We may detest flies, but our relationships with them are extraordinarily complex and often intimate. As well as vexing us, biting us, and making us sick, flies help feed us, remove our waste, and control our pests. They also consume us when we die—and many don’t even wait that long. Some indication of the fear, awe, and loathing we reserve for the Diptera, the insect order in which flies are classified, can be felt in the wonderfully onomatopoeic name given to Satan: Beelzebub, Lord of the Flies.
David Cronenberg’s 1986 sci-fi horror classic The Fly (a remake of the 1958 film of the same name) investigates the fly-human relationship in a thoroughly modern way. When a teleportation machine scrambles the genes of scientist Seth Brundle with those of a fly that accidentally enters the device, Brundle (magnificently played by the increasingly naked and bug-eyed Jeff Goldblum) transforms before our eyes into a fly/human hybrid. As he grows into “Brundlefly,” he becomes enormously strong and acrobatic, displaying Olympic virtuosity on the horizontal bars, as well as climbing vertical walls with terrifying agility. To the wonderment of his partner, Veronica Quaife (played by the adorable Geena Davis), he also becomes capable of feats of sexual endurance that leave her in a state of sweating, dehydrated exhaustion. But then, to everyone’s dismay, things start going wrong. Brundle’s body begins to rot away, as does his human empathy. Memorably, his feeding habits change: he loses his teeth and begins to eat doughnuts by vomiting copious volumes of digestive fluid on them, before slurping up the resultant mush.
If Cronenberg had had access to Jonathan Balcombe’s extraordinary book Super Fly, minor aspects of his movie might well have been different. Balcombe, a biologist of animal behavior who has written several books dispelling myths about other species, informs us that flies do not externally digest their food in the way Brundle does. They merely salivate on food to moisten it, in more or less the same way mammals do—except that we moisten our food when it is inside our mouths.
It turns out, however, that Cronenberg got much of the biology right in The Fly. (The same cannot be said of the 1958 original, in which the scientist ends up as a human head on a fly’s body.) As Brundlefly does, flies climb walls and cling to ceilings by extruding a glue-like substance from their feet. And true to the film, they also enjoy long bouts of sex. Lovebugs of the fly family Bibionidae are some of the most indefatigable copulators in the animal world, able to stay at it continuously for fifty-six hours. Other flies indulge in astonishing acrobatics, with males dangling females upside down from their genitals while clinging to vertical walls.
Ingenious experiments have shown that flies really do enjoy sex. In one, male fruit flies were paired with females that were receptive to sex, while others were paired with females that rejected sexual advances. Both groups of males were then given access to a solution laced with alcohol and another lacking it. In remarkably humanlike fashion, the sexually frustrated males consumed more alcohol than their sexually satisfied compatriots.
A second experiment involved male flies that were genetically engineered so that exposure to a red light led to ejaculation. (Human ingenuity seems endless.) When released into a cage with a red light at one end, the modified males crowded into the “red-light district.” And when the flies used in this experiment were given access to alcohol, the sexually satisfied red-light-district patrons drank less than their counterparts. A healthy sex life has, moreover, been definitively linked to fly health. Male fruit flies that are exposed to female fly pheromones but lack an opportunity to mate become stressed and prone to starvation, resulting in early deaths.
Flies are among the most diverse of all insects, as well as the most annoying. There are 160,000 known species of them. Biting midges and mosquitoes are flies, as are a myriad of other species you’ve almost certainly never heard of, the largest of which can hunt small hummingbirds. Some flies drink nectar and look like bees, while others parasitize creatures as diverse as ants and humans.
And they are abundant. According to one estimate, there are 200 million flies for every person on earth, and their lives and body forms are so varied that it’s difficult to understand why they are all considered to belong to the same order. But one thing they all have in common is a single pair of wings, the second set (possessed by most other insects) having become modified into small drumstick-like projections known as halteres, which act as flight stabilizers. The astounding agility of flies in flight owes much to their halteres.
