Welcome to Meet Our Little Neighbors, an occasional series about the little guys who live among and around us, but rarely get the attention they deserve.
Recently when a fly barged into my home, uninvited, I left them alone. I was feeling magnanimous and in need of some good karma. Perhaps, I reasoned, the mercy I showed this fly would return to me tenfold on some later date. I watched the fly zig-zag down my hallways and felt smugly philanthropic. How many flies got to live out the rest of their limited days on Earth in a habitat as temperate and well-attended as my pre-war apartment? Its constant buzzing, especially near my face, was annoying, and a part of me blanched each time my eyes strayed from my food, opening up the possibility for the fly to regurgitate its fly juice on my avocado toast. But these were all small, tolerable, annoyances, I reasoned. How long could a fly possibly live?
A week passed. The fly remained resolutely alive, fitter than ever, doing laps around the apartment. I opened some windows and attempted to waft it outside. I watched Melon, my prized bug-hunting cat, gallop after the fly as it buzzed from room to room. I swatted at the fly with my Croc, but it evaded me. Days later, my partner and I locked Melon and the fly in a room we temporarily nicknamed the "Killing Arena" and waited outside.
Several thuds later, we entered the arena to discover there was no body; it appeared that Melon, an honorable killer, had already consumed it. I felt a pang of regret. In what may have been the final moments of the fly's life, I had decided I wanted to write about it for this series, as flies are some of our most persistent and inescapable neighbors. I assumed it was a house fly, whose scientific name is Musca domestica, because it was literally a fly inside my house. But when I emailed a handful of fly researchers with a rough description of the victim—biggish, dark, and fly-shaped—I learned I was probably wrong.
"It’s possible that the fly in your house is a house fly," Jung Kim, a national taxonomist in the U.S. Department of Agriculture's Animal and Plant Health Inspection Service, wrote in an email. "But there are thousands of fly species, so it could easily be something else."
Just how many fly species are out there? Kim told me that our planet buzzes with 153,000 known fly species, which represent an amazing 10 percent of the planet's biodiversity. Of course, many of these flies live far away, in the tropics; it's not as if there were 153,000 possible identities of my late intruder. Luckily, I had a much more limited pool of suspects to match my half-remembered physical description of the victim. "Small black flies narrows it down to maybe 1000+ species," Erica McAlister, the principal curator of Diptera (flies) and Siphonaptera (fleas) at the Natural History Museum in London, wrote in an email.
I was at an impasse. How could I properly memorialize a deceased fly, let alone learn about its unique biology and lifestyle, if its species was unknown to me? I scrolled through pages of fly species on Wikipedia, half hoping that some photo would spark a moment of recognition as Law & Order: SVU wrongly taught me happened in lineups: That's him! That's the fly that broke into my house! But in all the days I spent alongside my fly, I had never gotten a good look at it. Maybe, I reasoned, I could still sleuth out its identity—if not down to a species, then at least a general family of flies. I turned to the experts for assistance.
Suspect No. 1: The House Fly
The house fly, Musca domestica, looks like the platonic ideal of a fly—not beautiful, per se, but the fly that would appear alongside the entry for "fly" in an encyclopedia. (Musca is Latin for "fly," and you can probably guess what domestica means.) They are small, dark, and slightly hairy, with two big brick-red eyes and a glossy set of wings. They are about as wide as a Ticonderoga pencil. Of course, there are minute details that you could marvel at if given the chance—the delicate fringe of hair framing their eyes like lashes, cushioned pads that allow them to scramble up walls, and four thick stripes on their backs. But most human encounters with M. domestica are likely antagonistic and fleeting, offering merely a glimpse at a buzzing black streak in the air.
Although M. domestica originated in Central Asia, it traveled around the world alongside us and can now be found on every continent except Antarctica. In 2016, Matt Bertone, an entomologist who directs North Carolina State University's Plant Disease and Insect clinic, published a survey of the arthropods living inside 50 houses in North Carolina. The researchers found flies, meaning insects in the order Diptera, in every single home. But there were fewer house flies than Bertone thought there would be.
(Some of the other flies include, but are not limited to: leafminer flies, wood gnats, root maggot flies, robber flies, March flies, blow flies, green bottle flies, cluster flies, carnid flies, gall midges, biting midges, phantom midges, non-biting midges, frit flies (not to be confused with fruit flies), fruit flies (not to be confused with frit flies), chyromyid flies, various flavors of mosquito, longlegged flies, dance flies, balloon flies, shore flies, mantis flies, lesser house flies (which are in a totally different genus than house flies), heleomyzid flies, lauxaniid flies, spear-winged flies, freeloader flies, shoot flies, tiger flies, garbage flies, face flies, fungus gnats, predatory fungus gnats, scuttle flies, big-headed flies, moth flies, drain flies, woodlouse flies, flesh flies, window flies, black scavenger flies, minute black scavenger flies, lesser dung flies, soldier flies, hover flies, horse flies, tachinid flies, crane flies, picture-winged flies, xylomyid flies, and, finally, house flies.)
