Vashta Nerada


 * For the inhabitants of the Howling Halls, see Elemental Shades.

Vashta Nerada were swarming, carnivorous beings. The name meant "the Shadows that melt the flesh".

Biology
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. These swarms were able to strip a creature to its bare bones in milliseconds and the Doctor once described them as "piranhas of the air". While these swarms were not common, they inspired a fear of the dark in most species across the Universe, since any shadow could be made of Vashta Nerada. (DW: Silence in the Library/Forest of the Dead) Both the Doctor and Jack Harkness believed they had few weaknesses and that the only thing to do was to run. (DW: Silence in the Library, WC: Captain Jack's Monster Files: Vashta Nerada) They move more slowly in the light, though it does not stop them. A sapient swarm became fearful of the Doctor upon learning of his exploits, suggesting they may have some as of yet undiscovered weakness.
 * The Vashta Nerada may instead have realized that they needed the Doctor alive, to stop the Reality Bomb among other things.

The Vashta Nerada lived in the darkness and cast shadows when they entered the light - even to the extent of stopping the reach of light from fission lamps. They would mimic the shadows of their prey in order to get close, which meant staying in the light was the only option for escape. (DW: Silence in the Library)

Despite being microscopic as individuals, Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate to the Doctor through the spacesuits' Data Chips. They were also able to coordinate their efforts and make the spacesuits walk, though only with some difficulty. (DW: Silence in the Library/Forest of the Dead)

Life Cycle
The Vashta Nerada lived on almost every planet which had organisms with meat. They were born as microscopic spores in trees and live their lives in forests. It was said that they were able to hatch and form into a swarm in minutes. (Captain Jack's Monster Files: Vashta Nerada) These spores were resilient enough to hatch even if the tree was processed, including if the wood was converted into paper and into books. (DW: Forest of the Dead)

History
Vashta Nerada had existed since the early Universe, and spread to many planets across the Universe. They were rare, and on some planets, such as Earth, they survived by scavenging meat from road kill.


 * How the spores originally got to different planets is unknown.

In the 50th century one planet suffered a large infestation. The Library was attacked by over a "million million" (1,000,000,000,000) Vashta Nerada, due to the spores inside the books' paper hatching. The 4,022 people were unsuccessfully evacuated by the Library's computer and the planet was empty for 100 years until an archaeological expedition and the Doctor with Donna Noble landed on the planet.

Though much of the archaeological team was eaten by the Vashta Nerada, the Doctor was able to convince them to stop and give him a chance to return the people stored in the Library's computer. They agreed to give him a day, in which time the Humans were rescued and left the planet to the Vashta Nerada. (DW: Silence in the Library/Forest of the Dead)