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A total of 383 fleas of 11 species were collected from 428 bank voles (Clethrionomys glareolus) near Babaevo Vill. (59 degrees 4', N, 35 degrees 8' E). Three species (Amphipsylla rossica, Doratopsylla dasycnema, Palaeopsylla soricis) are not typical for these rodent hosts, moving onto the latter occasionally from other animals, such as the common vole Microtus arvalis and shrews inhabiting adjacent or similar biotopes. Peromyscopsylla bidentata, infesting the bank vole in the most part of its range, was recorded only as a single female. Megabothris turbidus was also very rare, being sporadically recorded from May to October. Probably, study area is the northernmost border of its range. The most abundant species, Ctenophthalmus uncinatus, had two picks of average abundance: in April, when imagoes emerge from overwintering cocoons, and in July. Then the number of fleas decreased and the last solitary individuals were recorded till December. Amalaraeus penicilliger is a perennial parasite, being most abundant in winter. Other species were few in number. Imagoes of Megabothris rectangulatus were recorded from April to August, possessing two peaks of abundance: in April, when they emerged from overwintered cocoons, and in July, when the second generation emerged. Peromyscopsylla silvatica is the monovoltinous species emerging in July-August and parasitizing till September. Rhadinopsylla integella was the most abundant in October-December, but a few specimens were recorded in January. The polyxenous species Hystrichopsylla talpae emerged in late July and occurred till September.

The paper reports the results of eight-year investigations on the ectoparasites of rodents and insectivores carried out in southern taiga of the Ilmen-Volkhov lowland (Novgorod Region) and Kurgolovsky reserve (Leningrad Region). Twelve species of small mammals were captured including three dominate species--bank vole Clethrionomys glareolus (2722 specimens), common shrew Sorex araneus (1658 specimens), and wood mouse Apodemus uralensis (367 specimens). Parasite community of the bank vole comprises 34 species of mites, ticks, and insects, the community of common shrew comprises 25 species, and the community of A. uralensis includes 28 species. Taxonomic diversity of the ectoparasite communities was shown to be based on the diversity of types of parasitism and ecological nishes of the host body. Permanent ectoparasites are found to be represented by 2 species of lie and 14 species of acariform mites. The group of temporary parasites includes 13 species of fleas, 10 species of gamasid mites. 3 ixodid species and 1 Trombiculidae. There is a common pool of temporary parasites of small mammals in the ecological system of taiga. Significance of different shrew and rodent species as hosts were found to be dependent on the population density in possible hosts and many other factors. Species diversity in the parasite communities of different small mammal species is dependent on the number of possible ecological nishes in the host body. Actual infill of these nishes by ectoparasites is usually lesser than potential one. Species composition of temporary parasites, their occurrence and abundance changes according to season. Interspecific competition in the temporary parasite species can decrease because of the seasonal disjunction of their population peaks. Diversification of the ecological niches of ectoparasites allow simultaneous feeding of more parasite individuals on one host, than in the case of parasitising of single species or several species with similar ecological nishes. The distribution of parasites on their hosts was also studied. The aggregative distribution has been found in ixodid larvae only, and the distribution of fleas was close to the Poisson distribution. Deviations from the aggregative distribution can be an effect of several independent factors, including limited ability of small mammals for providing numerous parasites with food. On the most part of hosts simultaneous parasitizing of no more than 1-3 individuals of each tick, mite, and flea species was registered. Excessive infestation by ectoparasites may probably be limited by effective reactions of self-purification in the mammal hosts.

