O is for... Optera

A race of subterranean creatures encountered by the Doctor's companion Ian Chesterton on the planet Vortis. He and a Menoptra friend named Vrestin stumbled into their domain after falling down a crevasse, where they were captured by the grub-like creatures. They feared the world above, having lived below ground for generations, and they were going to sacrifice their captives by throwing them into a pit of fire. However, Vrestin recognised their origins. They had once been like her, but had retreated underground in the past and devolved into their present form - losing the power of flight and developing extra limbs for burrowing. They also made use of broken stalactites and stalagmites as tools and weapons.

They still recalled their former existence by making the Menoptra their gods, and when Vrestin spread her wings they recognised her as one of these.

Their leader Hetra explained that the acid pools used by the Animus to feed its Carsenome would leak down into their world, endangering them. When Ian and Vrestin explained that they sought to destroy the creature which they called "Pwodarauk", they agreed to help - taking them to a cavern directly below the Carsenome from which they could launch and attack. On the way, they encountered an acid leak, and one of their number - Nemini - sacrificed herself to block it by pushing her body into the crevice.

After the Animus had been destroyed, the Optera were encouraged to return to the surface, where future generations might have the power of flight once more.

Played by: Ian Thompson (Hetra), Barbara Joss (Nemini). Appearances: The Web Planet (1965).

• Thompson had previously appeared as the Aridian Malsan in The Chase, also directed by Richard Martin.
• Joss had featured in the same Dalek story, as Maureen O'Brien's double on the location filming for the Aridius scenes.
• The Optera did not feature in Bill Strutton's original scripts. They were created by Martin and story editor Dennis Spooner as they felt that the latter half of the story lacked incident.