As distinguished from true or natural hosts one must separate what may be termed casual or accidental hosts. All animals which come in contact with one another, or which live in close proximity, may exchange fleas. So even bird-fleas may be collected from mammals and typically mammalian fleas from birds. In this fashion puzzles may arise which tax the ingenuity of the collector to solve. Bird-fleas are sometimes found on bats, and this may be obviously attributed to the bats having inhabited a hole which was tenanted by starlings or an old loft infested with the fleas of pigeons. All beasts of prey are sometimes found to harbour the fleas of animals they have devoured. Rabbits’ fleas are found on wild-cats; hedgehogs’ fleas on foxes; mice fleas on weasels; and fleas characteristic of small birds on stoats. So also in the case of mice, rats and voles with holes and runs in the same hedgerow, the parasites usually peculiar to one are not uncommonly found on the others. It is sometimes difficult to determine the true host of a flea.
Much more puzzling to explain are the reasons which confine a flea to a certain host and which cause closely allied hosts to have different fleas. The fleas from the house-martin and the sand-martin are quite different; those from the domestic fowl and the domestic pigeon are distinct species. The causes which have affected the evolution of the various forms of flea are too obscure to enable anyone at the present day to offer any satisfactory explanation.
Speaking generally, the fleas found on birds have points in common, and they probably form a natural group to themselves. What may be called true bird-fleas have been collected from almost all European birds. An unwieldy genus (Ceratophyllus) comprises many species of different flea. Some species are very abundant and infest the nests of many different birds. Others are extremely rare. One of these rarities (C. vagabundus) is found in the nests of puffins and other sea-birds. Another has been collected on antarctic petrels. Penguins have a special genus of flea to themselves. A specimen, unique at one time (Ceratophyllus borealis), in Mr N. C. Rothschild’s collection was obtained from the gannet. It has now been found on rock-pipits in the Shetland Islands.
Two very rare fleas (C. farreni and C. rothschildi) are found in the nests of house-martins; yet the nests of these birds are infested with common species besides. A plague flea (Xenopsylla) has been found on an African swift.
Forty-six different species of flea have been found in the British Islands, but many of these are extremely scarce.
We know too little about the geographical distribution of fleas to lay down many accurate generalities. When a great deal more material has been collected and studied, it may be possible to show that certain groups are associated with certain regions of the earth or certain orders of animals. To some extent this is already seen to be the case. The fleas indigenous to the New World are distantly related to those of the Old World. Broadly speaking the geographical distribution of the parasite must follow that of the host. But sometimes the parasite is impatient of cold and cannot follow the host out of the tropics. The chigoes and their allies are fleas of hot countries. Different kinds of bats are found from the tropics to the Arctic circle, but the same bat-fleas are not found everywhere.
When a flea has a cosmopolitan range it is probable that it has travelled over the world in company with its host.
Monkeys have no fleas. This is an assertion that is commonly received with surprise and incredulity. Occasionally a gorilla or a chimpanzee may get a chigoe in its toe. And monkeys in zoological gardens or menageries are possibly exposed to the danger of catching an occasional human flea from the people who crowd round their cages. These are remote contingencies which may happen to anyone. Healthy wild monkeys are much too clean and active to harbour fleas. When they search one another’s fur in a fashion that must be familiar to most persons, they are clearing their coats of particles of scurf or of similar scraps of dirt and not of fleas. So, speaking generally, it may be said that no fleas have been found truly parasitic on monkeys.
Bats have fleas, but not in great abundance. All bat-fleas are rare on their hosts and extremely difficult to find and collect. The same species are not found on fruit-bats and the ordinary smaller insect-eating bats. The geographical distribution of some bat-fleas is puzzling. For instance, one species (Ischnopsylla unipectinata) is found on the greater horse-shoe bat in Europe; but it is, apparently, not found on the same bat in the British Islands. In Somaliland and in India it is found on other bats.
With certain exceptions, Ungulates are remarkably free from fleas. This great order of mammals includes a variety of hoofed animals: oxen, sheep, goats, deer, pigs, camels, giraffes and antelopes.