Species composition, abundance, annual cycles, and host association of fleas parasitizing small mammals were investigated. The problem of niche differentiation in these insects is considered on the base of the comparative analysis of their annual cycles. The annual cycles of the fleas are revealed to be similar in the case of few number of flea species in parasite community. Thus, two species parasitizing Sorex araneus (Doratopsylla dasycnema and Palaeopsylla soricis), as well as three species associated with Apodemus uralensis (Megabothris turbidus, Ctenophthalmus agyrtes, and Ct. uncinatus) have equal phenology of parasitizing. The fleas community of Clethrionomys glareolus is characterized by a large species number and high diversity of the annual cycles. The differentiation by the seasons of parasitizing is observed most clearly in the dominant flea species, namely Amalaraeus penicilliger, Ct. uncinatus, and Peromyscopsylla bidentata. The periods of imaginal life are overlapped significanly in these species, but they are differed by the season of dominance. Ct. uncinatus predominates in spring and summer, while P. bidentata predominates in autumn, and A. penicilliger predominates in winter and early spring. It may be noted also, that niche partitioning was not observed in the fleas having wide range of hosts. The imaginal life of such fleas usually does not go beyond the warm season.

Seven fleas species were revealed on Apodemus uralensis. Only three of them (Ctenophthalmus agyrtes, Ct. uncinatus, Megabothris turbidus) are the main parasites of this rodent species and have similar phenology of imago. They appear in April, parasitise during spring and summer periods and disappear in autumn, in September-October. The abundance of all three species shows two peaks in a year, which correspond to two generations. Four species (Amalaraeus penicilliger, Ct. bisoctodentatus, Peromyscopsylla bidentata, P. silvatica) are not peculiar to Apodemus uralensis. They pass on this host species occasionally from other animals inhabiting forest biotopes.

In the course of 5-year stationary investigations of the common shrew Sorex araneus in the north part of the Novgorod oblast, 12 flea species have been recorded on this host. Among them, Palaeopsylla soricis and Doratopsilla dasycnema are specific parasites of small insectivores, including the common shrew. Hystrichopsylla talpae is a polyxenous species, parasitizing both rodents and insectivores. Other 9 flea species are not common parasites of S. araneus and apparently have come to this host from other mammal species inhabiting the forest biotopes in the area of investigation. P. soricis and D. dasycnema have similar phenology of parasitism. These fleas appear in spring (April), are present during summer and autumn and disappear in winter. During a year, the abundance of these species shows three pikes, which correspond to three generations: spring (April), summer (June-July) and autumn (September-October) generation. The first species dominates in spring (April-May) and in the autumn-winter period, while the second species predominates in summer. These two species comprise over 90% of total number of fleas collected and determine general dynamics of the flea number on the shrews; during the year the flea number has a sinusoidal increasing from spring to autumn and minimum in winter.

Species composition, seasonal dynamics, and a load of ectoparasites per individual and population of the common shrew Sorex araneus have been examined in coniferous and mixed forests of the Ilmen'-Volkhov lowland (a neighbourhood of Oskuy village, Chudovo district, Novgorod Province) during the period 1999-2003. Trapping of mammals was carried monthly, with exception of few accidental gaps. The Gero traps were used for catching micromammalian hosts. The lines of trap were checked 2 times a day, places of lines changed each 3-5 day. Total number of micromammalians collected during the period of study is 3215, including 1115 specimens of the common shrew S. araneus and 246 ones of the pigmy shrew S. minutus. Parasite fauna on the common shrew included 23 ectoparasite species: fleas--12, ixodid ticks--2, gamasid mites--7, and myobiid mites--2 species. Among recorded species, 9 fleas species and some gamasid species are accidental parasites. These accidental ectoparasite species are common to the bank vole Clethrionomys glareolus in the territory investigated. Species composition, occurrence and abundance indices of parasites changed during the year. In total, about 55% shrew specimens are infected with ectoparasites. The infracommunity of ectoparasites on the common shrew usually consists of 6 species or less. Mean number of all ectoparasite individuals per one host specimens varies from 4 to 83. The greatest number of parasites (50 and 83) was recorded on the shrews, which carried 5 and 4 parasites species, respectively. Biodiversity of parasite species in the ectoparasite community on the common shrew and the load of parasites per one host specimen are lower than those in the bank vole. In forest biotopes explored, the most part of temporary ectoparasite species found on the common shrew was also recorded on other small mammals, which could have contacts with this host. It is possible to conclude that among the parasite supracommunity in the explored ecosystem, the temporary ectoparasites represent a "fond" of temporary parasites, which are common for most small mammal species. A role of different shrew and rodent species as main of additional hosts changes depending on a population density of potential mammalian hosts and other environment factors.

