Why is it that "fruit flys' seem to appear from nowhere?

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

Hi, you already have all the info you wanted (and then some!) but if you have a problem with the little buggers try putting an ounce or two of apple cider vinegar in a glass along with a couple of drops of liquid dish detergent. For some reason they can`t resist and swarm into the glass and drown!

Works like a charm ;-)

They are attracted to the fruit - and they are SO small, they can get through cracks or even very small breaches in screens. It's very hard to keep them from getting into your house, and they can smell tomatos and other fruit for miles. (okay, maybe not miles, but from a long way off)

I have heard that they can come out of the sink drain and that by covering the drain openings you can stop them. I currently have 5 baskets of vegetables in my kitchen and no bugs.

lol...I hate those damn things I think they appear in thin air...

lmoa