“Where there are humans,

you'll find flies,

and Buddhas.”

–Kobayashi Issa

Each day, in each country, a housefly is born. Lots of houseflies really. Houseflies have been being born around us for thousands of years. They are born of what everyone else abandons, corpses, cakes, and excrement. Yet their story is inescapably a version of our story. They spread early out of Africa, bound to us. You find them wrapped in mummies, their bodies held tight against the bodies of pharaohs [1]. You find them in ancient latrines, as larvae, tunneling through what we would rather be done with. At picnics they sit on hot dogs. In bedrooms, they look down from walls. In war and tragedy, they mouth what we cannot countenance. They brushed upon Gandhi, Mother Theresa and Caesar, but also Mussolini and you. And before they brushed upon you (or Mussolini) they brushed upon, well, you don’t want to know.

Actually, you might want to know. Or at least some scientists think you might want to know. So it is that there is now a large book worth of scientific studies of just what can be found living on flies. All of these studies are interesting, some are a bit disgusting, and one study from a pig farm in North Carolina is the kind of thing that might just change how you live your life.

Although we have seen houseflies for millennia, complained about them in a thousand languages using a hundred thousand adjectives, in some ways they are still among the least known guests at the table. No one knows for sure where they come from (only that they had already found us as of five thousand years ago). No one knows what they did before they found us (though one imagines it involved decay). What we do know about houseflies is that they gather a little bit of life from everything they touch and redistribute it, a sort of Robin Hood of germs.

Some of the bacteria living on houseflies are their partners. Housefly eggs and larvae depend on beneficial bacteria (such as the species Klebsiella oxytoca) bestowed upon them by their mothers. These bacteria produce compounds that kill fungi and, in doing so, help hungry young flies outcompete those same fungi for their otherwise rapidly decaying food [2]. Others though are hangers on, gathered by accident as the flies bump around the world. When a fly lands, its sticky hairs become covered in bacteria, which can then be transferred to whatever the flies land on next. Flies also store bacteria (gathered from their food) in their alimentary tract. These germs are brought to new places in fly poop, but also—as one treatise on flies delicately puts it— “in small droplets of regurgitated matter which have been called vomit spots.”

Just where do houseflies pick up these other bacteria, the ones the give back to us in vomit spots, feces and footsteps? Well, they find them in what we have abandoned, the remains on which they can survive. Once, houseflies emerged from horseshit by the billions. When that ran out (thanks to the invention of cars), they turned to our garbage and so we collected it more frequently and took it far away. When the garbage become rare (some places, though not everywhere), they found the dog waste we left behind in cities. Now that New Yorkers, for instance, in their fancy shoes and dark clothes, gather the dog poop in bags, the flies have found those places we have taken our waste to hide it (both from them and from ourselves). At garbage dumps flies flock in dense halos. They are born too out of the rough parts of towns—smoke signals of neglect. They have even found the places we have moved our animals, the modern mangers of chickens and pigs where waste is dumped into vast pools. Here, their naked children eclose as writhing maggots only to be born again later to their, hairy, winged forms.

It is among these last flies that my friend Coby Schal recently decided to spend some of his days [3]. Coby has studied insects at pig farms for a while. There are probably worse places to study insects, though I can’t think of them right now. At pig farms pigs accumulate, so, in great ponds, does pig waste. Coby has looked at the movement of roaches from one pig farm to another, but what he wanted to study with the flies was something different. Along with colleagues at Kansas State University, Coby wanted to know just what was being carried aloft as those flies rose. Flies, incidentally, take care in their rise. They bend their legs a little and, ever so gingerly, bounce, while flapping their wings.

Coby and his colleagues found fecal bacteria in 93.7% of the flies at the pig farm (The aptly named Enterococcus faecalis was the most common species). This came as no surprise. Houseflies the world over carry fecal bacteria. The surprise was many of those bacteria were resistant to antibiotics, such as tetracycline and erythromycin, antibiotics used to treat human bacterial diseases [4]. Such resistant forms, so-called superbugs, can kill, and while finding them on flies near pig farms does not guarantee they are making their way from the farms to our bodies via flies, it certainly suggests the possibility.

But why would the flies in pig farms tend to have antibiotic resistant bacteria? Herein lies the secret you might not have heard. Most pigs in the U.S., as well as most farm animals more generally, are fed antibiotics. By some estimates, eighty percent of antibiotics produced in the U.S. are used on animals. The antibiotics are not used to treat infections. Instead they serve solely to promote rapid growth, to make your bacon or burger cheaper and faster. As an evolutionary side effect when pigs are fed those antibiotics their weak bacteria--those susceptible to the antibiotics being used--die. Those most likely to survive are the lineages resistant to antibiotics, the tough mothers. If isolated on pig farms, all of this is imprudent but not tragic in as much as it seems isolated, faraway from our daily lives. Then the flies enter the story.

Houseflies can fly and they can do so more effectively than you might imagine. They fly with the wind, but even against it. Individual houseflies have been recorded having traveled more than ten miles [5]. Consider the geography of farms. Imagine the flies rising up from them and flying toward you. Whatever new resistant strains of bacteria they bear may be closer than you think. They might be tapping at your window now or, as Chekhov said of them, “brushing against the ceiling,” their bodies bouncing along as they leave their bacteria behind.

Humans tend to dislike successful animals. We scorn the murders of crows, the flocks of starlings and the even the ants that boil up around and into our houses. Their bodies seem vulgar. The flies though, we conclude, are not just loathsome but dirty and even, in the context of Coby Schal’s new study, potentially deadly. This is one lesson to take from the flies, but the wrong one. The real truth they offer, if we pay attention, is more about the nature of humans than it is the nature of flies. Anopheles mosquitoes are vectors of malaria, but houseflies, well, they are vectors of what we leave behind, carrying it back to us, as though to say, “Over here! You forgot something…” They are the messenger nobody asked for, bearing the messages nobody wants, whether about the overuse of antibiotics or some other of our failings. So go ahead and kill the messenger, but heed the message. Meanwhile, billions of fly eggs are ready to hatch out of whatever we leave behind.

1-Panagiotakopulu E, Buckland PC, Kemp BJ (2010) Underneath Ranefer’s floors—urban environments on the desert edge. J Archaeol Sci, 37:474–481

2-Zvereva EL (1986b) Peculiarities of competitive interaction between larvae of the house fly Musca domestica and microscopic fungi. Zoologicheskii Zhurnal 65:1517–1525, Lam K, Thu K, Tsang M, Moore M, Gries G. 2009. Bacteria on housefly eggs, Musca domestica, suppress fungal growth in chicken manure through nutrient depletion or antifungal metabolites. Naturwissenschaften, 96 :1127-1132.

3-Well, and to send his students, postdocs and technicians, to spend theirs.

4-Ahmad A., A. Ghosh, C. Schal, and L. Zurek. 2011. Insects in confined swine operations carry a large antibiotic resistant and potentially virulent enterococcal community. BMC Microbiology, 11:23.

5-Chakrabarti S, Kambhaampati Zurek L. 2010. Assessment of house fly dispersal between rural and urban habitats in Kansas, USA. J Kans Entomol Soc, 83:172-188.


Image 1. Fly foot (AKA tarsus)

Image 2. Horse and housefly. This is not really the relationship I was talking about, but this drawing was too funny to resist. Certainly, the idea of houseflies riding into cities on horses is right, it is just that they would be riding a little further back. From the funny houseflies collection.

Image 3. Canoe ride anyone? This is a typical waste pond at a pig facility. From a distance (or in a photo) it seems pleasant enough, but that pleasantness is an illusion.