera within the family Alloptidae are briefly discussed. Ibidocolus gen. n. (Alloptinae) includes: Ibidocolus sexangularis (Peterson et Atyeo, 1977) comb. n. (type species) and I. furcatus (Peterson et Atyeo, 1977) comb. n. Both sexes of this genus have characters as follows: prodorsal shield separated from scapular shields; setae vi, ve absent; setae c3 lanceolate with elongated apex, situated posterior to setae cp; ambulacral discs with rounded distal margin, its central sclerite transversal, with tridentate anterior margin; tarsi I, II with 3 ventral setae, setae d present on tarsi I—IV, on tarsi II, III these setae are shorter than respective e and f; setae kT III absent; setae mG of genua II lanceolate, acute; setae sR of trochanters III lanceolate or thick setiform. In male: opisthosomal lobes long, separated by deep triangular terminal cleft; interlobar membrane developed all along terminal cleft margin; lateral membranes absent; hysteronotal shield with narrow longitudinal lacuna extending anteriorly to setae d1 and opened posteriorly into terminal cleft; coxal fields III closed or open, fields IV always closed; pregenital transversal sclerite joins anterior ends of genital apodemes and anterior tips of epimeres IV; adanal shields variable in form, as small ovoid sclerites or as big inverted Y; setae h1 absent; setae e1 situated at basal level of opisthosomal lobes; setae h3 hair-like; legs IV slightly greater than legs III. In females: setae e1, h1, f2, ps1, ps2 present; dorsal shields with net-like pattern, hysteronotul shield with or without transversal slit-like lacuna, posterior end of opisthosoma with short rounded lobes; epigynium free, situated at level of epumeres III; setae g absent; medial tips of epimeres IIIa without additional sclerites; copulatory opening on apex of short external copulatory tube projecting into terminal cleft. This genus is distinguished from closely related Heterobrephosceles by having rounded ambulacral discs in both sexes, absence of setae h1, position of setae e1 at basis of opisthosomal lobes, hair-like setae h3, presence of pregenital sclerite in males, external copulatory tube and setae ps2 in females. Diomedalloptes gen. n. (Oxyalginae) includes only the type species Diomedalloptes pacificus (Peterson et Atyeo, 1972) comb. n. This species is most closely related to the genus Oxyalges, than to the Alloptellus. In both sexes: idiosoma greatly elongated, spindle-shaped; prodorsal shield separated from scapular shields by narrow S-shaped furrow; setae vi present; setae c3 situated slightly anterior from setae cp; ambulacral disc with apical spine; transversal part of central sclerite strongly sclerotized, longitudinal part scarcely sclerotized; tarsus I, II with 4 ventral setae; setae d present on tarsi I, II, IV, absent on tarsus III; setae kT III present; setae mG of genua I, II and sR of trochanter III hair-like. In male: Opisthosomal lobes completely fused, posterior end of opisthosoma widely ovoid; setae e1, h1 present; setae h2, h3 short, hair-like, situated on opisthosomal margin; setae ps2 and f2 almost contigious, situated ventrally; setae f2 represented as macrochaetae; genital apodemes fused as long inverted Y, its anterior tip free, posterior ends fused with opisthoventral shields; epimeres Ilia almost touch fused part of genital apodemes; legs I, II equal in size. Female. Setae e1, h1, f2, ps1 present; setae ps2 absent; opisthosoma rounded without lobes; epigynium free, situated al level of epimeres III; setae g present; genital valves free from epimeres IIIa; copulatory opening dorsal; adanal sclerite absent; macrochaetae h3 2—3 times longer than macrochaetae h2. The genus Diomedalloptes differs from the genus Oxyalges by having unpaired setae vi in both sexes, presence of setae h1, setae f2 as macrochaetae in males, and free genital valves in females.

The recent parasite fauna of the perch, Perca fluviatilis, and the roach, Rutilus rutilus, in the estuary of the Neva river (Finnish Bay) was studied. It was possible to compare the recent data with those obtained 36 and 62 years ago. The analysis shows that the parasite biodiversity has decreased as a possible result of antropopressure, but an infection with parasites, adapted to this pressure, has increased.