In The Fly, Davis’s character becomes pregnant by Brundle and dreams that she gives birth to an enormous maggot. Most flies lay eggs, but some give birth to live young. The dreaded tsetse, for example, gives birth to a single maggot that is three quarters the length of its mother. Among those flies that give birth to live maggots, Balcombe informs us, “there is some urgency to…letting them out because occasionally the ungrateful little tykes will start eating the mother from the inside.”
Even parasitic flies can become the victims of other parasites. When the spores of the Cordyceps fungus land on a housefly and germinate, they turn their victim into a fungus-serving robot. As its body becomes riddled with fungal hyphae, the fly gets an irresistible urge to ascend to a high place. Once there, it sticks out its feeding proboscis and glues itself to the surface. Firmly fixed into place, the fly then buzzes its wings for a few minutes before locking them into a vertical position, aiming its abdomen upward. As the fly dies, tendrils of the fungus push through its skin, releasing spores to infect other flies and begin the cycle anew.
Scuttle flies specialize in parasitizing ants, and some have a particularly gruesome modus operandi. The flies stalk their victims along ant trails, and when they find a vulnerable ant they lay a single egg on its thorax. When the egg hatches, the maggot enters the ant through the fissure between its head and thorax. From there, the maggot burrows into the ant’s head, where it feeds on powerful biting muscles. “After a couple of weeks,” Balcombe writes, “the full-grown maggot releases an enzyme that dissolves the membrane connecting the ant’s head to its body.” As the headless body stumbles around, the maggot turns the ant’s head into a protective capsule from which the adult fly emerges two weeks later. (I’m grateful that the makers of The Fly did not have access to Super Fly, if only because I could not bear the sight of Davis’s head rolling around on the floor of Brundle’s apartment.)
Botflies are large flies that parasitize mammals. Their eggs are sometimes carried by mosquitoes, which spares the large adult fly risk of death from a swatting tail or hand. They are common parasites on reindeer, cattle, and even dogs. Some, known as snot bots, squirt larvae into the nostrils of sheep, goats, deer, moose, horses, and camels, from which they migrate to the sinuses, mature, and get snorted out to pupate in the earth. The only known example of the species called the mammoth botfly was found fossilized in the stomach of a frozen mammoth. Before it vanished, along with the last of the mammoths over four thousand years ago, it presumably irritated their trunks and throats.
One species of botfly thrives today because its preferred host, humans, exists in unprecedented numbers. As Balcombe writes, in 1999 the life cycle of the human botfly was investigated in detail, albeit inadvertently, by Robert Voss of the American Museum of Natural History. He encountered the species in French Guiana while hiking shirtless through the rain forest. When back home in New Jersey he noticed slight prickling sensations on his back, and his wife, Nancy Simmons, saw red lumps that resembled irritated mosquito bites. Voss eventually sought the services of a dermatologist, who referred him to a “tropical specialist” Balcombe calls Dr. X.
Dr. X was eager to see Voss, exclaiming, “I think you have myiasis,” a parasitic infestation by a fly larva. “Ah yes, here it is!” he cried as he operated behind Voss’s back. Without explaining what he was doing, he cut out a conical hunk of bloody flesh that had a small fly larva wriggling at its apex. Voss had not given permission for the excision, and he suspected that Dr. X desperately wanted a specimen for his collection. When Dr. X moved to excise a second maggot, Voss objected, and when he asked for the excised maggot to be returned to him, Dr. X “squawked with indignation.” Finally, Dr. X offered to waive his fee in exchange for the first maggot. Voss still regrets that he accepted the offer.
Voss and Simmons started a “botfly watch,” and a bond began to develop between Voss and his sole surviving maggot. Hosting the maggot was obviously a deeply moving experience for Voss, who remarked to Simmons that it “was as close to pregnancy” as he’d ever get. When the larva and its pupal case emerged from Voss’s body, he put it in a safe place; when the adult fly hatched five weeks later, it and its puparium were donated to the collections of the American Museum of Natural History.
