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The effect of group was studied on cattle being attacked by horse flies of three genera. The method of simultaneous registrations of attacking horse flies in herds of 8 to 100 animals and on single cows was used. It has been shown that the effect of group reveals itself only when animals in the herd reach a certain minimum number, the effect rate depending on peculiarities of attacking of a given species of bloodsuckers, such as a part of responding individuals, distance of an attack, duration of contact with an object. These parameters tend to change with increasing number of animals in the herd. Therefore differences in the intensity of attacks on herds with different cattle stock cannot be explained proceeding only from differences in the occupied areas. The number of attacking horse flies decreases from the periphery of the herd to its centre and is not the same in different parts of the periphery. The effectiveness of attacking, ie the part of sucking individuals of a given species (genus) from the number of horse flies attacking for a definite period of time, is the highest in a large herd and increases in its ranges from the periphery to the centre. This dependence leads to a more even distribution of sucking individuals as compared to attacking ones.

10 elementary behavioural acts (EBA) were distinguished in the behaviour of the taiga tick during its spring activity. The acts inclyde repeatable and indivisible into more simple forms poses (active and passive waiting poses, resting pose, hiding pose), movements (scanning movement, Haller's organ cleaning, contacting movement) and locomotor acts (locomotion to a zone of contact, locomotion to a shelter, locomotion to a source of scent). EBA form 3 behavioural programs: entering a zone of contact, moving to a shelter and attacking a host. The first two of the programs are turned on by processes of water loss and uptaking in the organism of the tick, the third program — by signals of the host. The system of describing of ticks behaviour, offered by the author, can be used by investigators dealing with the ethology of ticks.

In four studied Dermacentor species at preimaginal phases sexual dimorphism hardly reveals itself in body size and mass of individuals. The formation of sexual distinctions at the nymphal phase concerns different characters as in different species so in populations of one species. In D. niveus male and female nymphs differ in the length of II—III palpal joints and width of gnathosoma, in D. ushakovae in the length of scutum and its proportions, in the width of gnathosoma and hypostome and in the diameter of peritreme. The sex of D. silvarum nymphs can be identified by the width of gnathosoma, length of hypostome and diameter of peritreme. Female and male nymphs of D. marginatus from the Stavropol Territory and Armenia differ in the scutum proportions and populations from the West Pamirs in the length of scutum and gnathosoma.

Virgin and cultivated biotopes of Donets Priazovje were investigated for parasite coenoses of small mammals. 469 animals and 210 nests were collected, Apodemus sylvaticus, Mus musculus, Microtus arvalis and Cricetulus migratorius being the most abundant species. In all 2940 larvae of trombiculids (18 species) were found on animals and in their nests (2846 specimens on animals and 94 specimens in nests). Two species, Hoffmannina theodori and Doloisia skljari, are new for science, the latter being the first find of the genus Doloisia member in the fauna of the USSR. The majority of parasites are collected from rodents inhabiting virgin biotopes (2328 specimens) while in cultivated biotopes 189 specimens were collected. More rich is the species composition of trombiculids in virgin biotopes (13 species) as compared to that in cultivated ones (5 species), Neotrombicula earis, N. japonica, N. autumnalis and N. vulgaris being mass species. 8 species of larvae are recorded from the nests of rodents. Seasonal dynamics of the abundance of mass species, their occurrence, index of abundance and infection intensity are shown.

Microsporidia of the genus Ameson were recorded from larvae of horse-flies of the genus Hybomitra in Karelia. Earlier these Microsporidia were recorded from crustaceans. The infection extensiveness ranges from 5.1 to 10.5%. The parasites develop in musculature, fat body and salivary glands of the host. The new species has uninucleate, single-located egg- and pear-shaped spores. The ultrafine structure of developmental stages and spores is studied.

A study of the distribution of Ancyrocephalus paradoxus in gill microbiotopes of Stizostedion lucioperca has shown that the main factors affecting it are not only the speed and direction of the ventilation stream washing the gills and active search for the places of optimal habitat by the parasite itself but also the density of the parasite population, host's age and biology, and season.

