Harold Russell

“ One may attribute some of the extraordinary strength and vital energy of a flea to the fact that, by the blood-system and the air-system, the tissues of the body are kept richly supplied with oxygen. The blood of a flea is a thin fluid and, of course, without red corpuscles. The blood that is shed when a flea is crushed comes from the stomach and not from the blood-vessels of the insect. ”

“ 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. ”

“ An animal which harbours fleas and which nourishes the adult insect with blood is called a host. No fleas are more than what is called temporary parasites; which means that they pass but a portion of their lives on their hosts and frequently take occasion to hop on and off. All fleas, apparently, go from host to host. The labours of diligent collectors have proved that the great [9] majority of mammals and birds have fleas. As a general rule, it is true to say that certain species of flea are associated with certain species of host. Thus man is the true host of Pulex irritans ”

“ Unlike many parasitic insects, fleas do not constantly pass their time upon the bodies of their victims. The greater part of their life is probably spent on the ground, in the house, or nest, of the mammal or bird which serves them with blood. In this respect there is considerable difference in the habits of different species of flea. Some attach themselves to an animal and actually burrow into the skin. These are the most parasitic species. Some only come to feed and leave to lay their eggs. Many probably do not suck blood more than once in their lives. ”

“ But the human flea is sometimes found on cats and dogs, and cat and dog-fleas occasionally bite human beings; and cat-fleas are found on dogs and dog-fleas are found on cats. All fleas, so far as we know, may occasionally pass from one species of host to another; but they do not, for the most part, seem to flourish in unaccustomed quarters. Some fleas are more catholic in their tastes than others. Some seem to be very strictly confined to one host, and even when starving only suck strange blood under protest. ”

“ We have in the cord of ganglia a series of brains, as it were, running from the head down to the extremity of the abdomen. Each ganglion is a mass of nerve cells, from each of which a fibre passes off to unite with the other fibres and make a nerve. The first ganglion in a flea is placed in the upper part of the head above the gullet. It may be called the brain since it receives the nerves of the antennæ and eyes. In the ancestral insect we may suppose that there was a pair of ganglia in each segment. ”

“ If a number of fleas be fed on a septicæmic rat and, subsequently, be kept under observation and nourished on healthy animals, the proportion found to be infected steadily diminishes day by day. It is remarkable that the existence of numerous plague bacilli in the stomach of a flea does not seem materially to affect the insect’s life. Fleas, in other words, do not suffer from plague though they can transmit it. ”

“ Fleas use their legs for leaping, for running, and for clinging to their hosts. They also use their mouth-parts for the last purpose and it is worthy of note, as we shall see later on, that in those fleas in which the mouth-parts are shortest and weakest the legs are most liberally supplied with bristles and possess the stoutest claws. The legs of a flea are unique in the insect world owing to the enormous development of the segment nearest the body called the coxa. Most leaping insects rely for their activity on the muscles of the lower joints. ”

“ The chigoes and their allies belong to a group of fleas sufficiently remarkable to deserve a somewhat detailed account. The reader may remember that they form a family to which the name of Sarcopsyllidæ has been given. They are the most completely parasitic of any fleas; and the South American chigoe (Dermatophilus penetrans) enjoys the distinction of being the first foreign flea ever described. This pestilent insect, of which the female has the habit of burrowing into the flesh of the host, soon made itself known to the early travellers in the tropics of America. ”

“ The mouth of a flea, as of any other insect, is merely an orifice which forms the opening into the alimentary canal. Around the orifice are the various mouth-parts which convey blood to the mouth, but these, the reader will doubtless remember, are the modified limbs or appendages of the segments that compose the flea’s head. The mouth, then, gives access to the digestive canal. The first part nearest the mouth is the pharynx which merges gradually into the gullet. ”

“ A large number of fleas possess what is called a frontal tubercle. It is a notch in the centre of the forehead but nearer to the mouth than to the antenna. Sometimes the tubercle projects from a groove. This is most marked in the genus of African fleas Listropsylla. The real nature of this organ is unknown. Some regard it as an organ of sense. Its homology is also uncertain. To some it suggests the egg-breaker of the larva and they regard it as a relic of the larval stage. To others it suggests an eye and they regard it as the remnant of an unpaired ocellus possessed by the ancestral flea. ”

“ Mankind is, occasionally, bitten by a variety of other species besides Pulex irritans. In hot countries the chigoe (Dermatophilus penetrans) is a serious and troublesome pest, particularly to bare-footed people. In temperate regions there are rat-fleas, cat-fleas, dog-fleas and bird-fleas which occasionally transfer themselves to man and feast on his blood. But, on the whole, hunger and propinquity rather than free inclination seem to actuate these fleas of which man is only the occasional host. ”

“ It has been suggested, with some show of probability, that the wide and strongly serrated mandibles were acquired after man became the host. The naked skin and rough garment of mankind would render the claws and legs of the flea insufficient to keep the insect in a steady position when feeding. Natural selection would, in due course, strengthen the mouth organs.
The division of mankind into different races, many of which are quite as distinct as the various species of some genus among other animals, leads one to expect various races among the fleas which are parasitic on them. ”

“ The human flea is a good deal more select in the choice of a host than some other species. The cat-flea (Ctenocephalus felis) has been found not only on the cat, but also on the dog, tiger, leopard, goat, horse, rat, hedgehog, kangaroo, deer, guinea-pig, rabbit, and on man. Many of these were specimens collected in zoological gardens. Although when hungry and confined in a test-tube the human flea will readily bite a rat or a guinea-pig, it has been found that human fleas kept with no other food-supply than rats and guinea-pigs soon die off. ”

