The genus most closely allied to that which contains the human flea consists also of a single species only. It is a large flea (_Pariodontis riggenbachi_) found on porcupines all over Africa and in India. Mankind is, occasionally, bitten by a variety of other species besides _Pulex irritans_. In hot countries the chigoe (_Dermatophilus penetrans_) is a serious and troublesome pest, particularly to bare-footed people. In temperate regions there are rat-fleas, cat-fleas, dog-fleas and bird-fleas which occasionally transfer themselves to man and feast on his blood. But, on the whole, hunger and propinquity rather than free inclination seem to actuate these fleas of which man is only the occasional host. There are besides very numerous species which have never under any circumstances been known to bite man. There is no doubt that some persons are more attractive to fleas than others. The reason for this we do not know. It may depend on the tenderness of their skin, the quality and taste of their blood, or their personal smell, or possibly all three combined. The various forms of rat-flea which are important in carrying plague from rodents to the human race are dealt with later on. Among the commonest fowl-fleas which bite man are _Ceratophyllus gallinæ_ and _C. gallinulæ_. Both species infest the nests of many common passerine birds besides the domestic fowl. A common parasite of the pigeon is _C. columbæ_, which also bites man. [Illustration: Fig. 7. The head of a female dog-flea (above) and a female cat-flea (below) to illustrate the difference in shape. In the males the difference is less strongly marked but quite perceptible. From _Novitates Zoologicæ_, Vol. XII, January, 1905.]