Tractators are harmless burrowing beetles that live in tunnels beneath a planet's surface. Sometimes, however, the species produces a Gravis, a super-intelligent Tractator with the ability to unite the lesser Tractators into an organized force.

The Gravis is the driving force behind the expansion of the species, which can grow only by colonizing new planets. With the ability to construct sophisticated and accurate mining equipment, the Gravis oversees the building of tunnels to turn the centre of a planet into a giant gravity motor, enabling the Tractators to travel through space.

Tractators, however, need a supply of humans, or other intelligent minds, to provide the living intelligence that powers and guides their mining machinery. If no such minds are readily available, the Gravis will seek to divert passing spaceships, causing to crash on the planet. Once the colonists have built up sufficient numbers to guarantee a permanent supply of usable minds, the Tractators move in, picking on weakened, sleeping or sick targets and dragging them through the very earth by dint of their gravitational powers. These people are then harnessed to the mining machines, which they operate until their minds burn out.

When awakened by the Gravis, Tractators stand upright and are about 2 metres tall. The back of a Tractator is protected by black chitinous segments; tufts of black fur stick out from the overlapping sections. Its underbelly is a pale brown colour as is the broad, sucker-like base which allows it to move. It has a pair of short, stubby arms with ridged, shovel-like hands. The Gravis is identified by its predominantly green skin, instead of the more usual brown. Large, glassy eyes stare out sideways from an almost fishlike head, and instead of teeth, short tendrils are visible between the Tractator's thick lips. Two antennas, about 40 centimetres long, stick out from the top of the head and provide the means of directing and channelling gravitational energy.

The Gravis is the Tractators' greatest weakness. Without him, the Tractators revert to harmless, purposeless creatures. Knocking the Gravis unconscious or moving him away from the same planet will break his control.