Tarchannen III Species

This is an unnamed species of non-sapient life forms from the planet Tarchannen III which reproduce by assimilating other species.

Biology
The Tarchannen III creatures are Human-sized bipeds with a hunched posture, three-fingered hands and three-toed feet. They're normally invisible, but exhibit a distinctive bioluminescence under ultraviolet light, revealing bright blue veins that crisscross their skin, and bright yellow eyes with vertical slit pupils. They flee from visible light, but not from ultraviolet. They produce low body heat and their life signs can't be detected by Federation sensors.

The Tarchannen aliens are non-sapient and act by instinct alone. They appear to live in small groups, and are generally timid creatures which prefer to avoid confrontation.

Assimilation
This species' method of reproduction is based on the assimilation of other species, including Humans, although the changes don't start to manifest until five years after infection. The method of infecting a new host hasn't been discovered, although it's clearly discreet and speculated to involve the consumption of some infectious agent, possibly airborne spores.

Once active inside the host, following a dormant period of five Earth years, the alien DNA strains settle inside the thymus and use the host's immune system to spread genetic instructions to other cells, inducing a form of metamorphosis.

The first symptoms of this result in a chemical blood alteration that resembles an allergic reaction. Other symptoms include agitation, light-sensitivity and an uncontrollable urge to return to the planet Tarchannen III. After the transformation commences, it may take only a few hours for the body to undergo drastic changes. Blue veins start developing all over the host's skin, while their index, middle and ring fingers start to fuse together. Their eyes glow yellow with vertical slit pupils and the sensitivity to light increases.

Appearances

 * Star Trek: The Next Generation, s04e18, "Identity Crisis" (1991)