Do Flies Really Vomit and Poop When They Land on You?

The Science and Danger Lurking in the Humble Housefly

House fly feeding.
Getty Images/London Scientific Films

Let's get to the bottom of a common belief about flies – do flies really vomit and poop when they land on you?

Where There Are People, There Are Flies

First of all, we need to be a bit more specific. We're talking about houseflies here, known by scientists the world around as Musca domestica. The house fly associates with people. Virtually anywhere on the planet where you can find people, you will also find Musca domestica.

Anyone who has ever enjoyed a backyard barbecue knows that house flies will crash your picnic table, walk all over your potato salad, and attempt to taste your burger, should you dare to leave it unattended for just a moment. And occasionally, those flies will come to rest on you. So you are probably wondering what they're up to while they sit there. It's a totally understandable and realistic concern.

Yes, House Flies Vomit (a lot)

Let's tackle the first bit of this question first—do flies vomit on you? The answer is a resounding sometimes. House flies do vomit, sort of, and they do so pretty often. Unfortunately for the house fly, it is not equipped to chew solid foods. Most insects that feed on solid food—beetles, for example—have chewing mouthparts, with which they can properly masticate their meals into tiny, digestible bits. House flies were instead blessed with sponge-like tongues. Only in flies, we call their tongues labella (the singular is labellum, but the fly has a matched pair).

House flies "taste" with their feet, so they have no choice but to walk on their food (and ours, should they be sampling our picnic menu). When a house fly comes upon something that seems like it might be yummy—keep in mind that dog poop is the kind of thing house flies find yummy—it will reflexively stick out its labella and press them against the potential food item to investigate. Liquids can be slurped up without much effort. Inside the house fly's head is a structure called a cibarial pump (or food pump), which generates a suction to draw the liquid up through channels in the mouthparts (called pseudotrachea).

So, how does the house fly make a meal out of meat, or any other solid food (like dog poop)? It uses those same mouthparts to liquefy the entrée. The house fly dabs the tasty morsel with digestive enzymes by bringing up a little regurgitated food and saliva. The enzymes begin breaking down the solid food, gradually turning it into a slurry the house fly can then lap up. Meat milkshake, anyone?

House Flies Also Poop (a Lot)

Now, think about the last time you had the stomach flu. Anytime you vomit repeatedly, you run the risk of dehydration, so you have to drink a lot of fluids to replace the ones you lost. Flies are no different. This liquid diet means flies require a lot of water. And when you drink a lot of water…well, let's just say what goes in, must come out, right? So flies do a lot of defecating, too.

Therefore, in answer to your original question—do flies really vomit and poop when they land on you? Yes, they do, but not every single time they land on you. It really depends on whether or not the fly thinks you are a potential meal. If the fly gets a message from its feet saying, "Hmm, this guy tastes pretty good. Take a lick!" you're probably going to get a little fly vomit on you. And hey, if the fly has got to go, it's got to go, so you might just get a little fly poop on you, too.

So Should We Worry?

Yes, you should worry. Houseflies are strongly suspected of transmitting at least 65 different diseases to humans, including typhoid fever, dysentery, cholera, anthrax, leprosy, and tuberculosis. A single housefly can carry over one million bacteria. Before modern antibiotics were invented, flies were definitely deadly.

It is important to control any type of fly that may be indoors or in your backyard, by killing the individuals and by eliminating the food waste in which they feed and reproduce. Practice good sanitation, indoors and outdoors, and wash your hands frequently.


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