The twelve flea species were revealed on bank voles. Only four of them (Amalaraeus penicillige, Ctenophthalmus uncinatus, Megabothris turbidus, Peromescopsylla bidentata) were abundant in some seasons of the year. Four other species (Ct. agyrtes, Hystrichopsylla talpae, P. silvatica, Rhadinopsylla integella) are the parasites of bank vole too but their numbers were always low in the study area. The four last species (Amphypsylla rossica, M. walkeri, Doratopsylla dasycnema, Palaeopsylla soricis) are not peculiar to bank vole. They occurs on it occasionally from other animals--the voules of Mucrotus and shrews. The most species diversity of fleas on bank vole was observed at the period from August to October, the least one--in late winter, spring and early summer. The total abundance indices of fleas on the voles (mean number of the insects per host) ranged in different months from 0.17 to 5.65. The time of minimum flea numbers was August-September. The peak abundance was reached in springtime (March-April).

A biodiversity, seasonal dynamics and parasite load of a single individual and local population of the bank vole (Clethrionomys glareolus) were studied in coniferous and mixed forests of the Ilmen-Volkhov lowland (neighborhood of Oskuy village, Chudovo region, Novgorod Province) in the period from June 1999 till May 2002. The Gero's traps were used for collecting the host. Lines of traps stood in each place during 3-5 days and were checked twice a day. Trapping of micromammalian hosts and collecting of parasites took place each month, except a few gaps. Total number of collected animals 2854 including 1405 bank voles. The 29 ectoparasite species were recorded on the bank vole in the area of study. Among some mites and fleas a few species are accidental parasites probably accepted by voles from other species of animals. In the Oskuy area, the bank voles are the main and additional hosts of 25 ectoparasite species: fleas--8, lice--1, ixodid ticks--2, gamasid mites--7, acariform mites 7. Species composition of ectoparasites, their occurrence and abundance change during the year. Seasonal changes of abundance and occurrence indices are most expressed in the temporary ectoparasites (ixodid ticks, chiggers, gamasid mites), while in the permanent parasites (lice, acariform mites: Myobiidae, Myocoptidae, Listrophoridae), the seasonal fluctuations of indices are displayed in a less scale. Most vole specimens were infected with this or that ectoparasite species. The parasitocenosis on an individual specimen usually included less than 10 species of the total number 29 species recorded on the bank vole in the area investigated. One ectoparasite species was recorded on 21% of host specimens, 2-5 ectoparasite species were found on 71% of host individuals. Maximal number of ectoparasite species (10 species) was registered on one specimen only. The parasite load was dispersed unevenly among the infected voles. Mean number of parasites of all species on a host individual varied from 124 to 295. The highest grade of parasites (237-297 parasite specimens) was found in the voles with 7-10 species of parasites.

In experiment, the mean diurnal fecundity of Leptopsylla segnis females increased during the first five days since the beginning of feeding on white mice and reached the maximum in the second five-day period (15.7-16.7 eggs per 1 female during a day). In subsequent period of stay on a host, an activity of egg laying gradually decreased. In flea females, which had lived on hosts for 40 days and more this index was 3.7 eggs. During the mean life longevity (23 days), the flea females lay about 260 eggs; the females, which have lived more than 40 days, produce above 400 eggs.

In experiments, the mean life duration of fleas Leptopsylla segnis on white mice (abundance of fleas within natural limits, up to 10 fleas per mouse) was 22.7 days in females and 18.8 day in males. Maximum life duration was 51 and 37 days respectively. In cases, when the initial numbers of fleas were 20 and 28-34 fleas, the duration of life was decreased. The maximum limit decreased greater than the mean duration of life. A survival dynamics of fleas depended upon the flea number. It was found out, that in cases of high abundance of fleas in the beginning of experiments, the mortality rate of males was lower than in females. During the stay on a host the fleas lost gradually an ability to endure a starvation. Possible mechanisms of the regulation of flea abundance are discussed.

