It is not clear what exactly the Macra are - they may be bacteria, they may be insects, but they certainly look like gigantic crabs with large, strong claws and slavering mouths. They are fairly impervious to blows, probably because of their hard shell. Most are black, except for the head Macra, which is white in coloration. The Macra breathe a kind of gas that is toxic to humans, and likewise, oxygen is toxic to the Macra. They can breathe an oxygen atmosphere for a time, but it causes them to become weakened, unsteady and sluggish, and they can only be revived with more of the gas. They apparently confine their space exploration to planets that are abundant in the gas, although on the planet where the Doctor encountered them, they could not find sufficient amounts of the gas in the atmosphere and were forced to have it pumped up from within the planet. They exist as parasites on the planet, keeping out of sight except at nighttime, when they venture out onto the planet's surface, crawling slowly over the ground.
In order to extract the gas, the Macra took control of a human colony that attempted to establish itself on the planet. They took over the colony's control room and enslaved the people's leader, known as Control. The Macra, who possess the ability to speak, then gave orders under the name of Control, using the man himself only as a figurehead.
As the colonists were conditioned to always obey Control, so they obeyed the desires of the Macra. As reinforcement, at night the colonists were put to sleep using a sleeping gas with the scent of sweet perfume, and were then subjected to subliminal control through deep sleep and thought patterns transmitted by a wire in the wall of their sleeping quarters. Thus convinced that the planet was a happy place with no troubles at all, the colonists began working to mine the gas from the planet and pipe it into the control room for the Macra to breathe.
Rumors of the existence of the Macra were quickly squashed by "Control", and anyone claiming to have seen them was locked up in a correctional hospital. Those who disobeyed the rules of the colony were sentenced to work in the mines as part of the danger gang, where the gas was at its highest density. Besides going out at night, spreading rumors about Macra, and disobeying Control, the colonists were also especially banned from walking on "forbidden ground", the shafts under the planet's surface where the Macra were living.
When the Doctor and his party got too close to the Macra's secret, the creatures attempted to kill them with the toxic gas. But eventually their sham was revealed, and the colonists were shocked to learn that Control had turned into an old, emaciated man and had not been leading them at all. The Doctor directed Ben to explode the gas pipes, thus killing the Macra on the planet.
