Elusive reptilian humanoids of Tarchannen III that reproduce by infecting other beings with a parasite which rewrites their DNA and compels them to return to the planet.
The native lifeforms of Tarchannen III are luminous, reptilian humanoids able to blend into their surroundings so completely that they are almost invisible. They reproduce in an extraordinary way: a parasite passed to visitors lies dormant for years, then rewrites the host's genetic code until the victim becomes one of them.
In the episode Identity Crisis, the members of a USS Victory away team who investigated a vanished colony on the planet five years earlier begin to disappear one by one, drawn back by instinct. Geordi La Forge and his friend Susanna Leijten are the last.
Doctor Crusher finds and removes the parasite in time to save Leijten. La Forge completes the change and flees to the surface, where Leijten persuades him to return before his mind is lost.