Tractators were a species that resembled giant invertebrates. They could control gravity. 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 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. It controlled the rest of the Tractators. If the Gravis was absent, 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 polished the walls of the cavern so that they were smooth to create a gravity motor. (DW: Frontios)
History
Sometime before the 20th century, the Tractators invaded Trion. The invasion etched itself into the racial memory of the Trions.
Millions of years later, a group of Tractators came to Frontios, and were stuck there. They waited for four hundred sixty years until a human ship came close to the planet. They made the ship crash using their gravity powers, then left them alone for ten years in order to rebuild. They used the asteroid belt in the Veruna system to attack and kill the humans to use their corpses in their machinery. This continued for thirty years, with few of the humans guessing the truth of the matter.
Eventually, the Fifth Doctor was caught near Frontios and was pulled in by the Tractators. While he tried to help the colonists, the Doctor's TARDIS was ripped apart by the Tractators. The Doctor went in search of the cause of the incident, 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)