The adult fungus gnat is a small (2-3 mm) black fly with clear wings. They are very slender and delicate and resemble a miniature mosquito.Fungus gnats lay their eggs on/in the growing medium in the pot. The larva feed on organic matter, algae and fungi in the potting soil. The adult flies are fairly easily controlled with an indoor insect control product. I have also used a vacuum cleaner to suck them up, since they are slow and weak flyers. The main and more serious problem is the potting soil.