Dr. Kassai Tibor szerk.: Parasitologia Hungarica 2. (Budapest, 1969)
that the fleas of the hosts mentioned above do not mingle on account of the different systematical categories of the host species. But then why do the fleas of the badger occur on the fox, and vice versa? - the two host species are assigned to two different mammalian families. The mice stand at least as far removed from the shrews as from the fox and the badger, but the fleas characterizing the one still occur on the other. The objection that the flea species of the mice and the shrews still differ may therefore be apparently justified - these hosts live in a common habitat and in subterranean burrows of not too differing types. In the course of making my collections, I have in fact found mouse and shrew species within the same habitat in many places but the shrews invariably occurred in the vicinity of the moister areas (smaller bodies of standing waters, shores of brooks), so the microclimate of their nests also is probably different from that of mice . Evidence supporting my thesis can be found in the mouse family. The flea Gt.a.assimilis is more or less specific to the field vole (Microtus a.arvalis Pallas). It is known that the field vole inhabits rather dry meadows, pastures, . and agricultural fields, and so the environment of its nests is drier than that of mice (Apodemus, Clethrionomys, Pitymys, Micromys). Now I have collected Ct «assimilis rather frequently on mouse species if their habitats were adjacent to dry areas, and even in cases when I found no fieldvoles in the vicinity. It might be assumed in this case - and with justification - that Ct.assimilis is bound not so much to M.arvalis as to the microclimate of its nests and their stricter environment. Though I have discussed here only occurrences jof fleas parasitizing mammals, I should like to substantiate my theory by an example drawn from avian fleas. A great number of hosts, not too far removed from one another systematically, of avian flea species are known in Hungary. There is one exception, namely Ceratophyllus styx styx Rothschild, demonstrated only from the sandmartin (Riparia riparia L.); nor did I find any other flea species on this host. Even PEUS's (1968) abundant research material failed to produce
