Dr. Kassai Tibor szerk.: Parasitologia Hungarica 2. (Budapest, 1969)

to participate in specific associations. Such are I.octactenus

, h.segnis

, A .erinacei

, and N.eusarca

. I have collected the hosts (Erinaceus europaeus roumanicus Barr

.-Ham

., and Mus musculus spicilegus Perényi) of two of these species (

A.erinacei and L

. segnis

) in moderate numbers, but I have never found them to occur together with other flea species, even though both hosts have ample opportunity, both in their nests and sphere of activity, to acquire a number of other parasites. One might therefore infer with some justification that these two flea species are for some reason less inclined to coexist with others or that they shun the presence of the latter, or, indeed, vice versa. One should also recall that Ct.caucasicus

, the flea of the mole-rat, has never been found on gophers though all flea species of that latter animal frequently occur on the mole-rat. The problem of host specificity in fleas, perhaps depending to some extent on the type of food (blood) may be clarified by laboratory experiments. 4. The la3t and most difficult question involves the actual ecological, or some other, cause of the coexistence of flea species on a host,or the possibility of its chance (spontaneous) nature. The specific fleas of the hosts are known, and we also have information on animals which could serve as hosts for the given flea species. Those fleas considered most specific appear incidentally though seldom, on other than their „true" hosts, but these are merely occasional phenomena. If, however, the host changes its haunt (mainly its site of reproduction) for any length of time,and if this be the habitat of hosts supporting other flea species, then those appearing later on the immigrant host can hardly be defined as merely occasional parasites. (The number of individuals of a flea species found on hosts and in their nests indicates the occasional or specific nature of the flea species.) It was many times my experience that the flea fauna of a host found in an environment different from its usual habitat (nesting site) is not composed of the species to be expected.