Fleas have adopted us, and it's too bad for them,
because we do all we can to get rid of them.
Yet, when you take a closer look at them,
these tiny creatures are really quite interesting.

Pulex irritans : the name says it all. The Latin name with which Linné baptized the insect summarizes perfectly its nature: an irritating pest. And one that we are going to have to put up with, because although Pulex irritans is threatened with extinction, thanks to the use of soap, the vacuum cleaner and insecticides, there are plenty of candidates to occupy its vacant position: the cat flea (Ctenocephalides felis), the dog flea (Ctenocephalides canis), the oriental rat flea (Xenopsylla cheopis), the hen flea (Echidnophaga gallinacea), etc. Around the world there are close to 2,500 species of hematophages, that is, creatures that feed on blood, but not just any blood: only the blood of birds and mammals, including humans of course. Moreover, they only develop properly in warm and humid conditions, in places where the larvae can peacefully feed on debris, waiting to become adults and rival Dracula. So almost all mammals that build or inhabit burrows harbor one or more species of flea. Thus, hoofed animals and primates are not affected, but rodents cannot escape them. Nor can humans, of course, who, since their appearance on Earth, maintain usually fleeting but sometimes dangerous liaisons with the flea.

Permanent or transient parasites

Long before it took up residence on the mammoth, the flea probably pestered small mammals, about 200 million years ago. Then it adopted humans when they sought refuge in caverns, close to the shelters of badgers and foxes. Ever since, the flea has continually adapted its lifestyle to its various hosts. Thus, cat and dog fleas have a permanent residence in animal fur, in which they move about incessantly. But the fleas of the beaver, the squirrel or the hamster attach themselves to the rodent only for the duration of a meal. The rabbit flea (Spilopsyllus cuniculi) pushes adaptation to the limit: unable to secrete its own sex hormones, this flea uses those of a pregnant rabbit. As it nourishes itself with its hostess' blood, the flea absorbs female hormones, which cause its own ovaries to develop. Several hours after the mother rabbit gives birth, the flea leaves its ears and hops onto the baby rabbits, whose urine contains a substance that attracts fleas. Only then does the flea mate and lay its eggs. Twelve days later, the flea returns to the ears of the mother rabbit, who is much more often infested than her male partner.

Wake-up call

In its contact with humans, the flea has used the special features of its cycle of metamorphosis to its own benefit. It begins with little white eggs that hatch after one to ten days. The young wormlike larvae, with their very sharp single teeth that facilitate their exit from the eggs, feed on dried blood excreted by the adult fleas. The fleas undergo two transformations before spinning their silky cocoons from their salivary secretions. Inside their cocoons, they transform first into nymphs then into biting insects. But an exterior element, a stimulus is necessary to set off the synchronized eclosion of all the cocoons. The mere vibration from a human footstep is sufficient. The future fleas are capable of waiting, camouflaged and hiding out between the floorboards or in carpets, for periods of up to a year, until their food parcel arrives. Which explains why people are often literally attacked by fleas when they return home from vacation.
These invaders, also known as "floorboard fleas" are, in fact, of the species Ctenocephalides felis, found in abundance on cats as well as dogs, and on a good fifty or so other animals: rodents, rabbits, sheep, coyotes, foxes, lynx, raccoons, possums, ferrets... And when there are no other victims around, they will not refuse human blood.

High jump champions

The flea can jump to heights that for humans would be the equivalent of the Eiffel Tower, with an acceleration fifty times that of a space shuttle, which represents a gravitational force of 140 G, whereas a fighter pilot finds 6 G hard to bear. This astonishing aptitude is due to the abrupt decompression of an elastic mass, known as resiline, which acts like a spring between the thorax and the hind legs. Resiline is also present in the flight mechanisms of numerous insects. It is possible that during their evolution, fleas lost their wings so as to facilitate their movement through their hosts' fur. They have however retained a clever landing mechanism: "airbags" in their legs, which inflate like balloons and allow them to slow down their descent once the target is in view. This mechanism is regulated by the respiratory system that activates the abdominal plates. The flea's body also has numerous hairs that act as captors. Every air movement tells the flea about the position of its target, and it can calculate the trajectory of its leap with precision.

Public enemy n°1

Each day, the flea can ingest fifteen times its weight in blood, which makes it a formidable vector of germs. Before gorging itself, it makes numerous trial bites, as if it were choosing the most favorable place for piercing the skin with its three blades. The meal is accompanied by the excretion of an abundant amount of partially digested blood. When a flea bites a plague-infected rat, it absorbs contaminated blood, and the bacilli multiply in its digestive tube until a blockage is formed. Unable to feed, the flea regurgitates the bacilli into its numerous victims, rats and humans. Around 120 species of flea are able to transmit the bubonic plague to humans, the main culprit being Xenopsylla cheopis, the oriental rat flea. Without treatment, death is inevitable in 50% of cases.
In the Middle Ages, half the population of London and a third of the population of Europe were thus exterminated. There are still seats of active infection today, notably in the Congo and Madagascar. Each year, more than 1000 suspected cases and thirty or so deaths are recorded on the large island, but the reality probably surpasses the statistics. During the Second World War, the flea's ability to transmit death prompted studies in many so-called civilized countries. With a view to bacteriological warfare, studies quite seriously looked at the possibility of raising fleas industrially and infecting them with plague bacilli of increased virulence, before dispersing them by air over cities in enemy territories...

No more fleas ?

