BITING BUG is a term used to describe any small insect that bites humans or animals. The three most common biting bugs are fleas, mosquitoes, and gnats. Fleas are small, wingless insects that feed on the blood of mammals and birds. Mosquitoes are small flying insects that feed on the blood of humans and other animals. Gnats are small, flying insects that feed on the nectar of plants. All three of these biting bugs can cause irritation and discomfort when they bite.