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No meat or dairy. Check on it every few hours. There should be a swarm of fruit flies in 3 hours or so. First, snap a lid on the jar, and then turn it upright. You can then follow the directions below to culture them, or feed them right away to your pet.
Put scraps of old fruit/vegetables in a large jar. Set the jar near a trash can or near a compost pile (without the lid, but make sure you've got one that fits tightly). Leave the jar there for several hours. When you've got a lot of fruit flies in the jar, you'll have to work quickly.