Where do fruit flies come from?

It's crazy. I mean they appear out of nowhere. How do they know the fruit is there? How do they get in your house if there are no windows or doors open? Do they live in the fruit?

Where do fruit flies come from?
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.
Reply:LOL! I've just got some grapes from the supermarket and my kitchen is swarming with the little ruggers! I blame the supermarkets!