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Interacting populations of the parasite and their hosts are regarded after Beklemishev (1956) as parasitic systems which are similar to other ecological systems such as predator-prey, herbivorous animal-plant, etc. A structure of parasitic systems formed by five species of acanthocephals of birds is considered herein. It has been established that the abundance of parasites is regulated by one of the definitive hosts as it was shown by Holmes e. a. (1977) for Metechinorhynchus salmonis. The regulation of the abundance is accomplished on the feedback principle. Functions of parasitic systems and their role as a model for a study of population biology of helminths are considered.

The water-saline metabolism in cestodes of Triaenophorus nodulosus and Eubothrium rugosum, parasites of the gut of fresh water fishes of various origin, were studied. Essential differences in the resistance of worms to the changes of salinity of external environment have been stated. It was shown that E. rugosum can stand higher saline concentrations (up to 20 g/l NaCl) than T. nodulosus (up to 10 g/l NaCl). The resistance of E. rugosum to salinity of external medium is absent in T. nodulosus. It is assumed that high resistance to salinity and the hypoosmotic regulation of E. rugosum infecting a fresh water host, the burbot, is the sign of their marine origin unlike the pike parasite T. nodulosus.

The paper presents data on the infection rate of larvae and young of anadromous fishes with metacercarians, agents of postodiplostomosis in the Volga delta and Northern Caspian. The reduction in the infection rate with Postodiplostomum cuticola in larvae and young of fishes after the regulation of the Lower Volga is considered.

1382 specimens of five species of squids were examined for the infection with anisakid larvae. Members of five genera of anisakids were recorded from the squids: Anisakis and Contracaecum (in most cases), Terranova, Raphidascaris and Acanthocheilus (rarely). Helminths have an uneven distribution in the squid's body: larvae of Anisakis are concentrated in general in the stomach's walls, larvae of Contracaecum — in the mantle. It has been established that anisakid larvae do not survive after 11 to 12 hr of squids storing at -20°C.

Radioisotopic method has shown that sporozoites of Eimeria tenella use exogenic ¹⁴Caspartic and ¹⁴Corotic acids for biosynthesis of pyrimidine nucleotides de novo, synthesize RNA. Metabolic pathway of biosynthesis of pyrimidine nucleotides in coccidia of the genus Eimeria are discussed.

Hystological study of hungry imagos of Dermacentor pictus Herm. and other ticks of the genus Ixodes has made it possible to improve the method of physiological age determination of ixodids (Balashov, 1961). The following criteria for age determination are suggested: 1) size of sections of midgut appendages and their structural peculiarities, 2) height of midgut epithelium, 3) amount of haemoglobin inclusions in midgut cells, 4) amount of haematin in midgut cells, 5) cellular structure of midgut epithelium. Four main physiological ages can be determined by means of the above method (Razumova, 1977), which correspond to four degrees of nourishing according to Balashov (1961). By means of the method a stereotype of seasonal changes in the age structure of D. pictus population has been found out — a younger composition in spring and old one in autumn.

The mite Haemogamasus ambulans is characterized by a mixed type of feeding such as haematophagy, zoophagy and schizophagy with a prevalence of haematophagy. Maximum fecundity of females took place when they were kept on complex diet (blood and living objects). When fed only with blood mites exhibited pronounced cannibalism that resulted in the reduction of their progeny. In addition to gamogenesis parthenogenesis was noted.

The paper presents results of approbation of the use of experimental areas (100 m² in size) for characteristics of the populational density of adult hungry Ixodes persulcatus. Data of a single catch (during the seasonal peak of abundance) of the ticks in these areas are used. The obtained results are suitable for characteristics of the abundance and spread of ticks inside the population. Correlations between indices of a relative and absolute estimation are given. It was concluded that the number of ticks over a vast territory can be in most cases estimated by means of 50 to 100 areas. The method of absolute estimation of ticks can be widely used for field ecological and epizootological investigations.

10 foals were artificially infected for the first time with eggs of Gastrophilus intestinalis,. G. veterinus, G. haemorrhoidalis, G. inermis, G. nigricornis. 63 to 67% of the 1st stage larvae die in the first period of life after emergence from the egg, at the moment of invasion into the skin and at the beginning of the migration. In 40 to 45 days after the infection most larvae (73 to 82%), depending on their specific belonging, were found in the stomach, duodemum and rectum.

Buccal capsule of males and females of Syngamus skrjabinomorpha was found to be surrounded by the cuticular corolla which is more developed in females. The cephalic end of these nematodes has two types of sensory organs: two pairs of submedian papillae and a pair of amphids. Submedian papillae of males rise above the cuticular corolla and their conjugate nature is clearly seen Papillae of females are plunged in largely spread cuticular corolla. Amphids are plunged into the cuticular corolla too. The corolla cuticule in females is smooth while in males has a thin transverse furrowed structure.

A new species of allantonematids, Psyllotylenchus rectangulatus sp. n, from the flea Ceratophyllus rectangulatus Wahlgr. collected in the Laplandsky State Reserve is described.

It has been shown that ticks possess cellular factors of organism protection which fulfil functions of seizure and digestion of different microorganisms in a way similar to phagocytosis. In response to the introduction of bacteria lysozyme quantity increases in tick's haemolvmph that exerts a bactericide effect on the introduced strains Micrococcus lysodeikticus 2665, Staphilococcus aureus 209 and Salmonella typhimurium IT-2. When administering phage FX-174 into tick's haemolymph a persistence of phage particles during 1—2 months is observed. Specific antibodies developing in response to introduced phage corpuscles were not found.

The tick Rhipicephalus pumilio P. Sch. is first reported in the West Pamir (Mountain Badakhshan, 2700 m above the sea level). Adult ticks and nymphs were collected on Tolai hare, in the flood land of the Gunt river. The abundance index is 28.4. The collections were conducted in the end of June. An ecological adaptation of the tick is compared to that in other landscape zones.

In the walls of the stomach of freshwater fishes Etroplus suratensis Bloch 1765 larvae of water mites of the family Hygrobatidae of the genus Unionicola sp. were found (the state of the available material does not allow the authors to give a more precise definition). Larvae were incapsulated unviable. In the authors' opinion the larvae of hydracarines are not parasites of fishes in this case.

Studies of natural enemies of bloodsucking Diptera have attracted recently a special attention. Aquatic arthropods and members of aquatic Hemiptera and Coleoptera, in particular, are most abundant regulators of the number of preimaginal phases of bloodsucking flies complex.

Two new species of black-flies, Hemicnetha tsharae sp. n. and Simulium vershininae sp. n., from Transbaikal Region are described. Both species belong to the groups including few (in the first case one) species.

A new method is suggested for obtaining cultures of ticks' organs in a simple microchamber constructed by means of ordinary cover glass and slide. The method permits long life observations, reduces expenditures of cultural medium to minimum and does not require the addition of antibiotics.

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