Why Did God Make Houseflies? (Part 1)

A close look at their lives will not make you want to stop swatting them.

By Richard Conniff

Marvel’s next big movie franchise: HOUSEFLY MAN! Weapon of choice: Vomit. (Photo: Kostümpalast)

With apologies, I’m planning to go quiet for two weeks to work on a book idea. I expect to be back here on July 12. Meantime, I’ll leave you with a doubleheader, a two-part piece about one of my least favorite animal life forms. Maybe yours, too. I wrote this piece for Audubon Magazine years ago, when editor Les Line allowed his writers freedom to take a story in the direction, and the voice, that we chose. One result was this antic tour of the housefly’s world.

Though I have been killing them for years now, I have never tested the notion, recorded in one collection of country sayings, that with a little cream and sugar, a fly "tastes very much like a black raspberry." So it's possible I'm speaking too hastily when I say there is nothing to like about flies. Unlike the poet who welcomed a "busy, curious fly" to his drinking cup, I don't cherish them for reminding me that life is short. Nor do I much admire them for their function in clearing away carrion and waste. It is, after all, possible to believe in the grand scheme of recycling without necessarily liking undertakers.

Among poets, I tend to side with Ogden Nash, who once wrote: "God in His wisdom / Made the fly / And then forgot / To tell us why."

A fly is standing on the rim of my beer glass as I write these words. Its vast, mosaic eyes look simultaneously lifeless and mocking. It grooms itself methodically, its forelegs twining together like the arms of a Sybarite luxuriating in bath oil. Its hind legs twitch across the upper surface of its wings. It pauses, well- fed and at rest, to contemplate the sweetness of life.

We are lucky enough to live in an era when scientists quantify such things, and so as I type and wait my turn to drink, I know that the fly is neither busy nor curious; the female spends 40.6 percent of her time doing nothing but contemplating the sweetness of life. I know that she not only eats unspeakable things, but that she spends an additional 29.7 percent of her time spitting them back up again and blowing bubbles with her vomit. The male is slightly less assiduous at this deplorable pastime, but one diligent researcher has reported that a well-fed fly may also defecate every four and a half minutes. Flies seldom trouble us as a health threat anymore, at least in the developed world, but they are capable of killing. And when we are dead (or sooner, in some cases), they dine on our corrupted flesh.

It is of course mainly this relentless intimacy with mankind that makes flies and particularly houseflies so contemptible. Leeches or dung beetles may appall us, but by and large they satisfy their depraved appetites out of our sight. Flies, on the other hand, routinely flit from diaper pail to dinner table, from carrion to picnic basket. They are constantly among us, tramping across our food with God knows what trapped in the sticky hairs of their half-dozen legs.

Twice in the twentieth century, Americans waged war against flies, once in a futile nationwide "swat the fly" campaign, and again, disastrously, with DDT foggings after World War II. The intensity of these efforts, bordering at times on the fanatic, may bewilder modern readers. "Flies or Babies? Choose!" cried a headline in the Ladies' Home Journal, in 1920. But our bewilderment is not entirely due to greater tolerance or environmental enlightenment. If we have the leisure to examine the fly more rationally now, it is mainly because we don't suffer its onslaughts as our predecessors did. Urban living has separated us from livestock, and indoor plumbing has helped us control our own wastes, thus controlling flies. But if that changed tomorrow, we would come face-to-face with the enlightened, modern truth: With the possible exception of Homo sapiens, it is hard to imagine an animal as disgusting or improbable as the housefly. No bestiary concocted out of the nightmares of the medieval mind could have come up with such a fantastic animal. If we want to study nature in its most exotic permutations, the best place to begin is here, at home, on the rim of my beer glass.

In North America, more than a dozen fly species visit or live in the house. It is possible to distinguish among some of them only by such microscopic criteria as the pattern of veins in the wings, and so all of them end up being cursed as houseflies. Among the more prominent are the blue and the green bottleflies, with their iridescent abdomens, and the biting stable flies, which have served this country as patriots, or at least as provocateurs. On July 4, 1776, their biting encouraged decisiveness among delegates considering the Declaration of Independence: "Treason," Thomas Jefferson wrote, "was preferable to discomfort."

The true housefly, Musca domestica, of course does not bite. (You may think this is something to like about flies, until you find out what they do instead.) M. domestica, a drab fellow of salt-and-pepper complexion, is the world's most widely distributed insect species and probably also the most familiar, a status achieved through its pronounced fondness for breeding in pig, horse, or human excrement. In choosing at some point in the immemorial past to concentrate on the wastes around human habitations, M. domestica made a brilliant career move. The earliest known human representation of what appears to be a housefly is on a Mesopotamian cylinder seal from 3000 B.C. Somewhat later, the ancient Romans used a poultice of mashed houseflies as a treatment for baldness. (Flies are hairy, and the theory was that this might rub off on bald men.) But houseflies were probably with us even before we had houses, and they spread with human culture.

The Appetite for Abomination

Like us, the housefly is prolific, opportunistic, and inclined toward exploration. It can adapt to either vegetable or meat diets, preferably somewhat ripe. It will lay its eggs not just in excrement, but in a rotting mass of lime peels, in bird nests, in carrion, or even in flesh wounds that have become infected and malodorous. Other flies aren't so flexible. For instance, M. autumnalis, a close relative, prefers cattle dung, and winds up sleeping in the pasture more than in houses or yards.

But while the housefly's adaptability and evolutionary generalization may be admirable, it raises one of the first great questions about flies: Why this dismaying appetite for abomination?