This review concerns the role of the fleas in survival and spread of the plague, their influence on the seasonal dynamics of the epizootics, and infection rates of these insects in different natural foci. The critical evaluation is given to the data which are used to calculate the flea transmission probability for mathematical simulation of plague epizootics.

In experiments the fecundity of Leptosylla segnis was depended upon duration of the preliminary contact of the host (white mouse) with fleas of the same species. As the duration of the contact was promoted the daily mean number of eggs per female at first (at time of contact from 1 to 6 days) increased, then (at time of contact 7-12 days) declined and decreased below original level at 13-30 days of contact. It is supposed that observed changes in fleas fecundity were connected with the immune reactions of the host to the bites of insects.

Within the boundaries of the former USSR, the northern part of the conformis group distribution is located. It spreads over the arid regions of the Trans-Cacucasus, Pricaspijckaja lowland, Kazakhstan and Middle Asia. In this area 10 species and subspecies occur. They are mainly parasites of gerbillins. Unlike many other Siphonaptera the conformis fleas, when in the host's home, do not concentrate in the nest but inhabit the passages of burrow and food chambers throughout the year. On this reason the preimaginal development and existence of the adults take place not at the temperature of the habitable nest, which is heated by the host body, but at the temperature of the soil at a depth of burrow. The temperature threshold for preimaginal development of conformis fleas is reported to be 10-12 degrees. Temperature below the threshold is fatal for all immature instars. On the contrary the imago can survive at freezing temperature. The annual cycle of the conformis fleas is characterized by the presence of adults throughout the year. They breed in the warmer season and overwinter in the state of reproductive diapause. In this state the fleas are able to attack the host and to feed but do not deposit eggs. In the north deserts the reproduction begins at the early April and terminates at the early September. Southern, the reproductive period is longer. Furthermore, the complete interruption of the reproduction in the autumn-winter time may be absent as it was observed in X. gerbilli gerbilli and X. hirtipes in the south of the Middle Asia. On the other hand it is noted that in southern deserts the rate of oviposition falls in the most hot time. The number of generation per year in the conformis fleas varies from 2-3 in north deserts to 6-7 in south ones. The flea populations peak in late autumn when the insects cease to bread. The high abundance is maintained until springtime. After the diapause is ceased and the fleas begin breeding their abundance declines. In the late spring and early summer the emergence of adults begins and the populations increase. In a mild of summer the second fall takes place and then the fall is replaced by the autumnal peak. On their main hosts the conformis fleas prevail over all other species of Siphonaptera especially for warmer time when their quota among other fleas does not descend as a rule below 90% but more oftently it approaches to 100%. The fleas of this group and especially species parasitizing Rhombomys opimus are remarkable for the high level of their abundance. In the northern deserts in the periods of the most high abundance (late autumn, winter and early spring) the number of fleas per burrow occupied by family of Rh. opimus exceeds usually 1000 specimens and sometimes it can reach several thousands. In the southern deserts the abundance of the fleas is lower but the period of their active parasitizing is longer. In the species parasitizing Rh. opimus it is shown that in a complex burrow only some part of fleas has the possibility to feed regularly. In the spring and summer the percentage of fleas daily attacking the host varied from 17 to 86% and from 10 to 150 ectoparasites feed daily on one animal depending on the quantity of fleas and of hosts in the burrow. The number of attacking fleas is regulated by behaviour of the hosts, which change the used parts of burrow when the fleas are crowded there.

The check-list of fleas of the northwest of Russia, includes 56 species. Among them, 40 species have been actually recorded in the region in question, while the occurrence of rest 16 species is suggested according to their distribution in adjacent areas. The fleas of the northwest Russia belong to 7 families. All flea species of the region in question including recorded and suggested species are distributed among these families as follows: Pulicidae--5 species, Vermipsillidae--3, Ceratophyllidae--23, Leptopsyllidae--5, Ctenophthalmidae--11, Hystrichopsillidae--1, Ischnopsillidae--8.

