- For the virus in DW: The Invisible Enemy, see The Swarm (The Invisible Enemy).
The Swarm were creatures that lived in swarms of billions and due to their sheer mass were able to open wormholes in space by moving at fast speeds. They were named "alien stingrays" by UNIT due to superficial resemblance to the stingrays that live in Earth's seas.
Biology
The Swarm had an exoskeleton of metal to enable them safe travel through the Wormholes they created. Their digestive system allowed them to eat anything, including metal, which was used to make their exoskeleton once digested. They were seemingly unintelligent and incapable of speech. They acted on the instinct to devour and move on. They moved from planet to planet by creating wormholes. They created these by swarming around the planet many times so their sheer mass would rip space creating a wormhole they then traveled through. Their exoskeleton could protect them against bullets but not explosives as shown when UNIT managed to dispatch three of their members using a mounted rocket launcher when ordinary rifles had no effect.
History
The Swarm opened a wormhole that led them to the planet San Helios. There they attacked the planet and devoured everything, turning it into a barren desert. They then began making a second wormhole with which to leave the planet. A number of individuals were trapped in the hull of the Tritovore's ship. Lying dormant due to its low temperature, but when Christina de Souza went to retrieve the ship's crystal nucleus, her body heat raised the temperature just enough to revive them, after which they tore through the ship and devoured the two Trivore occupants. Malcolm Taylor managed to close off their latest wormhole that lead to Earth. However, three individuals made it through. Conventional firearms were useless against them, but UNIT's mounted missile launchers proved more effective. The ones trapped on San Helios could easily make another worm hole, but the Doctor stated he will attempt to guide all their wormholes onto uninhabited planets, where they could feed on natural structures.