Biting gnats are parasitic organisms similar to mosquitoes and are capable of very annoying bites. However, while mosquitoes pierce the skin and feed with mouthparts similar to a hy- podermic needle, biting gnats have scissor-like mandibles that cut the skin to produce a small, bleeding wound. In the process of biting, saliva penetrates the skin causing an allergic reaction. This reaction is minor in most people, though it can be severe in rare cases.
Source: gnats.indd – Gnats.pdf