Houseflies not only defecate constantly, but do so in liquid form, which means they are in constant danger of dehydration. The male can slake his thirst and also get most of the energy he needs from nectar. But fresh manure is a good source of water, and it contains the dissolved protein the female needs to make eggs. She also lays her eggs in excrement or amid decay so that when they hatch, the maggots will have a smorgasbord of nutritious microorganisms on which to graze.

Houseflies bashing around the kitchen or the garbage shed thus have their sensors attuned to things that smell sweet, like flowers or bananas, and to foul-smelling stuff like ammonium carbonate, hydrogen sulfide, and trimethylamines, the products of fermentation and putrefaction. (Ecstasy for the fly is the stinkhorn fungus, a source of sugar that smells like rotting meat.) The fly's jerky, erratic flight amounts to a way of covering large territories in search of these scents, not just for food, but for romance and breeding sites. Like dung beetles and other flying insects, the fly will zigzag upwind when it gets a whiff of something good (or, as often happens, something bad) and follow the scent plume to its source.

Flight School

Hence the second diabolical question about the housefly: How does it manage to fly so well? Why is it so adept at evading us when we swat it? How come it always seems to land on its feet, usually upside down on the ceiling, having induced us to plant a fist on the spot where it used to be, in the middle of the strawberry trifle, which is now spattered across tablecloth, walls, loved ones, and honored guests?

When we launch an ambush as the oblivious fly preens and pukes, its pressure sensors alert it to the speed and direction of the descending hand. Its wraparound eyes are also acutely sensitive to peripheral movement, and they register changes in light about ten times faster than we do. (A movie fools the gullible human eye into seeing continuous motion by showing it a sequence of twenty-four still pictures a second. To fool a fly would take more than two hundred frames a second.) The alarm flashes directly from the brain to the middle set of legs via the largest, and therefore the fastest, nerve fiber in the body. This causes so-called starter muscles to contract, simultaneously revving up the wing muscles and pressing down the middle legs, which catapult the fly into the air.

The fly's wings beat 165 to 200 times a second, and while this isn't all that fast for an insect, it's more than double the wingbeat of the fastest hummingbird, and about 20 times faster than any repetitious movement the human nervous system can manage. The trick brought off by houseflies and many other insects is to remove the wingbeat from direct nervous system control, once it's switched on. Instead, two systems of muscles, for upstroke and downstroke, are attached to the hull of the fly's midsection, and trigger each other to work in alternation. When one set contracts, it deforms the hull, stretching the other set of muscles and making them contract automatically a fraction of a second later. To keep this seesaw rhythm going, openings in the midsection stoke the muscles with oxygen directly from the outside (the fly has no lungs). Its blood (which lacks hemoglobin and is therefore colorless) meanwhile pumps fuel for the cells to burn at a rate 14 times faster than when the fly is at rest. Flies can turn a sugar meal into usable energy so fast that an exhausted fly will resume flight almost instantly after eating. In humans . . . but you don't want to know how ploddingly inadequate humans are by comparison.

Once airborne, the fly's antennae, between its eyes, help regulate flight, vibrating in response to airflow. The fly also uses a set of stubby wings in back, called halteres, as a gyroscopic device. Flies are skillful at veering and dodging; it sometimes seems that they are doing barrel rolls and Immelmann turns to amuse themselves while we flail and curse. But one thing they cannot do is fly upside down to land on a ceiling. This phenomenon puzzled generations of upward-glaring, strawberry-trifle-drenched human beings, until high-speed photography supplied the explanation. The fly approaches the ceiling right-side up, at a steep angle. Just before impact, it reaches up with its front limbs, in the manner of Superman exiting a telephone booth for takeoff. As these forelegs get a grip with claws and with the sticky, glandular hairs of the footpads, the fly swings its other legs up into position. Then it shuts down its flight motor, out of swatting range and at ease.

My War Crimes

While landing on the ceiling must be great fun, humans tend to be more interested in what flies do when they land on food, and so I trapped the fly on the rim of my beer glass. (Actually, I waited till it found a less coveted perch, then slowly lowered a mayonnaise jar over it.) I'd been reading a book called To Know a Fly by Vincent Dethier, in which he describes a simple way of seeing how the fly's proboscis works. First, I refrigerated the fly to slow it down and anesthetize it. Then I attempted to attach a thin stick to its wing surface with the help of hot wax. It got away. I brought it back and tried again. My son Jamie, who was then four years old, winced and turned aside when I applied the wax. "I'm glad I'm not a fly," he said, "or you might do that to me." I regarded him balefully but refrained from mentioning the ant colony he had annihilated on our front walk.

Having finally secured the fly, I lowered its feet into a saucer of water. Flies have taste buds in their feet, and when they walk into something good (bad), the proboscis, which is normally folded up neatly inside the head, automatically flicks down. No response. I added sugar to the water, an irresistible combination. Nothing. More sugar. Still nothing. My son wandered off, bored. I apologized to the fly, killed it, and decided to look up the man who had put me in the awkward position of sympathizing with a fly, incidentally classing me in my son's eyes as a potential war criminal.

(to be continued tomorrow)

Richard Conniff’s books include Spineless Wonders: Strange Tales of the Invertebrate World (Henry Holt), now available as a kindle e-book. His most recent book, Ending Epidemics: A History of Escape from Contagion (MIT Press), is now out in paperback. He is a National Magazine Award-winning feature writer for Smithsonian, National Geographic, and other publications, and a past contributing opinion writer for The New York Times