Macra

The Macra were a gigantic crustacean race that fed on unclean gases which were poisonous to humans. The Macra resembled crabs. They would consume humans when possible.

Eventually, the Macra fell into evolutionary decline, reverting to mindless creatures in the depths of New New York.

Biology
There were two known types of Macra, the original sapient species and the later predators of New Earth. Both fed upon poisonous gases and organic lifeforms when available.

Sapient species
The sapient species of Macra were larger than humans, able to hold a human in one claw. They were stocky in build, with large eyestalks, large claws (with a second, smaller set beneath the mouth) and were a dark, mucky brown colour. They were also covered in hairs. (TV: The Macra Terror)

Predator species
The predator species of Macra were much larger in size, several hundred times larger than a human. They were more animal-like, having fallen into evolutionary decline.

They had gigantic claws, large enough to easily hold an entire New Earth car. The rest of the body was smaller in proportions, with glowing eyes on thin stalks and six spindly legs. The Macras were predators who hunted by movement and showed no sign of intelligence. They also had distinctive and terrifying roars. (TV: Gridlock)

History
The Macra were an ancient race with a large empire - "the scourge of this galaxy," as the Tenth Doctor put it. (TV: Gridlock) Their home planet was colonised by humans but the Macra took control of the colony and hypnotised the humans into mining gases for them.

They were defeated when the Second 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. (TV: The Macra Terror)

At some point before 2157, the Guild of Adjudicators dealt with the Macra. (PROSE: Lucifer Rising)

After many billions of years they devolved into feral beasts. (TV: Gridlock)

On New Earth, a colony of Macra survived and multiplied 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 by 5,000,000,053, the Tenth Doctor opened the roof of the Motorway, allowing the cars to escape to the Upper City, depriving the Macra of food and gases. (TV: Gridlock)

Following the opening of the Motorway, the Macra got a "new career" cleaning up the city's pollution. The Cybermen also used the Macra to dispose of all non-humans. (WC: In-Flight Entertainment )

Behind the scenes

 * The Doctor labelled the Macra as the "Scourge of this Galaxy" in Gridlock, a title he also gave to Baltazar in The Infinite Quest. However they are from different galaxies.
 * Approximately forty years passed between the Macra's first appearance on Doctor Who and their second. This is the second longest such interval between appearances for any character or creature in the franchise. The Great Intelligence is in the lead with forty-four years. Axos follows at a close third with thirty-nine years.
 * According to REF: Doctor Who: Creatures and Demons, a non-narrative source, on New Earth, the colony of feral Macra that preyed on the New Earths Cars in the Motorway escaped from the New New York Zoo when the power broke down and made their way into the Motorway.
 * Macra also feature in the Decide Your Destiny book, Claws of the Macra, where a group of the feral kind seen in Gridlock infest a gas refinery on Earth.