Investigations conducted during two seasons on one summer old carps have shown that the summer distribution of bothriocephalids corresponds to Poisson distribution while in autumn the parasites decrease in number and their distribution acquires an overdispersed form. The conducted crossings have made possible the determination of the role of hereditary factor in the variability of fishes on the infection with bothriocephalids. In summer when the distribution of parasites corresponded to Poisson distribution no differences in the infection of fishes with bothriocephalids of different genotypes were observed. In autumn, with an overdispersed form of distribution such differences took place. This fact proves that the initial infection, in July, is of non-selective character and in autumn changes for differential one.

In order to elucidate the functional role of cercomere in larvae of the monocercus type their transplantation from the specific host Chironomus obtusidens to Gammarus lacustris was conducted. At early stages after the transplantation proceeds an increase in the functional activity of the tegument of follicles of the cercomere followed by their complete destruction in 3 or 4 days. On the surface of the exocyst membrane an adhesion of the host's haemocytes occurs, which becomes more distinct in 3—4 days when the process acquires a character of local incapsulation. Within the same period, in the places of haemocytes aggregation, a local resorption of the exocyst external membrane takes place. Later intensification of the host response to transplant is associated with the destruction of follicles of the cercomere. In one case the occurrence of follicles of the cercomere in the cavity of cysticercoid was observed that is caused by the microbe affection of the latter. In the zone of contact of the tegument of scolex and neck with follicles of the cercomere an increased secretion (the microapocrine type) of the tegument, disturbance of the microvillous tegument of the cercomere's follicles and their destruction are observed. Incompatibility of the tegument of definitive departments and cercomere, which arises during differentiation of larvae, is supposed to affect the formation of scolex invagination in the evolution of larvae of Hymenolepidats.

Biological and ecological peculiarities of the little-known trematode Parvatrema duboisi, which uses Black Sea mussels as a second intermediate host, have been studied during several years. New data were obtained on the distribution of the parasite in rocky colonies of mussels, relationships in this host-parasite system, distribution of the trematode in the Black Sea, effect of some ecological factors on its distribution.

Intrastrain heterogeneity of muscular larvae of trichinellids has been revealed in experiments of free sedimentation and isopyknic division in the density gradient of saccharose, the presence of which is confirmed by the study of morphological characters and infection activity. The presence of intrastrain heterogeneity indicates the necessity of introduction of standard technique for studies of biological characters of different strains of trichinellids.

Hystochemical reaction with glyoxylic acid has revealed a specific fluorescence caused by the presence of indolamine (apparently serotonin) in the nervous system of larva and mature cestode. Fluorescence manifests itself in neurons and nerve fibres of the central ganglion and its commissure, in nerve cells of the proboscis, in longitudinal trunks and transverse commissures, and in the nerve elements connected with genital system.

Three different cell types are distinguished in the sarcocysts of Sarcocystis fusiformis from the water buffalo: metrocytes, intermediate cells, and merozoites. The former lines the cyst near the border, the two latter lie more inside forming groups of cells separated from each other by septae. The pattern of metrocyte division giving rise to another metrocyte population has not been observed, still remaining obscure. Merozoites do not divide asexually due to their gamont nature to be realized in thefinal host only. The intermediate cells divide asexually by endodyogeny giving rise, on the one hand, to another population of intermediate cells, and on the other — to merozoites which divide no longer. Cytophotometrical measurements (Gaibova, 1987) revealed the amounts of DNA per cell nucleus within 1 and 2 c. This quantity corresponds to the stable DNA content in the most numerous population of merozoites (gamonts), whereas the amount between those higher than 1 с and 2 с may be attributed to the nuclei of metrocytes and intermediate cells, respectively.

The life cycle of the cestode Batrachotaenia carpathica, parasitic in newts, has been studied. It has been established experimentally that different freshwater copepods serve as an intermediate host. Detailed descriptions and figures of developmental stages of this cestode are given.

Data are given on the infection of Black Sea and Far East crabs with Rhizocephala (Sacculinidae) and on the structural peculiarities and developmental rate of the surface of rhizomorphous processes in two members of the family Peltogastridae, Briarosaccus callosus and Peltogaster paguri. Their structure has a number of distinct characters: in Briarosaccus callosus these processes are more dense, their ramification in more complex, morphological parameters are some orders higher and tissues contain a greater amount of excretory fat.