Here is a fungus gnat. They are small flies - this one is about 2.5 mm long - that can infest house plant soil. Their larvae feed on organic matter, including plant roots, which can stunt the growth of plants, or kill seedlings. Watering the plants with 3% hydrogen peroxide will kill the larvae.
Equipment used: Canon EOS Rebel T5i with a standard zoom lens (EF-S 18-55 mm) on a 49 mm long extension tube.