If the Urban Composter lid has been left open it is possible for flies to come in and lay eggs. This is more common in warmer climates and during summer months. Very quickly the fly eggs will turn into maggots. If this happens to you, don’t worry as they are still natural. You can wipe them away and continue composting. The acidic environment will prevent them from living too much longer. Make sure the lid is closed properly, and food isn’t left open on the counter to help prevent again.
If they bother you then empty the bucket and start again. Use water and possibly vinegar to clean.