I know they are a nusiance but they are all part of the decompostion process, so I'd leave them alone personally.

Those tiny flies are just fruit flies that have hatched from eggs laid on fruit peels and skins. They're harmless, and as SBG says, contribute to decomposition as the larvae feed on moulds on the fruit skins. Don't worry about them, but you could leave the lid off the composter for a few hours to let them escape and let predators get in.

Seaburngirl is right Davidar. The 'fruit flies' are a part of your composters ecosystem. Try a layer of dry leaves on top of the material thats in there to balance things out. The flies will still remain, doing their duty, but wont attempt to invade your nostrils so readily each time you lift the lid!

Seaburngirl is right Davidar...they are all a useful part of your composters ecosystem. Try to view them as friendly assistants rather than tiny pests that try to invade your nostrils each time you open the lid..grrr! Chances are you have decomposing 'green' waste,ie veg or fruit at the top? A layer of 'brown' waste, dry leaves for example will help a lot. The 'fruit flies' will still be there doing the business,but not surging up to greet you merrily each time you lift the lid. Layering,thus, eliminates the majority of composting issues & maintains the correct moisture balance necessary for successful composting