My houseplants are being colonized by small flying insects. It started out with a couple of little flyers in one of my philodendrons. I figured it was no big deal, just a couple of strays that would lose interest and die off. Now they've increased and started to colonize a couple of my other plants! They're considerably smaller than a housefly, but larger than a flea. They seem to live in the top of the soil and wander around the plant. When I water, they get provoked and fly around the plant. Does anyone have any idea what they might be or what I can do about them?