More than likely they are fungus gnats. The larvae eat decaying organic matter/dead plant roots, etc. They need very moist soil to survive. If your plants are removable, take them out and let them dry out as much as possible with out killing them, then replace the plants and substrate and dont introduce anything to the cage with out drying it out first. Chances are good the plants came with the gnats, or they came from other plants in the house.
Another thing you can do to help reduce their numbers (but this doesnt necessarily eradicate them) is to add springtails to the enclosure. They eat the same things and seem to out-compete the gnats. However, springtails will also eat any moss or leaves you have as substrate. I have to replace my oak leaves at least once a month because of this, but better than gnats... and their cleaning capabilities are unmatched.
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