Houseflies Vomit On Their Food Before Eating It

If you've ever watched a housefly land on your sandwich and wondered what it's doing, here's the unappetizing truth: it's probably vomiting on it. And then eating that vomit. And possibly repeating the process several times.

Houseflies don't have teeth or the ability to chew solid food. Instead, they have a sponge-like tongue called a labellum at the end of their proboscis—basically a straw-mouth designed for slurping liquids. So when a fly encounters solid food, it needs to turn that food into liquid first.

The Vomit Strategy

When a fly lands on something (using its feet to "taste" the surface), it regurgitates digestive enzymes stored in its crop—a storage pouch in its digestive system. These enzymes break down the food, essentially pre-digesting it into what scientists politely call "a slurrable soup" and what the rest of us might call "barf soup."

The fly then uses its spongy mouthparts to mop up this liquefied meal. But here's where it gets even more disgusting: the fly may pass this dissolving food bubble back and forth between its crop and mouth multiple times, regularly applying fresh saliva, until the meal is fully liquefied and ready to send down to the stomach.

Why This Matters (Besides Being Gross)

This regurgitation behavior isn't just unpleasant to think about—it's a genuine health concern. Both the regurgitation and the fly's feces can contain dangerous pathogens. When a fly lands on garbage, feces, or rotting material, then lands on your food and vomits on it, it's potentially transferring bacteria like E. coli and Salmonella.

A single housefly can carry over 100 different pathogens. That innocent-looking fly doing its vomit-eat-vomit routine on your picnic potato salad could be depositing anything it ate from the dumpster it visited 20 minutes ago.

The peritrophic matrix—a protective membrane that forms around food in the fly's midgut—prevents food from being regurgitated once it passes a certain point in digestion. But everything in the crop and foregut is fair game for the vomit-reeat cycle.

So next time a fly lands on your food, remember: it's not just resting. It's probably having a multi-course meal that involves more regurgitation than a college freshman's first party. Your best move? Throw that food away.