On display in the Kremlin Museum is the majestic wire cage of a crinoline that belonged to some lady-in-waiting—I’ve forgotten which—of the tsar’s court. Fastened to the belt, or, perhaps I should say, to the blood-chilling metal hoop that served as a belt, are two small porcelain tubes, the same shape and size as a chemist’s test tube; the museum label tells us that these were flea traps. A teaspoonful of honey was poured into the bottom; the fleas, in their wanderings from one piece of cloth to another, would be attracted by the aroma of honey, enter the tube, slide down the smooth walls, and fall to the bottom, where they would be stuck.
This is just a chapter in the novel that describes the endless struggle between two forms of cunning: the conscious, short-term cunning of man, who must defend himself against parasites and invents his stratagems in the course of a generation or two, and the evolutionary cunning of parasites, which requires millions of years but attains results that leave us astounded.
Among animals, it is the parasites that we should especially admire for the originality of the inventions inscribed in their anatomy, their physiology, and their habits. We do not admire them, however, because they are distasteful or harmful; still, once we get over that preconception a realm opens out before our eyes in which, truly, reality surpasses imagination. Just think of intestinal worms: they feed, at our expense, on a food so perfect that, unique in all creation, along with the angels perhaps, they possess no anus. Or think of rabbit fleas, whose ovaries, by dint of a complicated interplay of hormonal messages, work in synchrony with the ovaries of their host: thus both the female rabbit and her guest give birth simultaneously, so that each little bunny is issued at birth its own ration of tiny larvae, and will emerge from the nest already provided with fleas its age.
These are necessary forms of cunning. We must keep in mind that the profession of a parasite (“he who eats beside you”) is not an easy one, in either the animal world or the human world. A good parasite must exploit a host that is bigger, stronger, and faster (or, in the human version, richer and more powerful), but it is indispensable that it make its host suffer as little as possible, at the risk of being expelled; and it must take care not to make the host die (in human terms: go bankrupt), because it would be ruined along with the host. Let us consider mosquitoes and vampire bats, which, though different from each other, both invented anesthesia, and utilize it to keep from troubling the sleep of their host during their modest withdrawals of blood. A human analogue to this anesthesia could be found in the adulation of the powerful dispenser of favors, but the parallel between human and animal parasites cannot be pushed much further; in our complex society, the freeloading dining companion has broadly made way for parasitic social classes and incomes, against which it is much harder to defend ourselves.
One essential difference remains in place between human and animal parasites. The old-school human parasite had to be intelligent, because he lacked the appropriate instincts: for him, parasitism was a choice, and he was obliged to invent his own artifices. The animal parasite, as far as we know, is all instinct, totally programmed, and its brain is minimal or entirely lacking. There is an economic consideration in this; the hunt for an enormous, rapid host has such an unpredictable outcome that the species has preferred to invest its creativity not in brains, not in the digestive apparatus, not in the sensory organs but, instead, in a prodigious reproductive apparatus: the tapeworm, devoid of brain, digestive tract, and locomotor apparatus, produces in the course of its adult life many millions of eggs. This enormous compensatory fecundity serves to inform us that a tapeworm’s “infant mortality” is extremely high, and that a larva’s likelihood of enjoying a career is on the order of magnitude of one in a million.
Human fleas, which is where we began, have fallen out of fashion, and no one misses them, but in fact in the past few years we have witnessed a mysterious revival of lice, and so we should be on the alert. It is important to remember that the flea, aside from being a vehicle of epidemics, was, just a few decades ago, part of European civilization and folklore, mixed with all social classes (as is shown by the crinoline described above), and was often mentioned by men of letters. Bernardin de Saint-Pierre, who had a boundless faith in providence, wrote that fleas are dark, and are attracted by light cloth, in order to ensure that humans can catch them: “If these little black, light, nocturnal animals didn’t have an instinct for white, it would be impossible for us to descry them and catch them.” Giuseppe Gioacchino Belli, in an 1835 sonnet, paints an oddly sensual miniature of the purciaròla (flea-infested girl), who finds no delight greater than that of getting rid of her fleas:
Everyone has his preferred tastes.
Mine is for fleas, you see—I like
to crush them and hear the crackling sound.
In Balzac’s Droll Stories, the nuns of the merry Abbey of Poissy explain to an innocent novice what she must do to tell whether a flea she has caught is male, female, or virgin, but they add that it is an exceedingly rare thing to find a virgin flea, “since these beasts have no morals, are all wild hussies, and yield to the first seducer who comes.”
In the popular mind, the flea, much like the fly, for that matter, has family ties to the devil. In Faust, in Auerbach’s Cellar, Mephistopheles is roundly applauded when he sings a song about a king who had a large-sized flea, cherished it like a son (not like a daughter: pulce, feminine in Italian, is Floh, and masculine, in German), and had a fine suit made for it out of silk and velvet.
Truth be told, the appearance that a flea presents under a microscope is so peculiar as to verge on the diabolical, and equally diabolical is its ability to avoid capture with a leap so rapid that it swiftly eludes sight and seems to vanish. It was precisely this leap that formed the subject of many decades of study by an amateur scientist rich in patience and talent, Dame Miriam Rothschild. It should come as no surprise that a naturalist would ignore our repugnance and our taboos; in fact, these studies have produced such surprising findings that even laymen deserve to know about them.
The leap of a flea is commensurate with need: the leap of a mole flea, or of any flea that lives on a permanently den-dwelling animal, is feeble or even nonexistent, because getting onto its slow or sedentary host presents no problems. On the other hand, when the host is fast-moving and lively, like a cat, a deer, or a human being, it is crucial that the insect, as soon as it has completed its metamorphosis, be successful in the fundamental undertaking of its life, that is, the leap that will take it from the ground to its destination. The human flea has made measured leaps of thirty centimeters in height, which is to say, at least a hundred times the length of the flea itself.
Now, the power required for such a leap cannot be supplied by any muscle, much less an insect’s muscle: insects are practically inert at low temperatures, and the flea must in fact make a “cold” leap, because it often completes its metamorphosis in unheated quarters, such as the floors of certain human dwellings, and as soon as it emerges from its larval stage it has a need for blood.
Given the problem, evolution has through millions of years of trial and error come up with an elegant solution. The powerful musculature that enabled the flight of the flea’s winged ancestors has been converted, and linked to a system of elastic storage of mechanical energy: basically, a tension, trigger, and release mechanism similar to that of the crossbows of olden days, or the spring-loaded spearguns used now by scuba divers.
The flea’s elastically deformable organ, analogous to the speargun’s spring and the crossbow’s bow, is made of a protein that is virtually unique in the animal kingdom, similar to rubber but with greatly superior performance. Thus, the energy needed for the prodigious and instantaneous leap is accumulated during a slower preparatory phase: between one leap and the next, the flea must “gather its strength,” store up energy in its springs, but even for this pause it requires only a few tenths of a second. This is the secret that allows the insect to leap even in chilly surroundings, and to leap so high and so far.
Dame Rothschild and her colleagues plumbed and reconstructed these subtle phenomena by fabricating ingenious tools, for example high-speed cameras operated by the flea’s own leap. There may be readers who wonder what the purpose of such research might be: a religious soul might reply that the harmony of creation is mirrored even in a flea; a secular spirit would prefer to point out that the question is beside the point, and that a world in which we studied only useful things would be grimmer, poorer, and perhaps even more violent than the world in which it is our lot to live. In essence, the second answer is not very different from the first.