In the fleas Leptopsylla segnis settled on white mice the steady egg-laying took place under conditions including three factors--darkness, high relative air humidity (80-100%) and the "smell of host nest". The absence of anyone of these factors reduced an egg-production by 34-56%. The removal of the "smell" source coupled with substitution of high air humidity on low one (50-60%) interrupted the egg-laying almost completely inspite darkness or light. Owing such reaction on joint influence of the "smell of host nest" and air humidity the fleas are capable to interrupt and to recommence egg-laying depending on either the host is outside or in the nest.

When infected with cultures of plague microbe reared at 28 C the experimental fleas showed no reliable differences in the mortality as compared to control ones. By contrast, the infection with agent cultures reared at 37 C as well as the infection on agonizing white mice caused a statistically reliable increase in the mortality in insects of both sexes. In the groups of fleas infected on white mice the most intensive mortality was observed during the first 24 hours after the infectious feeding that, apparently, is connected with the effect of toxins present in the blood of agonizing animals.

In colonies of wild rabbits in the south of the Ukraine (environs of the city of Kherson) and in the Crimea (near the city of Belogorsk) there were found in masse fleas of Spilopsyllus cuniculi (Dale). The highest abundance of these parasites in spring, its decrease in summer, minimum in early autumn and rise by winter, as well as great abundance of the parasites on pregnant doe rabbits, correspond to regularities which were observed in other parts of S. cuniculi distribution area.

The plague microbe of the Ulegei subspecies is able to cause the stable infection of fleas of X. cheopis, which is accompanied by the formation of "block" in most individuals. The preservation of the plague agent in these ectoparasites is characterized by its variation in the number that undergoes cyclic changes depending on the feeding rate and digestion. The bloodsucking is accompanied by the excretion of some microbes and by the reproduction of the remaining ones in the midgut and proventriculus which lasts the whole 1 st stage while dense clots of erythrocytes are preserved in the intestine. In the course of blood digestion the reproduction of microbes is replaced by their dying off and when the digestion is over the quantity of microbes increases again. In this period small cocci, which form dense clots in the midgut and proventriculus, are the dominant form of microbes.

In infected fleas the agent of intestinal yersiniosis underwents a complex cycle of quantitative changes after each feeding. A species belonging of blood consumed greatly affected the dynamics of the agent abundance. The general peculiarity of the development of microbes in insects, which fed on various animals (white mice, Sirian hamsters, white rats and guinea pigs), is characterized by the decrease in the abundance of the agent during the first hours after feeding. This was followed by an active multiplication of microbes replaced by a new fall after which the abundance maintained on the level close to the initial one. A comparison of obtained results with the data on the digestion in fleas has shown that the phases of the primary dying off and depression of the agent falls within the intensive-decay of the food clot. The active multiplication proceeds at the end of digestion that may be promoted by the decrease in the fermentative activity and abundance of products of blood decay easily assimilated by microbes. The next fall in the agent's abundance and the absence of multiplication are associated with the exhaustion of the nutrient medium in the process of absorbtion and vital activity of microbes.

Hystological investigations of experimentally infected X. gerbilli minax have shown that at the early stages of the digestion, while in the intestine compact clots of non-digested blood, are preserved, proceeds the reproduction of the plague agent. With the decay of the alimentary clot it changes into the dying off of the microbe population and after the ceasing of digestion the accumulation of bacteria renews again. Simultaneously with the changes in the number of microbes their morphology varies. At the initial stage of blood digestion bacilli typical for the agent are dominant. With the decay of the alimentary clot they are replaced by ovoid and spheroid forms. After the ceasing of the digestion cycle the microbe mass is represented by small coccobacteria. During the decrease in the plague agent abundance bacteria are absent mostly often from the midgut and posterior parts of the alimentary canal. Most favourable conditions for their preservation exist in this period in the proventriculus and in adjoining to it oesophagus part.

