How do fruit flies start?

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I get that they come from expired food, but what I don’t get is how they appear in the first place. Do their eggs just form or are they already in our food and hatch when the food expires? I don’t know why I don’t understand it but I just don’t haha

In: Biology

Fruit flies don’t come from expired food, they are only attracted by it, so when they smell it, they come rushing for it. They dont come FROM the food, they come FOR the food.

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You leave food out for a few days. You don’t see any flies around your house. But those suckers are small, and can hide. One sneaks in when you open your window. One sneaks in when you opened your door to accept your pizza delivery. One comes through when you carry your shopping in.

That fly eventually finds the food and start buzzing around it. They also lay a bunch of eggs on it while at it, since discarded food is the perfect spot for their larvae to feed. About a week later, you have your own fresh swarm of flies!

The eggs, adult flies, or larvae are already there in the food when you bring that food into your house.

The other thing you’re describing (“do their eggs just form”) is a long-debunked theory called spontaneous generation. Back around 1668, when people thought that maggots (and therefore flies) just magically appeared from meat, [Francesco Redi](http://www.scientus.org/Redi-Galileo.html) showed that if you cover the meat with cloth, no maggots appear (because the adult flies can’t get to the meat to lay eggs).

On another note: The best way to get rid of fruit flies in your kitchen is to put out some small open containers of apple cider vinegar. The flies will drown themselves in the vinegar.

I’ve asked this question, but about wine, for years.

Like “where WERE YOU, before I left out that finished bottle of merlot a few hours ago.”