Fungus Gnats (Bradysia pauper) one of the most common sciarid flies are small, 4 to 5 mm long dark brown flies with long legs and antennae. Fungus gnat larvae are slender (5 mm) with distinct black head capsules and transparent bodies. These flies are found in damp, humid environments and are commonly seen in or around houseplants. These pests feed on decaying organic matter and living materials such as roots, root hairs and stalk tissue. Fungus gnats have been known to spread detrimental mites, viruses and fungal spores to plants.