None of the researchers I spoke with believed that the fly I saw in my house was likely to be an actual house fly. Bertone offered me a bit of consolation. "It is technically a house fly, I guess," he said. "But it's not the house fly."
If I were living in the 1800s, however, my guess might have fared better. "Your tentative identification of Musca domestica found in a house could have been correct when we lived near livestock and drove carriages with horses," Kim told me. Horse manure and animal waste are M. domestica's preferred stomping grounds, so house flies are most reliably found near stables and farms. But this is not to say that any small black fly you might see in a stable or holding up the rear of a carriage ride is certainly a house fly. You might be surrounded by stable flies, face flies, or horn flies. These close, red-eyed relatives of Musca can often look exactly like house flies.
Although I felt enlightened in the matter of the house fly after these conversations, my investigation remained wide open. If I didn't see a house fly, what fly did I see in my house?
Suspect No. 2: The Blow Fly
Given that I do not live in a stable with a horse for a roommate, there are some prime candidates for the fly I did see. When it comes to identifying flies, Hannah Chu, a PhD student at the University of California, Riverside, told me first to ask myself a series of questions:
How big is it? If the fly is really tiny, just a few millimeters long, it could be a fruit fly, drain fly, or fungus gnat. My fly was at least the size of a pea, eliminating those options. Where is it? If it is around fruit, it's probably a vinegar fly. Houseplants, might be a fungus gnat. Your drains, a drain fly. Your trash? Then it might be one of the many flies sometimes nicknamed "filth flies," a catch-all for flies that associate with rotting plant and animal material, sewage, and waste. The larger filth flies include house flies, blow flies, flesh flies, and cluster flies.
Some other questions: Is it shiny? House flies are dull, but blow flies are metallic, their exoskeletons gleaming green, blue, bronze, or black. Their family name, Calliphoridae, means "beauty bearer" in Greek. Is it loud? Blow flies are loud; one species, the blue bottle fly Calliphora vicina, beats its wings around 150 times per second to create a seriously annoying buzz.
One of Chu's lines of research investigates forensically important blow flies. Insects are not just important for the environment, but also for criminal investigations—certain species of blow flies are among the first insects to access dead bodies, and measuring the development of their maggots can indicate how much time passed between death and the discovery of a body. "It's very morbid," Chu said. Of course, blow flies don't only breed on the flesh of the recently murdered; garbage works just fine.
Chu wants to change the minds of people who see blow flies as gross—not to convince them that the flies are cute, but at least to get them to acknowledge their vital roles in ecosystems. She often tells people that some blow flies pollinate flowers, just like the more popular honeybees, because people love pollination. (And unlike honeybees, blow flies are not invasive to North America.) But a blow fly's most important job, decomposition, is a gross one, so we might be thankful that it's doing the work. "If we didn't have them, you would be walking around and encountering a lot more dead things," Chu said. "That's not ideal."
Over the course of her research, Chu has become desensitized to rotting flesh, of any kind. "Once that initial feeling of disgust is gone, now it's like a free-for-all," she said. She started off baiting her flies with rotting liver, but recently switched over to rotting squid. "It's way stinkier," she said. The blow flies swarm to the smelly scraps and lay their eggs, so that Chu can put the egg-dimpled meat into a jar and wait for the babies to hatch.
I've had blow flies in my apartment before—heard them audibly thumping into my windows and seen their metallic gleam in a beam of sun—and I was sure the fly I was looking for was certainly not a blow fly. It was too small, too matte, and its buzzing far too muted. But Chu told me about a close relative of the blow fly just as just as obsessed with the dead things inside my trash. Could this be the fly I was looking for?
Suspect No. 3: The Flesh Fly
Of all the flies already discussed in this story, flesh flies—of the aptly named family Sarcophagidae—may have the most revolting common name. But not all flesh flies make their bed in dead flesh. Some simply "do their own thing," Bertone said, offering as an example flesh flies that lay their larvae in pitcher plants so their young can feed on the drowned insects inside.