A survey of literature on the Anguillicola nematodes found in the swimbladder of the eel Anguilla anguilla is given. It has been shown that 2 species of this freshwater nematodes were brought to Europe with the eel A. japonica, living specimens of which were intended for marketing. During 1.5 decades these nematodes mostly Anguillicola crassus have been spread within the area of the eel from West Europe countries to the Baltic sea region. A. crassus was the cause of high mortality of eels in the Lake Balaton, Hungary. Systematics, biology, life cycle in the new area, pathogenicity have been carefully studied. It has been stated that besides intermediate hosts (freshwater Copepoda and Ostracoda) their parathenic hosts are many freshwater fishes, especially the ruff Gymnocephalus cernua and bleak Alburnus alburnus. This phenomenon is not known in natural area of these nematodes.

The life cycle of a microsporidian from the midgut of imagoes of Oulema melanopus has been studied in light and electron microscopes. On the basis of ultrastructural peculiarities of spores, host specificity and tissue location this microsporidian is described as a new species Unikaryon oulemi sp. n. Spores of this microsporidian measured 2.7—2.9 x 1.2—1.4 µm on ultrathin sections, are short-cylindrical. They possess one nucleus and a polar filament forming 11.5—12.5 coils arranged in one row. The line connecting the anterior polar filament rows is tilted 55 degrees to the longest spore axis. Polaroplast is composed of thin lamellae. Posterior vacuole seems to be formed of concentric membranelike layers. Exospore is c. 100 nm, endospore is 50—70 nm thick. The spores of oval shape with less number of polar filament coils can be also rarely met. Both types of spores have the same internal structure except the number of coils. The linkage between of the membrane-like structures of polar sac, polaroplast and posterior vacuole are obviously seen on cross sections of the spores. Membranes of these organelles isolate the spore nucleus from the rest of the spore. All intracellular stages — meronts, sporonts and sporoblasts, possess a single nucleus and develop without a parasitophorous vacuole. Tubular structures of two types (50—60 nm and 30—40 nm in diameter) are visible around intracellular stages of microsporidians in the host cell cytoplasm. The comparative analysis of the species of the genus Unikarion from Coleoptera is given. In the Krasnodar region the extensivity of infection of the field populations of cereal leaf beetles with Unikaryon oulemi varied in different years from 1—2 to 80—95%%, that might indicate to the participation of microsporidians in natural control of this pest.

A seasonal occurence an maturation of acanthocephalans Echinorhynchus salmonis and E. truttae are analysed on a basis of observations carried out in the Baikal Lake (the Chivyrkui Bay). Differences in annual cycles of acanthocephalan abundance (extensity and relative density) and age structure of their hemipopulations in three species of definitive hosts (Coregonus autumnalis migratorius, C. lavaretus, Thymallus arcticus baicalensis) are shown. A seasonal dynamics of occurence and muturation of acanthocephalans is suggested to be induced by cyclic trophic connections between definitive and intermediate hosts.

The ultrastructure of sensorial tips in adult progenetic cestode Diplocotyle olrikii developing in gammarids was studied. 7 morphological types of sensillae were recognized, among them 4 types in apical part of a scolex and 3 types in a tegument of bothriae. Description of all types of sensillae and peculiarities of their disposition on cestode bodies are discussed.

The study of trematodes of the genus Himasthla was carried out in the bird and mollusks populations of the East Murman (Barents Sea). One new species, Himasthla larina sp. n. has been found. Its morphological description, differential diagnosis and life cycle are given. Descriptions of two species, which taxonomic status needs additional research, are also included.

A new monogenean, Gyrodactylus osmeri sp. n., is described form gills of the European smelt Osmerus eperlanus captured in the Ladoga lake.

The commensal fungus Palvasicia beaufortae is recorded in a new host, an isopod Sphaeroma serratum, for the first time in the Black Sea. The fungus is located in the intestine of the isopode.

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Summary is absent.