Bugs are sometimes an issue with the Nature's Head composting toilet. You can do a few things to get these little buggers taken care of.
1) If you don't have the mushroom vent or the PVC vent for structures, you should find a way to get a bug cover over your vent. Some people will use panty hose to cover the flange that the vent hose connects to. Other have taken mesh and glued or screwed it into the vent flange.
2)If you have done the above, then you can try to use Ortho Home Defense for flying bugs with Essential Oils or Wondercide Indoor Natural Pest Control for Home and Patio (Cedar scent) or moth balls to help control the pests. You can also try adding Diatomaceous Earth to your composting medium in the solids bin.
3) If the above two didn't work, try emptying the solids bin and giving it a good cleaning with vinegar and water. Put in a new batch of composting medium.
4) If you are using peat moss as your composting medium, try microwaving it for a few seconds. The process to package peat moss doesn't always kill all of the bugs that are in it when it is harvested. By microwaving it after bulking it up, it will kill any bugs that are alive in it. We use coco fiber as it goes thru multiple processes before it is packaged.
Here is a great resource regarding the Nature's Head and questions about it. Questions about Nature's Head