The Macra are a sapient extraterrestrial species native to a planet which has been colonized by Human settlers in the future.
The Macra are huge, slow-moving insectoids, covered in thick exoskeleton, with large grasping chelae (claws), eyestalks with glowing eyes, and long tentacle-like feelers for sensing the world around. They breathe an atmosphere rich in vaporous gases which are deadly for Humans, and in turn find Human atmosphere to be deadly for them.
The Second Doctor speculated that the Macra evolved early in the history of their homeworld, when the atmosphere was different, and went underground as it was replaced by an oxygen-rich one, surviving by mining the primordial gases trapped in the geological salt deposits of the planet. When Humans settled on the lush Earth-like planet, the Macra hypnotized the colony leaders with a special gas that tames the Human mind and makes it obey any order given by the "Control" (actually the Macra government in disguise). This way the Macra have enslaved Humans to work in the gas mines and processing units which guarantee new supplies of the substances the Macra need. The population, convinced by its leaders that the gas is essential to the proper functioning of the colony, does not even suspect being servants of an alien intelligence. The Macra are able to visit the surface for short periods of time at night, by filling their lungs with stocks of their native atmosphere.
The Tenth Doctor has described them as the "scourge of this galaxy" (referring to the Messier 87 galaxy in the Virgo Cluster), owners of a vast empire sustained by multiple enslaved Human worlds. Over five billion years in the future, all that remains from their once mighty empire are a race of degenerate descendants, more than ten times larger than their ancestors and almost devoid of intelligence. A colony of these new Macra is known to inhabit the toxic gas filled lower levels of New New York's motorway on New Earth.