Fig. 6.—Pulex irritans, female. The legs of the left side only are shown. Enlarged. (After a drawing by A. Dampf.)
Fleas are temporarily parasitic on many mammals and birds, but some mammals and some birds are much freer from fleas than others. As the flea is only on its host for part of the time, it has to put in the rest of its existence in some other place, and this, in the case of the human flea, is usually the floor, and in the case of bird-fleas the nest; from these habitats they can easily regain their hosts when the latter retire to rest. But large numbers of Ungulates—deer, cattle, antelopes, goats, wild boars—sleep in different places each recurrent night, and to this is probably due the fact that, with the exception of two rare species—one taken in Northern China and the other in Transcaucasia—the Ungulates have furnished descriptive science with no fleas at all. Both of these Ungulate fleas are allied to the burrowing-fleas or ‘chigoes.’
I know none of my readers will believe me when I say that the same is true of monkeys; but I do this on the undoubted authority of Mr. Harold Russell, who has recently published a charming little monograph on these lively little creatures. Monkeys in nature are cleanly in their habits; and although in confinement occasionally a human flea attacks them, and although occasionally a chigo bores into the toes of a gorilla or chimpanzee, ‘speaking generally, it may be said that no fleas have been found truly parasitic on monkeys.’ Whatever the monkeys are looking for, it is not fleas. What they seek and find is in effect little scabs of scurf which are made palatable to their taste by a certain sour sweat.
As a rule, each host has its own species of flea; but though for the most part Pulex irritans is confined to man it is occasionally found on cats and dogs, whilst conversely the cat- and dog-fleas (Ctenocephalus felis and Ct. canis) from time to time attack man.
The bite of the flea is accompanied by the injection of the secretions of the so-called salivary glands of the insect, and this secretion retards the coagulation of the victim’s blood, stimulates the blood-flow, and sets up the irritation we have all felt.
It is only a few years ago that the spread of bubonic plague was associated first with rats, and then with rat-fleas; and at once it became of enormous importance to know which of the numerous species of rat-flea would attack human beings. The Hon. Charles Rothschild, who has accumulated a most splendid collection of preserved fleas in the museum at Tring, had some years ago differentiated from an undifferentiated assemblage of fleas a species first collected in Egypt, but now known to be the commonest rat-flea in all tropical and sub-tropical countries. This species Xenopsylla cheopis—and to a lesser extent Ceratophyllus fasciatus—unfortunately infests and bites man. If they should have fed upon a plague-infected rat and subsequently bite man, their bites communicate bubonic plague to human beings. Plague—the Old English ‘Black Death’—is a real peril in our armies now operating in Asia and in certain parts of Africa.
Just as some fleas attack one species of mammal or bird and avoid closely allied species, so the human flea has its favourites and its aversions. There is a Turkish proverb which says ‘an Englishman will burn a bed to catch a flea,’ and those who suffer severely from fleabites would certainly do so. The courage of the Turk in facing the flea, and even worse dangers, may be, as the schoolboy wrote, ‘explained by the fact that a man with more than one wife is more willing to face death than if he had only one.’ But there are persons even a flea will not bite. Mr. Russell has reminded us in his Preface of the distinguished French lady who remarked, ‘Quant à moi ce n’est pas la morsure, c’est la promenade!’
Fig. 7.—Larva of Pulex irritans. C.f. frontal horn; d, antenna. Enlarged. (After Brumpt.)