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)

Biology
There were two known types of Macra, the original sapient species and the later predators of New Earth.

Original Macras
The original Macras were larger than Humans, able to hold a human in one claw. They were stocky in build, with large eyestalks and large claws, and were a dark, mucky brown. (DW: The Macra Terror)

New Earth Macra
The Macra of New Earth were much larger in size, several hundred times larger than a Human. They had gigantic claws, large enough to easily hold an entire car. The rest of the body was smaller in proportions, with glowing eyes on thin stalks and 8 spindly legs. The Macras were predators who hunted by movement and showed no sign of intelligence. They also had distinctive and terrifying roars. (DW: Gridlock)

History
The Macra were an ancient race with a large empire; "the scourge of this galaxy" as the Doctor put it. However, after many billions 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 existence. 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) How the Macra got here is simple; some had escaped from the New New York Zoo when the power broke down, made their way into the Motorway and they multiplied. (Doctor Who: Creatures and Demons)


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

Behind the Scenes
The Doctor labelled the Macra as "Scourge of this Galaxy" in Gridlock, a title he gave to Baltazar in The Infinite Quest. However, they are two different galaxies.