Tractator

Tractators resembled giant invertebrates and could control gravity with their minds.

Physical characteristics
The Tractators wear giant chitinous shells. Except for their leader, the Gravis, they do not speak.

History
Sometime before the 20th century, the Tractators invaded Trion, As a consequence, the very sight of a Tractator paralyzes some Trions (such as Turlough) with fear even if they themselves, had never personally seen a Tractator. The invasion etched itself into the racial memory of the Trions.

Millions of years later, Tractators burrowed underneath the Human colony on Frontios and secretly used Human colonists as living parts.

Culture and technology
A figure of authority, a Gravis, acted as central figure of the Tractators. Tractators use scavenged machinery as well as the parts of living beings, as machines. (DW: Frontios)


 * We do not know if Tractators actually possessed a hive mind, though that would seem very possible.

Behind the Scenes
Writer Christopher Bidmead based the appearance of Tractators on insects known as wood lice. The production team toned down some of the more gory deciptions of Tractator technology, thinking them too horrific. The Gravis would have used a floating globe-shaped translation device, which would speak for the Gravis, cobbled together from metal parts and also a severed human head, but the production team vetoed this idea, too. Aside form the difficulty of making the globe float convincingly, they felt that the globe would make distract from one-on-on interaction between the Doctor and the Gravis. Many of these ideas turn up in Bidmead's novelizatoin of the story.