During fixation of fleas infected with plague agent (with preliminary cut heads, limbs and posterior part of the abdomen) in 2.5% glutaric dialdehyde or 2% osmium tetroxide their disinfection was obtained in two days. After double fixation with 2-hour exposure in each of these fixators disinfection of the material was acheived only after additional 2-day maintenance in 70% ethyl alcohol. Fleas, which had been placed into fixators or in 70% ethyl alcohol after double fixation without preliminary dissection, were disinfected completely only in 10 days.

Being inoculated parenterally various microbes cause the fleas X. cheopis a stable infection which, as a rule, is preserved in experimental insects to the end of life. Reproducing intensively Listeria monocytogenes and Salmonella typhimurium caused the death of all ectoparasites in 3 to 5 days. The increase in abundance of Escherichia coli, Yersinia pseudotuberculosis, Y. enterocolitica and vaccine strains of Y. pestis "EV" and Francisella tularensis went on gradually and infected fleas lived up to 15--20 and more days.

Data are given on the blood amount engorged by the fleas Xenopsylla conformis, X. cheopis, Leptopsylla segnis, Nosopsyllus laeviceps, N. consimilis, Ctenophthalmus golovi, Neopsylla setosa, Citellophilus tesquorum and Coptopsylla lamellifer. The average weight of blood portion in females of different species engorged for the first time ranged from 0.05 mg (X. conformis) to 0.72 mg (C. lamellifer). Females of most species, which had multiple blood meals, engorge more blood. Males engorge less blood than females and blood portions do not increase with age. In all sex-age groups most ectoparasites cease feeding spontaneously after having incomplete blood meal. Indices are suggested for the estimation of saturation rate in fleas during feeding.

Xenopsylla cheopis fleas infected parenterally with the virulent strain of plague microbe of gerbil variant preserved the agent to the end of their lives. In the body cavity the microbes retained their ability for reproduction which was, however, limited. During the first seven days after the infection the number of microbes slightly increased and later became stabilized. Its mean indices (mean g) varied within the limits of 500 to 2000 microbe cells per 1 individual, maximum index rarely exceeded 30 000 microbe cells. Parenteral infection with plague agent did not affect essentially the longevity of fleas.

Histological studies have shown that in experimentally infected fleas (C. consimilis) Listeria are preserved in the alimentary canal. In infected individuals microbes occur along the whole intestine, from oesophagus to the rectum but most often they are recorded from the mid-gut and proventriculus. Listeria are able to penetrate into its muscular tissue and in some cases enter the body cavity of the insect. The finding of Listeria in the oesophagus confirms the possibility of the transmission of the infarction by fleas through the bite without blocking.

Under experimental conditions fleas X. cheopis engorged and assimilated blood of 4 species of rodents, man, pigeon and two species of reptiles. Histological investigations have shown that the structure of food clot, destruction rate of blood cells and general duration of digestion are changed considerably depending on the host's blood. In females the digestion of white mice blood to haematin at 23-24 degrees lasted not over 12 hours, the digestion of blood of hamsters and man - 12 to 20 hours, of guinea pigs and reptiles - 18 to 25 hours and of white rats and pigeons - from 20 to 30 hours. In males the digestion carries out somewhat slower. The above differences in blood digestion can apparently create unequal conditions for pathogenic microbes in the intestine of fleas that affects the preservation and reproduction of the agent in the organisms of these carriers.

Histological studies of C. consimilis infected with S. typhimurium have shown that the microbes are preserves along the whole extent of the alimentary canal, expecially in the midgut and pyloric portion. Bacteria are concentrated in the gastral cavity; in addition, they were found to penetrate the alimentary epithelium cells and pyloric valve tissue. The number of microbes and their distribution in the intestine undergo great variations that depends largely on the insects feeding rhythm. During bloodsucking a "lavenment" of the alimantary canal takes place; as a result, numerous microbes accumulated here are periodically excreted outside. The infection with Salmonella typhimurium is often accompanied by pathological changes in fleas tht is expressed in the destruction of the midgut epithelium and pyloric valve tissue.