SI Vault
Bil Gilbert
August 15, 1977
Fleas have been putting the bite on man and beast for millennia, and even in this modern age of sophisticated pesticides, not much can be done about it
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August 15, 1977

I've Got You Under My Skin

Fleas have been putting the bite on man and beast for millennia, and even in this modern age of sophisticated pesticides, not much can be done about it

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Fleas are oblong, neckless and waist-less creatures, ideally designed to slip through an underbrush of fur, feathers or thermal underwear. Their flanks are smooth and slippery, which facilitates their skipping about. However, hooks on their legs and feet enable them to cling tenaciously to a stump of hair when they come to a glade of inviting skin and decide to stop and browse.

Fleas are of a respectable size in comparison with other parasites. Generally they are five millimeters long or a bit less. Looked at sideways, fleas seem almost nonexistent, being what entomologists call "severely compressed laterally," or about as flat as an animal can be. Any dog that has tried to nip a flea, any person who has tried to pinch one between fingernails or for that matter smash one between bricks knows all about the lateral compression of fleas. They are so compressed that it is next to impossible to compress them any further.

It is hard to hold a flea and hard to do anything to him once you have him, but it is even harder to catch one to begin with. In comparison to fleas, crickets, kangaroos and David Thompson are all but earthbound. The flea world records for the long jump and high jump are 13 inches and eight inches, respectively. This may not seem like much, but if fleas were as big as men and retained their prowess, they could long-jump 450 feet and high-jump 275.

Actually, if fleas grew up and entered sanctioned competition they would probably be disqualified for failing a drug test. Housed inside an arch near the base of the flea's hind legs is an elastic-like clump of protein called resilin that can be stretched and contracted back to its original shape much faster than any known rubber. As the flea begins his jump, it crouches like a sprinter in the starting blocks, lowering its head and contracting its body. These actions compress the resilin, and the flea can unleash the stored energy at will and leap out of reach in a great burst.

The potential uses of resilin among other species are various and obvious. For example, a basketball coach who was somewhat less than acutely interested in a discussion about fleas came immediately awake when the magical properties of resilin were mentioned. "O.K., where can we get it? Do you think they can put it in a pill or would you have to shoot them in the fanny before each game? Maybe you could build it into shoes. I mean, you wouldn't want to give them a lot. You wouldn't want those clowns hitting their heads on the press box. Just enough so you could get 15 or 20 out of your low-post man or maybe in a tight game from the power forward."

Jumping, high or low, doesn't seem to affect a flea's life span. If the external situation is ideal, the whole cycle from adult to adult—egg, larva, cocoon, adult—can be completed in less than a month. However, if environmental conditions are unfavorable, it may not be completed for a year, the cocoons simply remaining intact, marking time until it is propitious for the adult to emerge. Adult fleas are relatively long lived; the world record is held by a Russian flea that lived 1,487 days.

Although fleas can survive in one stage or another in a fairly wide range of environmental conditions, they do not like extremes of heat, cold or aridity and do not cope well with standing water. All of which accounts for the fact that they have evolved as parasites on nest-building animals—the nest being a creation that modifies the environment. Fleas are most fertile and active in situations where they are protected from precipitation, where the temperature remains between 65� and 80� and the humidity hangs at about 65% or 70%. You will immediately note that the place that meets these requirements and also provides lots of hiding places and quantities of warm blood is the modern family dwelling. So perfectly are our homes suited to fleas that it is almost as if through the centuries the insects had employed us to invent tight roofs, central heating and shag rugs.

By way of example, the golden retriever who instigated this investigation is a confirmed house dog. In clement weather she moseys around outside for a few hours a day, but otherwise hangs out under tables, desks and on rugs. Her son, a hearty, clumsy, unruly beast, seldom comes inside, largely by popular demand. He has more or less improvised his own living arrangements, bedding down on a pile of leaves on the lee side of the barn. During the summer, when all the world, or at least temperate Pennsylvania, is a marvelous place for fleas, he has a few of them, but from October through April, the time of sleet, snow and flood, he is virtually flealess. His dam, on the other hand, does a little scratching most of the year. The moral of the story is that anyone who is serious about avoiding fleas should close up the house and go live by the side of the barn.

Fleas have evolved specialized preferences as to their nest mates. In addition to human fleas, there are, among many others, dog, cat, pig, hen, bat, rat, mouse, bird and squirrel fleas. The various species may show some anatomical or behavioral differences (as does the rabbit flea) but in the main they are categorized by their choice of hosts. However, gastronomically speaking, fleas look at life much as we do. Some of us may find lamb chops more tasty than calf's liver. If there are no chops and we are hungry we will take the liver or even frozen breaded shrimp. So will a flea. If he is a cat flea but can find no cats, he will lunch on a dog or a man.

The cosmopolitan taste of these insects was demonstrated by an entomologist named George McCoy, who once combed fleas out of 606 brown rats. His bag was as follows: 1,936 Oriental rat fleas, 1,822 European rat fleas, 650 human fleas, 181 mouse fleas and 28 dog fleas. McCoy then counted the fleas on four dogs and found 54 dog fleas, 25 human fleas, one cat flea and one squirrel flea. He concluded the hunt by defleaing 29 humans, who yielded 337 human fleas, three squirrel fleas, one cat flea and one dog flea.

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