Selection of an appropriate habitat with necessary and exploitable resources is one of the main tasks for any living organism. If an organism succeeds in fulfilling this task, its reward is translated into an increase or, at least, non-decrease of its fitness. If, however, it fails to find a necessary habitat, its fitness decreases. Continuous fitness decline may finally result in extinction.

The evolutionary motivation of fleas does not differ from that of any other living organism. Fleas have to select carefully their host organisms at both evolutionary and ecological scales. At the evolutionary scale, the selection of an appropriate host or hosts results in a host spectrum for a particular flea species. This spectrum represents a portion of the resource space used by this species, i.e. its fundamental ecological niche. At the ecological scale, an individual flea has to be able to locate and identify an individual of an appropriate host species and to distinguish it from individuals of often similar but less appropriate or even inappropriate species. Furthermore, even if a flea succeeds in finding an appropriate host individual, the selection task is not yet complete. The area of the host body from which (a) a blood meal may be most easily taken and (b) host grooming may be most easily avoided still remains to be found.

In this chapter, strategies of host selection adopted by fleas are discussed. I start with the issues related to differences among hosts from a flea's evolutionary point of view.