“ During the plague investigations in India many careful experiments have been made proving beyond doubt that the disease may be transferred from rat to rat by the transference of fleas from a septicæmic to a healthy animal. It was first shown that when fleas were present the plague could be transferred from rat to rat, kept in proximity, but carefully screened so as to avoid any possibility of contact. Next, fleas were collected from rats dead or dying of septicæmic plague and transferred to healthy rats living in flea-proof cages. More than half of the healthy rats contracted plague. ”

“ They are strong, recurved, chitinous, structures which prop up the body of the larva when it creeps and wriggles. There are similar props in the larvæ of certain beetles and no exactly similar organs are known in dipterous larvæ. But caudal stylets are of small taxonomic importance.
In one respect the mature flea is certainly nearer to a beetle than to a fly: the three joints of the thorax are free as in a beetle and not fused as in a fly; but when one studies the mouth-parts, the true view seems to be that the mouth-parts of a flea are equally unlike those of a fly and those of a beetle. ”

“ 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 [13] 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. ”

“ 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. ”

“ The conclusion seems justified that the human flea is descended from an ancestral form with combs. To discuss whether the combs became useless and were lost when the host lost the hairy covering of its body would lead into regions of vague speculation and occupy time unprofitably.
The nearest allies of the human flea, which are found on various animals, are all inhabitants of the Old World. The indigenous fleas of America are only distant relatives of Pulex irritans. ”

“ Some fleas have what may be called by comparison large heads and others very small ones.
A small head is never found in a flea with powerful mouth-parts. The head being the bearer of piercing and sucking organs, which require strong muscles, there must be room not only for the organs but for their extensors and retractors.
There are normally three rows of bristles on a flea’s head which divide the head into four sections. It is possible that these correspond to the four segments of the ancestral insect which are now fused together. ”

“ In many species the male flea is sufficiently different in outward appearance from the female to be easily distinguished. The male is usually smaller and the last segments of the abdomen are so shaped as to give the look of a tail tilted into the air. The frontispiece represents a male flea and shows this well. The internal organs of reproduction (testes and ovaries) in the male and female are placed near the end of the abdomen. The seminal outlet and common oviduct open to the rear of the sensory plate on the ninth segment of the abdomen. ”

“ In some species the cuts which divide the different segments appear to be confined to one side of the club. In others a sort of central core holds the segments of the club together. The antennæ, therefore, are undoubtedly exceedingly complex organs. Such an insect as a flea may well be far more sensitive to movements of the air, vibrations of the earth, smells, light rays and sound-waves than a human being. In their origin the antennæ, like the paired mouth-parts, are modified appendages of the fused segments which compose the head of the insect. ”

“ On one occasion two guinea-pigs placed in a house which had been vacant for some days, and in which fleas must have been short of food, failed to attract any of this species; while a man who entered the house shortly afterwards acted as an admirable trap. Those who have not had experience of the abundance and voracity of fleas in oriental countries can hardly believe the numbers of human fleas that may be captured by sending a bare-legged man into a deserted house and then picking the fleas off him. ”

“ The human flea (Pulex irritans) appears to occupy an isolated position. The genus Pulex which Linnæus established has now been reduced until it contains one species only. The human flea belongs to the group with eyes and without combs. In some [63] respects it is the most specialized of all the Pulicidæ. The chigoes (Sarcopsyllidæ) resemble it and are doubtless derived from the Pulicidæ. The chief structural character of this interesting insect is the greatly reduced thorax. ”

“ The common mouse-flea (Leptopsylla musculi) is blind. The bat-fleas are also destitute of eyes. The nocturnal habits of their hosts would render eyes of little or no use. If eyes are present they are large and placed on either side of the head. Each is a simple eye or ocellus; the compound eyes, divided into a great number of hexagonal facets, which are characteristic of many insects, are never found in fleas. ”

“ In the Old World this flea is essentially a parasite of man. It occurs only occasionally on other mammals. In America it certainly appears to occur more frequently on mammals, other than man, than it does in the Old World. Human fleas can propagate in deserted human dwellings. The larvæ find nourishment in any refuse that has been left behind, and the adult insect can apparently continue for some time to reproduce itself without a meal of any sort and certainly without human blood. ”

“ Men of science were at the same time engaged in collecting and examining the fleas from many thousands of rats. The great success of the work confirmed the soundness of the theory on which it was based. The spread of the most terrible of [95] epidemic diseases was controlled and prevented by knowledge. At San Francisco the fleas of man, rats, mice, dogs, cats, ground-squirrels and gophers were studied. It was found there, as elsewhere, that while each species of flea has its particular host few are unwilling or unable to attack man and other animals when the host dies. ”

“ It is suggested that the soil and climate in these regions of Africa are, for some reason, unsuited to fleas. In other parts of the Dark Continent, where there are European settlements, the human flea seems to thrive surprisingly well and to attack Europeans and natives, as well as wild and domestic animals. In those parts of Asia where there are European colonies and much intercourse between settlers and Orientals, Pulex irritans is a [65] well-established and thriving parasite. ”

“ It will be best to say, at once, that the fleas are not educated and that the performance can only be attributed to their desire to escape. It is stated that a performing flea may be broken of the habit of jumping by being put in a pill-box with glass sides which is made to revolve like a lottery wheel. A short course of this tread-mill teaches the flea that the objectionable practice of hopping is useless and exhausting. It is said that the life of performing fleas averages eight months, which seems surprising. ”