“We know you, Doctor! ….. at least ….by reputation!” The Gravis
The Tractator’s are large standing cockroach -like bipedal 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 speech and who controls and organises the other Tractator’s, via a hive mind.
The Fifth Doctor first meets the Tractator’s in Frontios – 1984 when the Tardis crash lands on the planet Frontios, 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 colonist’s are being attacked by intelligently controlled showers of meteorites, which hit their camp without warning.
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 Tractator’s to other planets.
We also discover that companion Turlough’s home world was invaded by the Tractator’s 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 monster’s (in this case, The Gravis) that the Doctor willingly invites into the Tardis control room to travel.
All part of a ruse though to separate its mental contact from that of its Tractator bethren and render the race inert, (without the leader the others go dormant!) standing the Gravis on another planet – without other lifeforms to bother.
The Tractator’s have since stayed dormant on Frontios and have not reappeared in Doctor Who, to date.
Season Twenty One – 1984