Where do fruit flies come from?

Dear Straight Dope: Where do fruit flies come from? Are there eggs in every piece of fruit I buy, and they hatch when it reaches a certain point of ripeness, or do they live outdoors and invite themselves in when I have an overripe peach sitting on the counter? I’m hoping it’s the latter, or I’m going to regret every piece of fruit I’ve ever eaten. Tristan, Baltimore, MD

Doug replies:

It’s a little of both, actually. Fruit flies – at least those most familiar to us (Drosophila) – wait to lay their eggs, seeking out fruit that has begun to ferment. The fruit fly’s whole sensory apparatus is devoted to sniffing out the distinctive odors of fermentation – that’s why you frequently find fruit flies hanging around beer bottles. They generally don’t lay their eggs on fresh fruit, though. The larvae feed mostly on yeast, and until a fruit has started to go beyond ripe, there just isn’t enough yeast there. The eggs hatch quickly and the larvae develop quickly to make the most of the little time available; a rotting fruit doesn’t last very long. If a fruit has started to ferment while in transit or in the market, then it might possibly arrive in your home with eggs or larvae already in it. But if it ripens in your home and then the fruit flies appear, they’re new arrivals.

Doug

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Where do fruit flies come from?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD