The Tractators are an insect-like race, bearing some similarities to pillbugs. Unlike the average Earth insect, the Tractators are very intelligent, with highly refined powers of abstract reasoning. They make a gargling scream when hurt. They live underground, where they construct funtionally advanced excavation machines to create extensive and elaborate tunnel systems. To drive these machines, they need the minds of living beings. The individual's body is hooked into the front of the excavation machine.
Tractators can produce a gravity beam with the aid of their antennae. They can even exert this force as far as outer space, if they want to snatch a passing spacecraft or batter the surface of their current planet with asteroids. This force is more commonly used, however, to bring living bodies from the surface for use in their excavating machines. To do this, they first aim their gravity beams upward at a being who is weakened - for example, unconscious or wounded. Pockets of earth around the body begin spiralling downward, and then the body itself is pulled down into the earth. For those living on the surface who see only a screaming man being pulled into the ground, this fate results in fear and paranoia, and phrases like "the earth is hungry", "the appetite beneath the ground" and "Frontios buries its own dead."
The Tractators expand and populate planets beneath the soil, being likened to an infection. The Tractators once attacked Trion in such a manner, either when Turlough was young or far enough in his past to constitute a race memory. On Frontios, they are led by the Gravis, who is capable of speech. Other Tractators are guards. Having been marooned on Frontios for 500 years after their old planet neared the end of its useful powers, the Tractators disabled the first ship that passed and pulled it down to the planet's surface. They allowed the Earth colonists ten years to establish their settlement, then began bombarding the planet with asteroids and culled the weakened humans. With their excavating machines, they smoothed the rock walls to a polish in order to act as wave guides and concentrate the force of their gravitic beams. They intended to build a mathematically-precise ring of tunnels, forming a gravity motor. This would enable them to pilot Frontios through the galaxy using the power of gravity, so that they could infest new planets for breeding, stealing and plundering wherever they travelled.
The Tractators draw their strength from the Gravis; without him, they become harmless, burrowing earth creatures. Isolating the Gravis from the rest of the Tractators, then, is the best way of defeating them. On Frontios, the Gravis coveted the Doctor's TARDIS, thinking it an excellent means of transportation and a way to spread themselves around the galaxy even more quickly. Although the TARDIS had been broken apart under the Tractator's gravitic force, the Gravis reassembled it around himself, cutting himself off from the others. The Doctor transported him to the rocky planet Kolkokron.
