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On the basis of the analysis of morphology and biology of representatives of the Parasitengona, mostly trombiculids, trombidiids and water mites, a new attempt is made to clarify probable evolutionary scenario in this group of the higher trombidiform mites (Actinedida). It is supposed that the very old ancestral group of terrestrial arachnids, having bite-sucking mouth-parts, poorly differentiated sac-like midgut and capability to extra-oral digestion, fed predatory on different small soil arthropods at all phases of the life cycle. They were small segmented orthotrichous homeomorphic arachnids at the rank of genus or family. The favorable feeding conditions of the adult phase have led to the small eggs rich in yolk and the small larva. The latter have led in turn to the necessity of intensive feeding at the larval stage to complete the ontogenesis. Further in evolution, this group gave rise at once to two or even more large paraphyletic branches. Most of them retained feeding on arthropods with transition of larvae to much more effective parasitic feeding provided with the additional specialization of the larval stage. This branch comprise divergently radiated paraphyletic terrestrial and secondary-water water mites each having long course of evolution resulted in the recent groups of Calyptostomatoidea, Erythraeoidea, Trombidioidea and several superfamilies of water mites. Another branch of the ancestral Parasitengona has followed the way of adaptation of larvae to feeding on vertebrates, which were being attacked by the larvae in the environment of pasture. The parasitism on vertebrates has lead to several radical specializations of these mites and their significant evolutionary progress. At the same time, the similar ontogenetic dynamics, as well as synchronous reduction of particular developmental stages in all parasitengones, inevitably indicate the monophyletic origin of the whole branch of Parasitengona with Pterygosomatidae as the most probable sister group.

Seven mosquito species of the genus Ochlerotatus ("cantans" and "dorsalis" species groups, family Culicidae) were found in the Novgorod Region as a result of our field investigations, which have been carried out in 1996—2004. One species, Ochlerotatus annulipes, was for the first time recorded in the Novgorod Region. Distribution, biotopic and seasonal preferences of all discovered species are considered.

Twenty one coccidian species from the genera Eimeria and Goussia are found in the perciform fish hosts from the continental waters of Russia. Names, authors and dates of the species descriptions, synonyms, brief figured description of the morphofunctional organization of the endogenic developmental stages of coccidian, Latin and Russian names of the fish hosts are given.

Investigations of the indigenous microflora associated with the mucous intestines of fish and its cestode parasites have been for the first time carried out using the methods of transmission and scanning electron microscopy. New data on the bacterial biodiversity in the cestode and its fish host are obtained. Nanobacteria and spirochaetes are for the first time revealed in a fish host together with the previously known bacteria forming the intestinal microflora of fish. Spirochaetes were shown to be associated with the intestines of a pike host only, while nanobacteria cover abundantly the surface of the apical parts of the intestinal microvilli and the apical parts of the microtriches in the cestode tegument. The similarity of the bacterial floras associated with the apical surface of the parasite tegument and the intestine of the host should be noted. At the same time, deeper bacterial communities represented by obligate symbionts are specific. Thus, there is a normal indigenous microflora in cestodes, associated with the tegumental surface. This symbiotic microflora has specific morphological features and provides the balance of relations in the parasite-host system.

Expanded and more detailed version of the conceptions concerning functional organization of the tick-born encephalitis natural foci, which were proposed by the author recently, is given in response to the critical review of his monograph "Functional organization of natural foci of zoonosis infections (by an example of tick-born encephalitis foci in the south of the Russian Far East)" (Prof V. K. Yastrebov, Parazitologiya, 2005, 6).

Summary is absent.