Spiridon (species)

Spiridons, also known as Zalerians, were the invisible humanoid natives of Spiridon.

Biology
Anti-reflecting light waves made the Spiridons naturally invisible at all times to all but the eye-plants. This ability made them the target of the Daleks, although the high energy demands and the risk of light wave sickness makes this invisibility difficult to achieve in other species. After death, a Spiridon would become completely visible. (DW: Planet of the Daleks) The Thal scientist Codal believed that their invisibility was a trait evolved as the ultimate survival strategy on a planet where all other life was unrelentingly hostile. (DWN: Doctor Who and the Planet of the Daleks) It was possible for them to make themselves visible by undergoing many painful treatments including the regular ingestion of certain chemicals and nutrients which added pigmentation and caused cellular decay. A Spiridon’s genetics, however, would constantly attempt to correct these changes. (BFA: Return of the Daleks)

History
The Spiridons had a civilisation, which fell into ruin by the 26th century, when the Daleks saturated the forests with deadly rays (DWN: Doctor Who and the Planet of the Daleks) and released bacteria which killed off most of the population. The survivors cooperated with the Daleks and were viewed as little more than slaves. A few, like Wester, did what little they could to fight back. With the planet conquered, the Daleks hoped to discover the secret of the Spiridons' invisibility and reproduce it for their own use on an army kept in suspended animation. During the occupation, the Spiridons wore thick purple fur coats that covered most of their bodies to protect themselves from the cold. (DW: Planet of the Daleks) The Daleks ordered them to wear these coats at all times so that they remained visible but there was some indication that this order was sometimes disobeyed. (DWN: Doctor Who and the Planet of the Daleks) Whilst the Daleks were developing a deadly bacteria, the Spiridons were to be innoculated against it. Wester, the Doctor, Jo Grant, and a small group of Thals, sabotaged the Dalek operation. The Dalek Supreme only considered this a delay and promised that the Daleks would return. (DW: Planet of the Daleks)

Fearing that their invisibility made them a target, the Spiridons undertook many painful treatments to remove this trait. The Spiridons renamed themselves Zalerians and their planet, Zaleria, hoping it would disguise them from the rest of the universe. In future generations, a council of Elders were the only ones on Spiridon to remember the dormant Dalek army on their planet. Their religious texts recorded their ancestors' complicity but most only knew the Daleks had invaded. (BFA: Return of the Daleks) Centuries later, an army of Daleks from the Seriphia Galaxy began the second occupation of Zaleria. (BFA: Return of the Daleks)

Davros spent a year on Spiridon, amassing

At the time Samantha Jones was on Hyspero, Spiridons in purple furs were present in Hyspero, its capital city. (EDA: The Scarlet Empress)

Behind the scenes
In the novelisation of Planet of the Daleks Vaber explicitly says that the Spiridons’ civilisation is in ruins but no one says when this occurred. The crumbling ruins, which the forest has largely overtaken, appear to be thousands of years old but it is not unreasonable to believe, as Jo Grant supposed, that there was something built more recently.