INTRODUCTORY
Fleas form a group of insects that have, until recently, been little studied by zoologists. We call them insects because they are jointed animals, or Arthropods, with three pairs of legs in the adult condition. The reader will best understand the position which fleas occupy in the general classification of animals by remembering that the arthropods, or jointed animals, are one of a dozen subkingdoms, or phyla, to which the various members of the great animal kingdom have been assigned. There is good ground for believing that all the animals included in each phylum trace their ancestry back to a common primitive form which lived in more or less remote ages. Besides (1) Insects, the arthropods, or jointed animals, include (2) Crustaceans, such as crabs, lobsters, shrimps, wood-lice, water-fleas and barnacles; (3) Myriapods, such as centipedes and millipedes; and (4) Arachnids, such as spiders, scorpions, mites and ticks. To all these varied forms of animal life fleas, and other insects, are therefore more or less nearly related.
The animals belonging to this large and important collection, which compose the arthropod phylum, have certain common characteristic features. We find a body made up of a series of more or less completely similar segments placed one behind the other. In this they resemble certain worms which are far less highly organised. The body is elongated, symmetrical on either side, and the mouth and anus are at opposite ends. There is, however, an important advance on the segmented worms. Each typical segment carries a pair of appendages which are very different from the foot-stumps that are found on certain worms. These appendages of arthropods are divisible into distinct limb-segments, separated from one another by moveable joints, and acted upon by special muscles.
The common ancestor of all the various arthropods which are found living on the earth to-day, was probably composed of a series of segments each very similar to the last and each bearing a pair of very similar appendages. In the course of ages, these appendages have been astoundingly modified in form and in function. So it happens that we find in the arthropods of the present day pairs of antennæ, of mandibles and other mouth-parts, of pincers, of legs, of swimming-feet and of tail pieces which on close examination can all be traced back to a common structure. The body-segments, also, have been strangely fused together and modified. All that has been so far said applies equally to fleas and to other insects.
It is of great interest, when one comes to make a minute study of the form and external structure of a flea, to try and trace the modifications that must have taken place in the course of descent from the ancestral arthropod; but the relationship of fleas to other insects living at the present day is of more immediate concern. Insects are highly specialized arthropods and fleas are highly specialized insects. This means that they have become vastly modified from the primitive ancestral type and fitted thereby for a life among certain defined and peculiar surroundings.
It will be unnecessary to remind the reader who knows anything of zoology or of botany that all classification is now based on descent. Since naturalists have abandoned a belief in the special creation of the various species of animals now living on the earth and have conclusively shown that they have arisen by descent and modification from other forms, the problem is to reconstruct a vast genealogical tree. What then were the ancestors of the fleas and to what other insects, in consequence, do they appear to be related?
It is probable that the ancestors of the fleas were winged insects, and that the organs of flight were gradually lost, as they became useless, when a partially parasitic life was adopted. At one time entomologists regarded fleas as wingless flies and placed them in the order Diptera. Certain supposed scaly plates on their bodies were regarded as the atrophied relics of wings. It is, however, more than doubtful whether this view is correct; and all modern entomologists who have given any special study to fleas are agreed that they are sufficiently unlike any other living insects to deserve a place in an order by themselves. To this order the name Siphonaptera has been given: which means that the insects comprised in it are provided with sucking mouths and are destitute of wings. Another name for the order is Aphaniptera, but this is gradually falling into disuse. Linnæus (1758) only mentions two species of flea: the human flea which he appropriately named Pulex irritans, and the chigoe of hot countries which he called Pulex penetrans, from the habit which the female has of burrowing under the skin of her victims. At the time of writing, about 460 species of flea have been described and named; but some of the names are doubtless synonymous, and the actual number of separable species that have been discovered is somewhere about four hundred. The vast majority of these have been described within the last few years, which shows what can be done when attention is turned to any neglected group of animals. There can be no doubt that many undiscovered species still remain, and will now, in due course, be collected, described and named.
The position which should be assigned to the order Siphonaptera in the general scheme of insect classification is a question on which the most learned modern entomologists have disputed with considerable vigour. Some see the nearest relatives among the beetles, others among the flies. The majority, as we shall see later on, would place them near the Diptera: but since no convincing arguments have been produced on either side it may be wisest to regard the question as still at present unsolved.
Fleas belong to one of the groups of insects which go through a complete metamorphosis. Their life-history consequently falls into four divisions: egg, larva, pupa and imago. If the climate permits, the female flea lays her eggs all the year round, and from one to five are dropped at a time. Unlike those of many other parasites they are never attached to the hairs of the hosts, but appear to be deposited indiscriminately on the floors of houses or in the nests and sleeping places of their hosts. The eggs generally hatch in a few days, and a minute, white, wormlike larva emerges ([Fig. 1]). The larvæ, of some, and possibly of all, fleas are provided with a wonderful adaptation in the shape of an egg-breaker or hatching-spine. This is a thin plate, like the edge of a knife, where the point of the head comes in contact with the shell. The movements of the prisoner make a slight split in the egg-shell, which then bursts asunder. This organ has vanished in later larval life, and it is probably lost after the first moult. The larva is legless and has thirteen segments. It grows rapidly, and, as it grows, moults its skin several times. It is provided with mouth-parts adapted for biting, and eats any decaying organic refuse. The larvæ may be reared on the sweepings of an ordinary room or the dirty scurf which collects at the bottom of old birds’ nests. It is hardly necessary to add that the mother takes no interest whatever in the larvæ and that the belief that she feeds them on dried blood is not based on any sound foundations.