Flies are annoying, especially on warm weekends spent outdoors. They land on us and our food, they buzz in our ears, and some of them bite. Mosquitos are a kind of fly and they transmit some of the world’s deadliest pathogens.
But consider for a minute that you may not really know flies. Or rather, the flies you likely do know — the house flies, the mosquitos, the gnats — are just a tiny, tiny fraction of an enormous group of insects that is, on the whole, quite wonderful. It also supports our very existence.
No, a fly didn’t write this. Flies do, however, have advocates among humans, and recently, one got to me.
Last fall, I met Emily Hartop, a scientist who studies flies at a natural history museum in Berlin. A lifelong bug lover, Hartop told me the world is home to hundreds of thousands, if not millions, of fly species. And they fill pretty much every ecological role imaginable. Flies are superb pollinators, shrewd parasites, and exceptional janitors — they literally clean up our shit.
Flies are also anatomical marvels, Hartop said. In addition to a pair of wings, they have special balancing organs called halteres that function like gyroscopes, allowing flies to turn sharp corners, hover, and land upside down. “They’re called flies for a reason,” Hartop said. “They are amazing aerial acrobats.”
Some flies are bioluminescent and glow in their larval form. Others can get “pregnant,” hatching larvae in their bodies that feed on a milk-like substance, according to Erica McAlister, a fly expert at London’s Natural History Museum. (Nearly all insects lay eggs instead.)
None of these facts make the flies we encounter in our homes and at our barbecues any less obnoxious. But this insect group — which is still largely unknown — comprises far more than just pests. Flies help us, and they can even inspire wonder.
Let’s meet some of them.
The flies that decapitate ants
The world’s coolest group of insects, Hartop argues, is the family of flies she studies, known as Phoridae. (She acknowledges her bias.) Phoridae contains tens of thousands of species that exhibit every behavior imaginable, making it perhaps the most ecologically diverse group of organisms in the world.
Many species, for example, are parasites — they live at the expense of other creatures.
A parasitic fly in the genus Pseudacteon is particularly savage. It lays its eggs inside an ant, and when they hatch, the larvae migrate into the ant’s head. There, the larvae release hormones that kill the ant and cause its head to fall off. The larvae then pupate in the ant’s detached head (as you can see in image F below).
Female flies in a different genus, meanwhile, will hunt down injured ants and then, using specialized mouth parts, manually saw off their heads, into which they’ll lay their eggs, Hartop said. You can see this in the video below, showing a small fly (on the left) removing an ant’s head.
The flies that don’t look like flies
Then there are flies that look more like spiders, bees, and other insects, in some cases to avoid being eaten. (Predators are more likely to avoid insects with painful stingers.)
A parasitic fly called the bee fly, for example, looks like a bumblebee, an insect that is not at all a fly. McAlister calls them “fluffy flying narwhals.” I prefer “wanna-bees.” The pointy structure sticking out of its head isn’t a stinger, it’s a proboscis, a straw-like mouth part that flies use to feed.
Female bee flies spray their eggs “like a machine gun” around the nests of ground-dwelling bees, McAlister said. The larvae then hatch, worm their way into the nest, and eat the bee larvae.
A large number of other flies mimic the appearance of bees, likely so they look threatening to birds and other predators. These include hoverflies, which people often call sweat bees. They really do drink sweat, but they don’t bite or sting.
Many bat flies, on the other hand, look like spiders, such as those in the images above. They spend most of their lives nestled in the fur of bats, subsisting on their blood. Some of these flies have evolved to be wingless, whereas others ditch their wings and have bats fly for them. (These are the flies that don’t lay eggs like other insects but rather give birth to a single live larva.)
Another impressive spider-lookalike is a rare fly in Kenya known as the terrible hairy fly, shown below. Their larvae are known to live in and eat bat poop.
One more: Females in a genus of Phorids called Vestigipoda mimic ant larvae. Their disguise is so convincing that ants feed the flies as if they were their own young. “They look like things that you would never recognize as flies,” Hartop said.
The flies with weird appendages and body parts
Other flies just look totally bizarre.
My favorite? The stalk-eyed fly. In males, the eyes are on the ends of thin stalks that can be longer than their bodies. These eyes function a bit like moose antlers or sheep horns; the flies likely use them to assert dominance, according to the Natural History Museum, London.
Flies in the family Pipunculidae also have remarkable eyes. They’re enormous. Their whole head is basically just eyes. Unlike humans and other mammals, flies have compound eyes made of multiple light-detecting parts; those eyes see in low resolution but are exceptionally good at detecting sudden movements.
