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Analysis of epidemiological and general biological aspects of modern concept of natural nidality of diseases of man has shown that obligato-transmissible diseases hold a prominent place in the phenomenon of natural nidality. The greater the role of the transmissible factor in the ecology of an infection, the better the main characters of natural nidality are displayed on its example. Population-ecological, immunological and genetic investigations of relations between bloodsucking arthropods and agents of natural-nidal infections are given much attention.

Immunity, especially aquired one, was found to play an important role in preventing the animals against falling ill with hypodermatosis. According to the results obtained, the intensity and duration of immunity, as the main factors of non-susceptibility, depend on the infection intensity, recurrence of the desease, periods of mortality of warble flies larvae and fatness of animals.

The necessity is substantiated to consider A. leucomelas and A. implicatus as one holarctic species on the basis of morphological characters of the larva, female and male, their distribution and ecological peculiarities. It is suggested to assign the name A. leucomelas Meigen to the species and the name A. implicatus Vockeroth regard as a synonym.

The age structure of the natural population of X. conformis was studied by means of luminiscent microscopy. The method has made possible a detail differentiation of females into age groups and can be used in field conditions. All age categories constituting a scale developed on a laboratory culture of fleas can be found among individuals caught on animals and in their nests. The autumn population consists of females that fed and did not reproduce and of individuals that interrupted their reproduction. The spring population is composed by reproducing females at different stages of reproduction. It has been established that in October begins the autumn—winter interwal in the reproduction of X. conformis and in the first half of April their spring activation that is in good agreement with published data.

The louse Hoplopleura edentula Fahrennolz occurs not only on the vole Clethrionomys glareolus Schreb. but also on the other members of the genus widely distributed in the USSR, C. rutilus Pall. and C. rufocanus Sund. The characters distinguishing H. edentula from the close species H. acanthopus (Burm.) described from Microtus arvalis Pall. are the differences in the shape of the dorsal lobe of the pleural plate of the third abdominal segment and in the size of the male copulative organ.

During the feeding of N. pomeranzevi larvae on their natural hosgs, large-toothed redbacked voles (Clethrionomys rufocanus Sandev.), in the skin of the latter are formed tubular structures or stylosomes characteristic of all trombiculids. During the formation of the stylosome the saliva of the mites and the interstitial fluid of the host are supposed to interact. The walls of the stylosome consist of two layers, are homogenous and do not include cellular elements. Around the stylosomes are formed the foci of necrosis and destruction of tissues. Behind the distal ends of the stylosomes are observed light interstitial cavities, which of then contain a great number of cellula relements of lymphoid and epithelioid nature. These cavities serve apparently as a reservoir of nutritive substratum for larvae. The feeding of trombiculids is accompanied by a reactive response of the connec tive tissue, the hyperemy of the superficial capillaries and the cellular infiltration of the affected area. The mass parasitism of larvae causes the ulceration of the host's skin and on its surface appear crusts from necrotic tissues.

A serological study of Leishmania clones from experimentally and naturally infected sand flies was undertaken. The cloning was done by means of Fonbrun's micromanipulator. Serological studies were conducted according to Adler's method in Safjanova's modification. Among the clones of Phlebotomus papatasi experimentally infected with Leishmania tropica major and L. gymnodactyli four were found to be serologically identical with L. tropica major, and two had close antigene relationships with both initial strains of Leishmania. No clones identical with L. gymnodactyli were found. By means of serological comparison to museum strains of Leishmania heterogeneity of the strains isolated from naturally infected sand flies and great variety of antigenic properties of clones obtained from them have been shown. Thus, among clones of Ph. caucasicus four are identical with L. donovani, one is closely related to Leptomonas pessoai and one displays no antigenic relationships to any standard strain of Leishmania. Two clones from naturally infected Ph. papatasi are identical with L. tropica major, one displays close antigenic relationships with L. tropica major and L. donovani and one could not be identified. The question is raised on the necessity to study possible mechanisms of genetic information exchange to explain the appearance of clones with mixed properties.

The spleen of 73 white mice was studied (15 mice were studied by electron microscopy) from 24 to 120 hours after intraperitoneal infection. The successive stages of interaction of parasites with cells of mononuclear phagocytes system were observed. At the early stages of the infection the activation of these cells takes place. During the dissemination of endozoites over the host organism they penetrate the activated mononuclear phagocytes. Activation, however, is not effective enough since in these cells the parasites preserve the ability to hinder the fusion of lysosomes with phagosomes containing endozoites and at later stages of the infection to lyse the membrane of parasitophore vacuole. The ability to reproduce in activated mononuclear phagocytes and to lyse the membrane of parasitophore vacuole is regarded as a factor of the virulence of endozoites of RH strain toxoplasms.

It has been established that all peculiarities of the structure of bothridia and additional suckers in adult tetraphyllids are characteristics of their plerocercoids too. In most cases longitudinal sections of the scolex must be done in order to elucidate the real structure of the above adhesive organs.

The hatching process of miracidia of Ph. rhionica is described. Mature eggs are contained in the uterus of the fluke. Hatching of the eggs and liberation of the miracidia take place in the water immediately after the eggs have left the flukes. After hatching the size of larvae increases and they begin to swim actively. The main reason blocking the hatching is the high osmotic pressure in the uterus of the fluke which is created by some substances secreted in the secretory cells. The same cells are found only in the uterus of "viviparous" trematodes.

On the basis of original and literary data more precise descriptions of species, their distribution on hosts, diagnoses of genera, family and superfamily are given. The paper describes biological peculiarities of nematodes of this family and gives some instructions concerning the study of larval and mature skrjabillanids.

Data are given on the life cycle of the nematode Abbreviata kazachstanica. New intermediate hosts of the species have been established as follows: 10 species of Goleoptera, 8 species of Orthoptera and 1 species of Mantoptera. In the intermediate hosts larvae of A. kazachstanica moult twice and in 20 to 23 days (in Orthoptera) and 26 to 29 days (in Coleoptera) reach the invasional stage. Rana ridibunda and Gymnodactilus russovi served as experimental reservoir hosts. The scheme of the developmental cycle of the nematode is given.

Hypopigia of males of the species C. abchazicus and C. filicinus (subgenus Avaritia), are compared. Distinct differences in the structure of aedeagus are aveliable character for distinguishing these species.

The necessity of every day collections of I. persulcatus in the second half of the day for a determination of their absolute number in registration areas has been shown.

Davisia amoena sp. n. has been recorded from Hippoglossoides platessoides from north-western Atlantic (in 36% of fishes). It differs from closely related D. longibrachia in the structure of vegetative forms, a smaller size of polar capsules and longer (1.5 times) lateral appendages. D. newfoundlandia Yoshino a. Noble, 1973 firstly recorded from Coryphaenoides holotrachys in the region of Falkland Islands (in 4.5% of fishes) is redescribed.

A new species, Gyrodactylus editus sp. n., is described. The species infects gills and fins of Schizopygopsis stoliczkai Steindachner, 1866 from the basin of the Piandzh river (the Pamirs).

M.M. Zavadovsky (1891—1957) embriologist and geneticist is one of the founders of the experimental helminthology in the USSR. Using original methods he in twenties— thirties studied the structure of the egg membranes in some parasitic nematodes, prooved the necessity of the access of free oxygen for the development of their eggs, studied the role of some environmental factors in the ontogenesis of nematodes. On this basis Zavadovsky worked out certain reeomendations for the prophylaxis of nematodoses of agricultural animals. His works have not lost their significance up to now.

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