eli5: How do flies find your food so quickly?

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I can be sitting outside without a fly in sight and the second I open a drink or food there is a swarm. How the heck to they find good sources so quickly?

In: Earth Science

Flies have olfactory hairs covering their bodies. They can smell better than we can. So once you open food packaging, you create a plume of molecules that you can’t sense for being too dilute, but the flies can. And the further that plume expands outward, the more flies in the area can smell it and go looking for it. There may not be any flies in sight, but that merely means you can’t see them, that doesn’t mean they aren’t there.

They have a sense of “smell” that compels them towards certain chemicals in the air that come from food and liquids with nutritional value. It’s particularly intelligent, either — they’re compelled towards them. It’s more like water flowing through the path of least resistance. This is the life of a fly, they’re compelled to follow smells for their entire existence, only taking a break for a moment when they sense danger.

They then forget the danger, almost immediately, being perpetually compelled back to the source of the smell.

What you do is put something out that will attract them not far from where you’ll be sitting. Do it before you sit down. They’ll all congregate there and be less annoying where you are. Works for us when we eat lunch outside.