Tractator

Tractators were a species that resembled giant invertebrates and could control gravity with their minds. Themselves mindless, they required the Gravis to act intelligently.

Biology
The Tractators were insect-like in general appearance. Their backs were covered in chitinous shells. Tractators had two long antennae on their head, which they used to control gravity. This power could be extended in the presence of other Tractators, as well as in properly made caverns.

Tractators were normally unintelligent animals, except in the presence of their leader, the Gravis. The Gravis could be identified by its darker skin, external nose and larger ears. The Gravis was the only Tractator who could speak and who controlled the rest of the Tractators. If the Gravis was removed, the Tractators would lose their focus and intelligence and return to their usual state. (DW: Frontios)

Technology
The Tractators used scavenged machinery as well as the parts of living beings, as machines. On Frontios, they created a Excavation machine that was powered by a human colonist through the use of their minds. They planned to turn the planet into a giant ship and covered the walls of the cavern so that they were smooth so as to create a gravity motor. (DW: Frontios)

History
Sometime before the 20th century, the Tractators invaded Trion. As a consequence, the invasion etched itself into the racial memory of the Trions.

Millions of years later, a group of Tractators came to Frontios, but were stuck there. They waited for 460 years, until a human ship came close enough to the planet. They made the ship crash using their gravity powers, then left them alone for 10 years in order to rebuild. They then used the asteroid belt in the Veruna system to attack the humans, killing them and allowing them to use their corpses in their machinery. This continued for 30 years, with most of the humans not guessing the truth of the matter.

Eventually, the Doctor was caught in the area around Frontios and was pulled in by the Tractators. While trying to help the colonists, the Doctor's TARDIS was ripped apart by the Tractators. The Doctor went in search for the cause of the incidents, and encountered the Gravis. He managed to trick him into repairing the TARDIS while he was inside it, causing him to collapse from the effort and isolating him from the other Tractators. (DW: Frontios)