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 can 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 some stranded Earth colonists, attempting to survive Frontios’s harsh environment. The colonists are being regularly attacked, by intelligently controlled showers of meteorites; which hit their camp without warning amd they cannot explain how this is occuring.
It turns out, that the underground dwelling Tractators ; are behind this venture. Using their powers, to kill the colonists to remove them from theor planet. The Tractaors are using their knowledge of space-flight to turn Frontios itself – into a giant spaceship; to transport the Tractators to other planets.
We also discover, that companion Turlough’s homeworld 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 for the fact – that they are one of the few monsters (in this case – The Gravis itself), which the Doctor invites into the Tardis control room.
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 Tractators have since stayed dormant, on Frontios – and did not reappear in Classic Doctor Who, again.
Season Twenty One – 1984