The only true fleas found on these are two species of the genus Vermipsylla, which resemble the chigoes in so far that the pregnant females burrow into the host and expand there. One species has been found on camels and horses in Transcaucasia; another on roe-deer in Northern China. The female of the last is often found ensconced on the inside of the nostrils of the deer. Of course chigoes may attack domestic Ungulates of all kinds; but no other members of the family Pulicidæ or typical fleas except those two above mentioned have been found on hoofed mammals.
Insectivora such as moles, shrews and hedgehogs are the hosts of a great variety of species. The same thing may be said of the Rodents, which include porcupines, squirrels, rats, mice and a vast number of other small mammals whose geographical distribution includes almost the whole of the habitable globe. Probably more different species of fleas have been collected from Insectivora and Rodents than from all the other orders of mammals grouped together.
The Carnivora, excluding the Pinnepedia, or seals, sea-lions and walruses, harbour numerous species.
Among the Edentata a very remarkable and highly specialised genus of fleas is parasitic on armadilloes in South America. This genus (Malacopsylla) consists of two species only, which are confined to South America and are found on the armadilloes and on carnivorous animals which probably have preyed on them. The thorax of these fleas is much reduced and very small in size. Their piercing organs are slender and weak, but they possess enormous spines on the legs with which they hold on to their hosts. These two South American fleas (M. grossiventris and M. androcli) will be referred to again later as striking examples of fleas with strongly developed legs and weakly constructed mouth-parts. The contrary combination of powerful mouths and degenerate legs is also found in other groups of fleas, as will be seen in the chapter on the chigoes.
The Marsupials of Australia and South America have special fleas which were probably associated with this strange order of pouched mammals before they became divided into the American and Australian groups. Fleas have been collected on the spiny ant-eater (Echidna) which belongs to the lowest order of Monotremes or egg-laying mammals.
On almost every form of bird, including the most aquatic kinds, fleas of various species have been obtained.
Only one instance has been recorded of a flea occurring on a reptile. A female of one of the species of burrowing chigoes (Echidnophaga ambulans) from Australia was collected by Dr Woodward from the Brown Snake (Diemenia superciliosa). This reptile, which is well known in Australia, belongs to a sub-family that contains some of the most deadly poisonous snakes and is allied to the cobras. The Brown Snake is a terrestrial snake, and one must regard the presence of the flea on such a host as a rare and chance occurrence. The snake was captured at Herdman’s Lake, near Perth in West Australia. The same species of flea has also been obtained from the phalangers (Trichosurus) which live in the tops of the Australian gum-trees; from the little terrestrial and nocturnal rat kangaroos (Bettongia); and from the banded ant-eater (Myrmecobius), another Australian Marsupial. It is possible that the flea moved from some small mammal which was being devoured by the snake and managed to fix itself between the scaly plates of the reptile.
When fleas are hatched in a nest they have no choice but to attach themselves to the young mammals or birds. But even in that case they frequently leave their hosts and do not for very long remain stationary. Moreover, when a host dies and becomes cold the fleas invariably leave their quarters, which explains how it may happen that Carnivora get infested with the fleas of their prey. This change of hosts which is always occurring makes it impossible to draw conclusions from material collected in zoological gardens where many animals are herded together. In menageries, too, the normal conditions of breeding are absent. A German naturalist collected 2036 fleas from theatres, concert-halls, ball-rooms, schools and barracks in the grand-duchy of Baden and found that more than fifty per cent. were dog-fleas (Ctenocephalus canis). What the proportion may be in other parts of Europe we have no materials from which to form a judgment. In zoological gardens cat-fleas (Ct. felis) are generally numerous in most of the cages.
It is, of course, well known to every zoologist that species are not fixed or constant and that various forms of mammal or of bird tend to show geographical variations. When a long series of skins are laid out on a table and carefully examined it is seldom that those from the west of any great region cannot be picked out and distinguished from those obtained in the east. So we also get northern and southern forms of the same species varying slightly. These variations are perceptible in many forms of insects, and zoologists now describe these local races as subspecies and designate them with trinomials. No one, however, knows enough as yet about all the various forms which are assumed by fleas to attempt, except in a few instances, to do so in the case of these animals.