As a tropical biologist myself, I was astonished at Voss’s benign attitude toward his parasite. I’ve never had a bot, but I was once parasitized by an unknown organism that migrated under my skin for weeks. Starting at my left elbow, it crossed my chest and belly, finally getting as far as my right knee. It left large lumps in its wake that were soft at first but eventually hardened. My medico, perhaps lacking Dr. X’s expertise, never suggested excising a lump. When I asked what they were, he looked puzzled and proclaimed that they were a sign of a man who had spent too much time in New Guinea.
Is it more acceptable to suck blood than consume flesh? Mosquitoes are simply specialized, bloodsucking flies. According to one estimate they collect 1.6 million gallons of American blood per year. But they take only a small amount from each victim—so small, in fact, that it would require between 200,000 and two million bites to exsanguinate an adult human. Sometimes characterized as the most dangerous creatures on earth, mosquitoes transmit a variety of diseases, including malaria, which makes us loathe them. But they also have their mysteries. They fly at a mere three miles per hour, making them relatively easy to swat, and they produce an annoying whine that alerts victims to their presence—a siren that must cost a lot of mosquito lives. So why haven’t they evolved to be silent? It turns out that it’s mostly females that whine, and the noise is generated in order to attract males. But only females suck blood, which they need in order to lay eggs, so the buzzing both makes breeding possible and endangers the future mother and her offspring.
The tribes of bloodsucking flies are numerous, and some make the stealthy mosquito, which administers anesthetic and anticoagulant saliva to its victims with surgically precise cutting mouthparts, seem positively civilized. The tiny biting midges, otherwise known as sand flies, punkies, and no-see-ums, are in some places so numerous and impossible to avoid that they can drive one to the brink of madness. They attack in multitudes, crawl into the smallest openings left in protective clothing, and leave large, itchy welts that can bleed profusely and cause irritation for weeks. Stable flies, blackflies, horseflies, and March flies are much larger, and when they bite they can inflict real pain. Thankfully, some, more used to biting deer or cattle, are not used to being swatted and are easy to crush against the skin before they strike.
Robber flies are, according to Balcombe, the Rolls-Royce of flies. Huge, burly, and equipped with a venomous sting, they are the ones that occasionally catch and kill hummingbirds. None—including the gigantic Satanas gigas, or great Satan fly (what a wonderful inversion, naming a fly after the devil!)—are particularly aggressive toward humans. One entomologist who is fascinated by robber flies told Balcombe, “The only way I can get them to bite me is to hold them between my fingers and press them against my skin, and even that doesn’t always work.” He added that the bite is “not very painful, nothing like a wasp sting.” Such is the enthusiasm of some entomologists for these majestic insects that they have successfully lobbied for a World Robber Fly Day, observed annually on April 30.
One of the many valuable services that flies carry out is waste removal. A dead animal seething with maggots is a revolting sight, but there’s no denying that the maggots are doing their bit to help keep the world clean. In fact, some maggots are so good at consuming dead flesh and outcompeting or killing deadly bacteria that they are used by doctors to treat wounds. Dr. Ronald A. Sherman, an expert in the use of maggots in medicine, says that 40 to 70 percent of patients whose wounds have failed to respond to all conventional care and who were scheduled for amputation respond so positively to maggot therapy that they either heal and avoid amputation or require much less aggressive surgery.
Over a lifetime, Balcombe claims, the average American produces 25,000 pounds of poop, a volume that would see the world awash with feces were it not for the fact that flies and other organisms consume it. It takes a certain type of person to study poop-eating flies, and they are not always popular among their colleagues. One of their most vital pieces of equipment is the poop-baited flytrap. A possibly costive researcher was using such traps in the tropics (where the traps are quickly cleared) when, lacking bait, he was forced to take desperate measures. He tracked some colleagues (who were doubtless studying birds or some other such delightful creatures) into the bush each morning, and when the coast was clear he dug up their poop to use in his traps. The ploy worked well enough for a few days, but when his colleagues discovered what he was up to, their indignation almost led to a fistfight, with one exclaiming, “If you want my shit, you can ask for it.”
