Spiridons, also known as Zalerians, were the invisible humanoid natives of Spiridon.
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 made this invisibility difficult to achieve in other species. After death, a Spiridon would become completely visible. 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. 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. (Planet of the Daleks 1973) more