Origin : Frontios
“We know you, Doctor! ….. at least ….by reputation!” The Gravis
The Tractator’s are large underground dwelling, bipedial cockroach-like insect from the Planet of Frontios, who have power over the force of gravity itself.
They are led by ‘The Gravis’, an intelligent hierarchical Tractator; capable of physical speech and who organises the other Tractators, via a hive mind control.
The Fifth Doctor meets the Tractators in Frontios – 1984 when the Tardis crash lands on the planet Frontios, literally disassembling itself into the planet’s surroundings.
Here, the Doctor meets a bunch of stranded Earth colonists attempting to survive Frontios’s harsh environment. The colonists are being attacked by intelligently controlled showers of meteorites, which hit their camp without warning amd they cannot explain how this is occuring.
It turns out the Tractator’s are behind this venture, using their powers, hoping to kill the colonists but not before using their knowledge of spaceflight to turn frontios into a giant bio spaceship; to transport the Tractators to other planets.
We also discover that companion Turlough’s home world was invaded by the Tractators in some unspecified time period, in the past (before his birth).
As far as monsters in Doctor Who go, they are pretty unremarkable, save that they are one of the few monsters (in this case, The Gravis) that the Doctor willingly invites into the Tardis control room to go on a jaunt.
All part of a ruse though, to separate Gravis’ mental contact from the Tractator bethren and render the race inert, (without the leader’s presence the others go dormant!) stranding the Gravis on another planet – without other lifeforms to hurt.
The Tractator’s have since stayed dormant on Frontios and have not reappeared in Doctor Who, to date.
Season Twenty One – 1984