A swarm of midges
Midges spawn in swarms in the Midgewater biome. They are very annoying but harmless, and can be killed in a couple of swats. Part of their annoyance is due to the fact that they have an attraction to nearby players, buzzing above the player's head and following them relentlessly. They are also very noisy.
Midges will spawn more midges over time if the amount of nearby midges is below a certain limit, inevitably leading to a very densely midge-populated Midgewater biome. However, while midges may reproduce outside Midgewater, the chance of this happening is much lower and the midge limit will be reached sooner.
There is also an achievement for shooting down a swarm of midges with a hired unit, called "Anti-Air Support".