Centipedes and millipedes are normal soil fauna, eating other bugs and decaying vegetable matter, respectively. The flying gnats are probably fungus gnats. They are more of a nuiscance than anything else, but they can be a symptom of overwatering. To get rid of them, use a mosquito control granule containing Bacillus thuringiensis israelensis. That bacterium attacks only larvae of members of the fly family--including gnats. In the few cases where that doesn't work, you can water the plant with a solution of carbaryl. At the same time, you might want to pour some chlorine bleach down the drains, since fungus gnats often live there, too.

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