The Swarm originates from Etter Syra, a planet covered with oceans of poisonous acid in the Tau Ceti system. The Swarm is literally a mega-swarm of bugs. They have no use for technology, but they can modify themselves as they need. Every unit or 'building' in the Swarm is just another bug with a special set of abilities.
Walls can't keep these huge bug bats out. They fly free around the enemy base, spitting acid on everything they meet. Note that you’ll have to perform certain quests before the Megabat unit is unlocked in your Recruiter.