Midge Swarms are pestilent mobs that spawn exclusively in the Ulterior Outback. They are comprised of five individual Midges that share a central hitbox. For every 3 damage the swarm takes (every heart and a half), one of the Midges will die, but the hitbox will remain the same.
Midge Swarms deal minimal damage-only half a heart-but inflict 14 seconds of Weakness when they do so. Combined with a harder-hitting mob (which is a common occurrence in the Ulterior Outback, being one of the-if not the-most dangerous biome(s) in the Erebus), Midge Swarms can be very problematic.