If you noticed that I wrote that flesh flies lay their larvae, not their eggs, that is not an error. "When you kill one, like a mother that's pregnant, the larvae will abandon ship," Bertone said. "So they'll crawl out of her while she's dead." A flesh fly's ability to give live birth offers their offspring a head start in life, although this boon is somewhat grotesque. "It's maggots instead of a little cute baby," Bertone caveated.
Flesh flies are almost always carbon-fiber gray. They are bigger and sturdier than actual house flies and often have tessellated patterns on their abdomen. But these differences are far from discernible from our perspective, especially when the fly is, well, flying. Plumbing the depths of my admittedly ephemeral memories of the fly in my home, the flesh fly seemed like the likeliest candidate so far: about the right size, the right color and sheen, and generally the right vibe. Was that enough to make an (admittedly rough) identification?
Kim advised against it. "Identifying flies can be fun, but it's not easy for most people," he said, adding that experts rely on detailed books and reference materials, as well as tools like a stereomicroscope. Still, Kim encourages anyone who either possesses a good photo of a fly or an intact fly corpse to attempt to identify it. He recommends Bugguide.net or the Manual of Nearctic Diptera as good online resources. If I had a picture of the living fly, perhaps one that captured features such as wing veins and the position of its hairs, it might be possible. But even if I had managed to pry the fly's body from Melon's jaws, that would not have been enough. "One cannot easily see important characters when a fly was captured and dismembered," Kim cautioned.
The Investigation Remains Unsolved, But Open (All Leads Welcome)
So it had come to this. I'd reached the end of my investigation with nothing to show for it. I had identified the killer (Melon), the time and place of death (afternoon in the Killing Arena), but had failed to put a name to the victim. Had I spent months on a fool's errand? No one wants to read a story with a headline "The Victim Could Have Been A Flesh Fly, But It's Hard To Say For Sure."
In college, Chu had originally planned to become a criminal defense lawyer. She'd grown up watching CSI and Law & Order, an extension of what she describes as her childhood fascination with "death, and how beautiful death can be." She only started learning about flies when she took associated classes in forensic science. "It turns out I really disliked law," she said. But Chu realized she did like flies. Her subjects were incredibly diverse, under-appreciated, and pretty much everywhere. And the longer she worked with flies, reading about death and the processes of decomposition, the more she felt connected to her childhood obsessions. The first time she visited the office of the chief medical examiner to pick up samples from bodies at the morgue, the impact of her work hit her. "We are helping, even if it's one person, through that process of grief, with bugs of all things," Chu said. "That felt really rewarding for me."
You can't study flies, especially some of those living in your house, without thinking about death—the rot scaffolding their past, present, and future. And I'd even wager to say you can't be around flies without thinking about death. A house fly can be an omen. You are unable to ask the fly what brought it here, if it followed the pungent whiff of an unseen rodent dragged in by the cat, or if it accidentally flew through a window, mistaking your lamp for the moon.
I asked the fly researchers, who presumably have a certain appreciation for flies, if they feel the urge to kill the small intruders in their home. They shared a range of opinions. McAlister, offended by my confession that I kill house flies, pointed out the hypocrisy of my question (asking for "guidance for people who want to coexist with other species but also don't want flies in their home"). "Flies, as with most animals on the planet, don’t understand the human obsession with calling things ours," she said. Bertone rarely kills the flies in his home. "I just generally don't kill insects just for existing, unless they are causing harm," he said. And these flies are often buzzing at the window, attempting to escape. "They're looking for the light," he added.
Kim and Chu, however, took harsher stances. Kim, who works on public health issues, finds flies "a bit disgusting to have in your home," as the insects can carry up to 100 different pathogens. "There is no capture-release program in my house," Kim said. "Any flies that invaded my house are met with a fly swatter." Each summer, Chu experiences a puzzling home infestation of blow flies—the ultimate case of work following you home. But given her familiarity with the flies, which tend to fly upward and are not good at seeing slow-moving objects, she's developed her own signature fly-killing trick: "Move your hands really slow above them, and then clap really fast," she said. The fly should fly up, and you'll be able to catch it with your hands. If the fly has been in your house for a while, stuck with limited resources, even better. "They'll be easier to kill," she said.
Oddly enough, this gave me some comfort. House flies, blow flies, and flesh flies all live around a few weeks to a month. The fly in my house, whoever they were and however they came there, had likely reached its golden years. It had spent days doing the dirty, thankless job of any decomposer—living and breeding in what we call filth so that our streets would not teem with slowly decomposing carrion. Maybe Melon, a somewhat ineffectual hunter, was not a killer but a last companion to a fly dying of old age, an elder that had completed the work of its life.