Another fly, called Moegistorhynchus longirostris, also has an impressive body part: a proboscis that can be longer than 8 centimeters, or about 3 inches. The fly itself, meanwhile, is only about 1 centimeter long. The flies use these appendages to reach the nectar in tube flowers.
Perhaps even more wonderful (and ingenious) are a handful of cave-dwelling gnat species in the genus Arachnocampa. As larvae, they glow to make their own bug traps. Their luminescence draws in moths and other insects, which then get ensnared by sticky threads that the gnats produce, McAlister said. The larvae then eat them.
Flies pollinate our plants and clean up our shit
Flies are cool, yes. But are they important? Also, yes.
It’s true that some flies are extremely harmful to humans, including female mosquitos in the genus Anopheles. They transmit malaria, which kills several hundred thousand people each year. Other varieties, like tsetse flies, also carry parasites that can be painful and sometimes deadly. These are very serious concerns.
Yet only a tiny fraction of the world’s flies harm us. We depend on many of the rest.
For example, flies are fundamental to the production of many of our favorite foods, such as chocolate, McAlister said. While roughly two dozen insects are known to pollinate cacao plants — the seeds of which are used to make chocolate — nearly all of them are flies, she said. So no flies, no chocolate.
Did you know? We wouldn’t have *Chocolate* if it weren’t for insects? The Chocolate Midge #fly (Theobroma cacao) is the main pollinator of the cacao plant. No chocolate if it weren’t for this little fly! #SaveInsects #NatureEducation pic.twitter.com/mII0agD3id— Dr. Akito Kawahara (@Dr_Akito) April 30, 2021
Altogether, more than 100 cultivated crops are largely dependent on flies for pollination, including mangos, cashews, and avocados.
Some flies can also help farmers and home gardeners deal with pests. The larvae of some hoverflies, for example, have a voracious appetite for aphids, small insects that infest crops. (If you buy organic produce, chances are you’ve encountered, or accidentally eaten, aphids.)
“We massively underestimate the impact these tiny little creatures are having,” McAlister said.
Gross as they look, fly larvae also help clean the world of waste — they eat our garbage, our roadkill, and our feces. None is perhaps more impressive than rat-tailed maggots, the larvae of certain kinds of hoverflies. On one end of their body, they have an extendable “breathing siphon” that functions like a snorkel, allowing the maggot to feed in a pile of waste even if it’s deprived of oxygen. “These things scuba dive,” McAlister said.
Yet we know very little about them
Considering flies are fascinating, important, ubiquitous, and in some cases cute, you might think everyone would be jumping at the opportunity to study them.
They are not.
Aside from pathogen-carrying mosquitos, flies are a massively under-studied group of organisms, in part because science tends to focus more on conventionally charming insects, Hartop said, like bees and butterflies.
“So much attention is dedicated to charismatic groups like pollinators and pretty things,” she said. “Flies take a bit more time to get to know and love.”
That’s one reason why fly science still has enormous gaps. In the genus Hartop studies within the family Phoridae, known as Megaselia, it’s unclear, for example, if there are 20,000 species, 100,000, or closer to 1 million. In other words, large sections of the fly family tree have yet to be filled in.
This is a problem, Hartop said.
Because flies occupy so many different habitats and roles within those habitats, the status of their populations is a useful indicator of how the environment is doing — i.e., whether it’s healthy or not. Even baseline numbers are missing. That makes it challenging to understand how ecosystems and the services they provide are changing due to threats like climate change.
“I’ve been to so many talks on things like bees and butterflies where they’re described as a proxy for how an environment is doing as a whole,” Hartop said. “I think that’s a really broken way to look at things.” Studying bees is useful for understanding how pollinators are doing, she said, but not the broader environment.
Scientists like Hartop are helping fill these gaps, such as by surveying different landscapes and streamlining the time-intensive process of sequencing fly DNA to describe new species. She has come across hundreds of new species, about 60 of which she’s formally described.
That’s what’s so exciting about studying flies, she said: There’s so much opportunity for discovery. You don’t have to travel to the deep sea to find a new species; there could be undiscovered species of flies in your backyard, perhaps even in a place like New York City, Hartop said. Each discovery is a chance to understand an influential member of the ecosystem.
“We really need to shift our thinking and we need to look at some of these groups that we’re not paying attention to,” Hartop said. “I don’t expect people to relate to very small flies in the way that they relate to a butterfly. But understanding the role that they play, understanding their importance, I think that is critical.”