Macra

The Macra were a gigantic crustacoid race who survived on unclean gases which were poisonous to Humans, whom the Macra would happily consume when they could. (DW: The Macra Terror, Gridlock)

Physical appearance
As stated above, the Macra were massive crustaceans. They had large eyestalks and gigantic claws, and were a dark, mucky brown. In all other respects, they were extenally identical to normal Earth crabs. (DW: The Macra Terror, Gridlock)

History
The Macra were an ancient race with a large empire, "the scourge of the galaxy" as the Doctor put it. However, after many millions of years they degenerated into feral beasts. (DW: Gridlock)

On one planet colonised by Humans, they used the Humans as slaves to mine the gases that were vital for their existance. They were defeated when the Doctor effected an uprising on the planet and destroyed the control centre with an explosion, killing the Macra in charge and apparently dooming the Macra on the planet. (DW: The Macra Terror)



On New Earth, a colony of several million Macra survived in the depths of the Motorway in New New York, feeding on the poisonous fumes and stray people who came down in their cars to the "Fast Lane". They were apparently again doomed when the Doctor opened the roof of the Motorway, allowing the cars to escape to the Upper City, depriving the Macra of food and gases. (DW: Gridlock)


 * It is not specified whether these two planets were the same, and therefore possibly the homeworld of the Macra, however it seems unlikely.