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 - plant s . This ability made them the target of the Dalek s , although the high energy demands and the risk of light wave sickness made this invisibility difficult to achieve in other species . After death , a Spiridon would become completely visible . ( TV : " 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 . ( PROSE : " 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 . ( AUDIO : " Return of the Daleks " ) History . The Spiridons had a civilisation , which fell into ruin by the 26th century , when the Dalek s saturated the forests with deadly rays ( PROSE : " 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 . Whilst the Daleks were developing a deadly bacteria , the Spiridons were to be inoculated against it . Wester , the Third Doctor , Jo Grant , and a small group of Thal s sabotaged the Dalek operation . The Dalek Supreme only considered this a delay and promised that the Daleks would return . ( TV : " Planet of the Daleks " ) Davros spent a year on Spiridon , successfully reviving the frozen Daleks and building them into an army of four million . During the fighting between the two factions of Daleks , the Spiridons aided the Seventh Doctor in releasing his allies from their mind control . ( COMIC : " Emperor 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 . Centuries later , an army of Daleks from the Seriphia Galaxy began the second occupation of Zaleria . Although the Zalerians fully cooperated with the Daleks , the Daleks began to handle the natives with more aggressive methods backed by Ogron troops . It soon became apparent that the Daleks realised they could revive the army and continue their invisibility experiments . Daleks experimented on the Zalerians by placing them in light - wave chambers , hoping to reverse their visibility and reverse - engineer the secret . Although Kalendorf and Susan Mendes attempted to stop them , the Seventh Doctor agreed to help the Daleks unlock the secret to invisibility in exchange for Kalendorf ' s life . Years later , the Zalerians ' invisibility was restored when the Doctor released a contagion . The same contagion gave the Daleks a fatal light - wave sickness which the Spiridons thought also killed the Doctor . ( AUDIO : " Return of the Daleks " ) At the time Samantha Jones was on Hyspero , Spiridons in purple furs were present in Hyspero , its capital city . The " Aja ' ib " apparently referred to the Spiridons during one of their periods of Dalek enslavement . ( PROSE : " 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 .