Solonian

The Solonians are a sapient species indigenous to the planet Solos, in the Milky Way Galaxy. Their homeworld has been conquered by Humans and annexed to the Earth Empire at some point prior to the 30th century.

Unbeknownst to most people, the Solonian species goes through periods of massive metamorphosis every 500 Earth years, accompanying the environmental variations of the planet Solos, which takes approximately 2,000 Earth years to orbit its sun.

Biology
Because each season in Solos' annual cycle lasts for 500 Earth years, the native Solonians have adapted themselves to change along with their environment. However, their individual lifespans are comparable to that of Humans. Therefore, a considerable number of generations pass between each period of transformation. As a result, the Solonians themselves are completely unaware of their species' periodic cycles of maturation.

Humanoid stage
The Solonians encountered by the Humans who first landed on Solos were humanoid, physically indistinguishable from the Humans themselves, save for the Solonians' natural ability to breathe their planet's misty nocturnal vapors, which Humans regard as toxic.

Mutt stage
At first believed by some to be a kind of mutation triggered by environmental pollution; the creatures called Mutts are actually an intermediary stage that Solonians typically go through when they're maturing, comparable to a mobile pupa. They resemble man-sized, bluish-dark-skinned, yellow-eyed insectoids with pincer-shaped hands and a thick exoskeleton, and are no longer capable of speaking.

It is possible that not all Solonians pass through this stage, and it's known that not all of them pass through it at the same time in any case. The Third Doctor has compared the early Solonians who morphed into the Mutt stage to "butterflies that hatch out on a cold spring day", and also noticed that their instincts had become confused due to the premature transformation.

Advanced stage
After the maturation is completed with the help of a local crystal, the Solonians reach their advanced stage, which appears to be an incorporeal life form capable of passing through solid walls, becoming invisible and shooting energy beams powerful enough to easily disintegrate targets.

History
As the Earth Empire gradually became weaker and decadent, Solos was granted independence in the 30th century. Interestingly, it appears that at some point in the far future, the Mutts have established themselves as a culture of their own with access to space cruisers. The Fourth Doctor has referred to these Mutts as "a mutant insect species, widely established in the Nebula of Cyclops". This raises the possibility that, away from the Solos crystals, the spacefaring offshoots of the Solonians might remain in their insectoid Mutt stage permanently.

Appearances

 * Doctor Who, Season 9, Serial 4, "The Mutants" (1972)
 * Doctor Who, Season 13, Serial 5, "The Brain of Morbius" (1976)