The study of Siphonaptera is still quite in its infancy. We know little or nothing of the minute geographical variation of fleas. That there is such a thing can already be seen in a few species. In the meantime the study of variation must be postponed until collectors have amassed a more plentiful amount of material; and it is best to treat all forms which are to all appearance constantly different as being specifically distinct until more is known about variation.
Any classification of fleas that may now be attempted can only be tentative. It will be enough for present purposes if the reader will remember that the Order Siphonaptera can be divided into three groups or families: (1) the chigoes and their allies, which are the most parasitic fleas (Sarcopsyllidæ); (2) the typical fleas to which the majority of species belong (Pulicidæ); and (3) the bat-fleas (Ceratopsyllidæ), which have certain peculiarities that will be described in a later chapter.
Of the antiquity of fleas, and of the period in geological history when the order made its appearance, little can be said. When it was thought that fleas were confined, as parasites, to warm-blooded mammals and birds, evolutionists were inclined to say that the parasites could not have appeared before their hosts. The discovery of a flea on a reptile opened the vista of possibly enormous antiquity stretching back to Permian or Carboniferous ages. The fossil record is most meagre. If we reject as too doubtful the supposed remains of a flea from the lower Oligocene strata at Aix in Provence, only one undoubted fossil has been discovered. Nor does it seem certain that fleas are entirely restricted to preying on vertebrates. Dr Dampf introduced a number of common bird-fleas (Ceratophyllus gallinæ) of both sexes to some hairy caterpillars. He observed that several of the fleas buried their heads in the hairy covering of the larvæ and remained some time in the attitude of sucking blood. While this was going on the victims made violent demonstrations of annoyance and discomfort. He also observed that a naked caterpillar was not attacked.
Mr Boden has also recorded how he found in a seed-warehouse some peas that were being eaten by two species of Lepidopterous larvæ. On bringing these home and keeping them in a jar, he found among them some small larvæ which ultimately turned into fleas, probably Pulex irritans. These fleas, being confined without other food, were observed to prey on the Lepidopterous larvæ and to feed freely on their juices. The larvæ which were attacked by fleas pined and died. The fluid from the stomach of the fleas when they were crushed was transparent and not red like vertebrate blood which often exudes when a mammalian flea is pinched and cracked open.
A French entomologist has also reported that the numerous fleas which swarm in the dwelling-houses of Corsica, for want of other nourishment turn their attention to flies that may be incapable of flight.
The only fossil remains of a flea that have, so far, been discovered are a single insect in a piece of Baltic amber of Oligocene age. Many organic remains have been preserved in this fashion, but this is the first mammalian parasite that has been found. The flea is admirably protected by its semi-transparent surroundings, and the most minute details of structure, the arrangement of bristles on the body, and the number of segments to the labial palpi can be discerned. This unique object is in the collection of Professor Klebs. The first point to note is that a flea of this antiquity hardly differs from the existing insects of the present day. It has been referred to an existing genus (Palæopsylla) of which there are at least four species living. Three of these are parasites of the mole, and the fourth is found on shrews. There is good reason to suppose that the host of the fossil was some insectivorous mammal. The early specialisation of fleas is strikingly illustrated. This insect is already adapted for life on some warm-blooded animal. It has a thoracic comb, and its mouth-parts are in all respects like those of a modern flea. It belongs to a genus which is still commonly distributed over Europe. When we consider how remote are the chances that a mammalian flea should first get embedded in amber and should, subsequently, be detected and described by a naturalist, we may well understand that the owner of the fossil asked, though without success, £1200 for it.
The ordinary person regards fleas as a subject for humour of an obvious and familiar kind. The utilitarian despises a man who can cheerfully spend his time in collecting fleas. Yet it seems probable that a study of their forms and habits may be of immediate benefit to the human race. The discovery that fleas are connected with the spread of plague is an instance of apparently unprofitable scientific labour proving of direct advantage to mankind. An accurate knowledge of the structure and habits of fleas is now seen to be of importance to all who are engaged in fighting one of the most dreaded infectious diseases. When plague breaks out men of science now at once turn their attention to the fleas. This is likely to prove more directly efficacious than the mediæval custom of marking the house with a red cross and inscribing the legend, “God have mercy on us.”