Although mankind is the true host of this flea, it has been obtained in various parts of the world on various mammals and occasionally on birds. But in England, and probably in other parts of Europe as well, Pulex irritans is an undoubted parasite of the badger. A good series of the insect has been got from wild badgers freshly captured near Reading in Berkshire and Hastings in Sussex. In other parts of the world it has been obtained from a variety of small carnivora: cats, dogs, foxes, jackals and polecats. It has also been found on Rodents (Gerbillus) and on Insectivora (Erinaceus). In South Africa it has been taken off a caracal and in North America off a lynx.
Sandy places such as sea-beaches and picnic grounds, where humanity congregates for pleasure or business, frequently swarm with this species of flea waiting an opportunity to feed. The larvæ are bred in the sand and feed on organic refuse.
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.
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.