In the United States, 70% of the turnover of certain pet shops comes from anti-flea products. In Florida, no fewer than 300 companies specialize in flea extermination, and in Miami, the "Fleabusters" brigade receives up to 1000 distress calls each week. No wonder some people have taken up flea hunting as a vocation… like Mike Dryden, known as Dr Flea, researcher at the University of Kansas, who for many years has studied the behavior of the cat flea, with a view to perfecting the most efficient flea trap. His research program includes a whole range of tests: evaluation of the trajectory, distance of the leap, sensitivity to heat, light…
In his laboratory, he has sacrificed more than one million fleas in developing a first generation of traps. Here is one of his bright ideas: a green light bulb, suspended in a wire mesh box on a transparent and very sticky plate, flashes on and off. When the green light goes out, the fleas think that an animal is passing before them, casting a shadow. So they jump onto what they believe to be an animal and get stuck in the glue.

The billion dollar flea

Each year, Americans spend over one billion dollars in the battle against fleas in cats and dogs. This has incited a veritable race for patents by laboratories. Among the arsenal of unique products are formidable new molecules that, when inserted into the parasites' cuticles, kills them by overexciting them. To test future anti-flea products, and despite the protests of "Animal Rights", an American association for animal protection, there are twenty or so secret laboratory flea farms producing fleas more or less industrially. At ten cents per flea, it's a flourishing business. Eggs are laid on cats, and then the eggs develop in greenhouses reproducing the ideal conditions of heat and humidity for the cycle of metamorphosis. For even greater productivity, "artificial dogs" are used. This is a sort of incubator consisting of rows of petri dishes and tubes. The egg-laying fleas feed directly by piercing the membrane on a blood-filled container. Each dish can produce 100 to 200 eggs per day. Could the industrialization of flea farming be the next American miracle?

Sex bomb

Apparently, the flea has multiple partners, and copulates as soon as it leaves its cocoon. The female flea climbs onto the back of the much smaller male. The male flea possesses an elaborate genital apparatus consisting of two appendages measuring a third of his length. One serves to hold the female during the act, the other is the reproductive organ. The male flea also has two sensory suckers just above his eyes, which he uses to hold the female in position during copulation.
The female possesses a sperm bank, at the back of the abdomen. She can therefore keep the spermatozoids well sheltered for several months, while procreating as she pleases and with a high fecundity rate, producing more than 800 eggs during her life.
Not surprising that female fleas were used to test the first contraceptive pills for women.

Fleas as collectors' items

The world's most remarkable flea collection, conserved in the Natural History Museum in London, is the Rothschild collection. Today, 270,000 fleas of great diversity have been inventoried: from the first, collected by the pioneers Charles Rothschild et Karl Jordan, to the smallest, the sticktight measuring a mere millimeter, to the biggest, an Australian flea measuring over eight millimeters.
Several hundred species of flea were discovered by the Rothschilds and Karl Jordan, during numerous expeditions that took them all over the world. It was in Egypt that Lord Rothschild discovered the flea responsible for plague, naming it Xenopsylla cheopis, in honor of the Cheops pyramid. Miriam Rothschild has published several hundred anatomical plates, as well as eight books retracing forty years of research.
She was the first to discover the secrets of the flea's propulsion, and notably performed tests on a rat flea that jumped 30,000 times without stopping.
To analyze and break down the mechanism of the leap, she used an ultra high-speed camera, turning at 3,500 images per second, that is, about 90 km/hour, compared with 24 images per second for a classical camera. No other material would permit this world first event. Unfortunately, despite the high speed, the flea appeared in only seven frames on the reel.

Living syringe

The idea to use fleas to vaccinate rabbits against myxomatosis came from Miriam Rothschild, the renowned British entomologist. This technique has been tested on Australian rabbits and more recently in the Marseilles region. Vaccinator rabbit fleas were released in well-circumscribed test zones. To enable the recognition of vaccinator fleas, they were marked with a fluorescent yellow powder.
The vaccine was first transmitted to the fleas by contact with their mouthparts during feeding. Several weeks later, with the help of hunting ferrets, rabbits were captured to verify that they harbored the vaccinator fleas.
The scientists then used an ultraviolet light detector to find the fluorescent yellow marks on the fleas collected from the rabbits. As soon as a carrier rabbit was detected, the scientists took a sample of its blood to evaluate the immunization effect achieved through the flea's biting. In fact, this technique is so complex and difficult to control that it is unlikely that fleas will ever become effective health workers.

Circus animal

The flea's leaping abilities have always stimulated the imagination of animal trainers and fairground performers. During the 19th Century, flea circuses were to be found at every fair, and they attained such notoriety that they were asked to perform before princes and heads of state.
In 1830, Signor Bertolotto offered performances with a troupe of 100 "artists". They would begin with a scene where fleas, dressed as Don Quichotte and Sancho Panza, mounted little paper horses and manipulated lances with agility. Then came a golden chariot pulled by a flea, another would drag a ball and inch-long chain, and finally there was the ball, where a dozen couples danced to the sound of an orchestra, in which each musician correctly held an instrument. It was only when the first chords of music, provided by a music box, played that the performers, waiting immobile for the signal, would begin to move …
Today, most flea circuses have had to close due to lack of performers. Thanks to soap, the human flea, Pulex irritans, the only species, according to the "trainers", capable of pulling a heavy load, have become rare.
But behind the pleasant folklore surrounding flea circuses hides a clever subterfuge, consisting of controlling their natural aptitudes. In fact, with a thread tied around its metathorax, the flea could no longer jump, and given its instinct to flee the light, it was obliged to walk. Besides, there are very serious doubts that fleas can hear let alone obey orders. The illusion was created when the "trainer" moved the "circus ring" in an undetectable manner to give the impression that the flea was performing the desired gesture.