- You may be looking for the Kahler doctor.
The Jex were a sentient insect species from Jexa.
Jex had large domed skulls with antennae, small red eyes, and claw-like pincers. They had an intergalactic empire spanning several galaxies, but by the 20th century it had shrunk drastically from constant attacks by the Canavitchi for the previous two thousand years.
The standard invasion methodology of the Jex was in three steps. First was infiltration, where they insinuated themselves into a planet's population. Next was economic conquest, taking over businesses around that world. Then came bombardment, where any resistance was dealt with militarily.
On Earth, they planned to detonate plutonium bombs to alter the atmosphere to be more conducive to the Jex. They also were going to use a gene-bomb to mutate humans to be able to survive in the new environment and be better slaves. Their plot was stopped when a Canavitchi fleet arrived and the two races annihilated each other in orbit, after the Fifth Doctor had set up a force field around Earth to protect it from the fallout from the battle. (PROSE: The King of Terror)