There are, I’m relieved to report, a large number of flies that abjure feces and dead bodies, and instead pollinate flowers. We often imagine that bees are the greatest pollinators, but in the beautiful, flower-filled Alpine valleys of Europe, two thirds of the pollinators are flies. In the Arctic, hoverflies and houseflies carry out 95 percent of the pollinations. Without pollinating flies, we wouldn’t have tropical plants like cacao and jackfruit, or as many geraniums, violets, or irises.
Some pollinating flies are such marvelous bee mimics that they have appeared in advertising. The label of one brand of honey features a pink flower on which sits a fly rather than a bee. (Presumably the manufacturers were unaware of their mistake.) An article in a major newspaper lamenting the decline of bees showed an image of a drone fly, prompting one entomologist to quip that “bees are getting so scarce that the newspaper couldn’t find a photo of one.” Conversely, a printing of William Golding’s classic novel Lord of the Flies features a cover picture of an insect that is not a fly.
As forensic science advances, flies are becoming the detective’s best friend. In the US, around fifty species of flies feed off dead humans, and they follow a strict schedule that assists pathologists in determining the time of death. The blowflies arrive first—often within minutes—while the last comers are the so-called cheese skippers, which arrive when the corpse is little more than a dried-out husk. The cheese skippers, incidentally, are vital to the production of one of the world’s most unusual cheeses: Sardinian casu marzu is a type of pecorino deliberately infested with maggots that digest the cheese’s fats (a process thought to enhance its flavor) and that are eaten, preferably alive, along with the cheese.
I would be surprised to discover that any creature has suffered more in the pursuit of human knowledge than the fly. For decades the fruit fly has been the creature of choice for studies of genetics. Drosophila researchers have won seven Nobel Prizes, and entire journals are dedicated to the publication of results from fruit fly studies. Around 100,000 strains of fruit flies have been created in the lab, many of which carry genetic defects that allow us to better understand diseases. The Ken and Barbie mutants lack external genitalia, Tin Man lacks a heart, and Cheap Date loves alcohol. Drop Dead, Sponge Cake, Swiss Cheese, and Egg Roll fruit flies all carry hereditary diseases that manifest in patterns of brain degeneration similar to those seen in humans. And fruit fly studies are becoming ever more sophisticated and focused on the nexus between nature and nurture. Marla Sokolowski of the University of Toronto is examining fly behavior that might shed light on the causes of autism, and she’s also studying flies that suffer chronic defeat at the wings of their rivals, in the hopes that their experience might inform studies of depression in humans.
The final chapter in Balcombe’s surprising book, titled “Caring About Flies,” examines ethics as they pertain to relationships with these creatures. The philosopher Jeffrey Lockwood’s paper “Why Is It (at Least a Small) Wrong to Harm a Fly?” resonates with Balcombe: Lockwood’s answer is that flies can feel pain. Some laboratory-bred fruit flies are born without anuses. I was surprised at the empathetic response of a supervising entomologist when shown one such unfortunate creature: “You’ll have to kill it, it’s in excruciating pain.” Balcombe, whose earlier books also explored the inner lives of animals,* can empathize with the agony of a fly that is unable to shit. He thinks that flies are rather like us in that their tiny brains seem capable of experiencing states akin to not only pain but sexual frustration, joy, and perhaps many other feelings and emotions familiar to humans.
What a wonderful book Super Fly is! Well written and full of fascinating facts, it urges us to appreciate one of nature’s least favored groups. Even if you can’t empathize with flies, Super Fly suggests good reasons for not reaching automatically for the swatter or bug spray at the first sign of buzzing. Without flies, crimes would go unsolved, flowers unpollinated, and garbage unremoved. Indeed, flies are clearly so vital to life on earth that a world without flies would quickly experience ecological collapse. Perhaps we are due for another remake of The Fly, with Balcombe as an adviser, that reveals more holistically the human-fly relationship.
See, for example, What a Fish Knows (Scientific American/Farrar, Straus and Giroux, 2016); reviewed in these pages by Elizabeth Kolbert, June 23, 2016. ↩