Deltan
The Deltans were a humanoid species originating from the planet Delta IV. They were members of the United Federation of Planets.
Physiology
Deltans largely resembled Humans with the exception of having no surface hair except for eyebrows, lashes, and (occasionally) a very thin layer of soft down along arms and legs. They are, on the average, slightly slimmer than Humans, and their cheekbones, eye shapes, and jawlines are slightly more symmetrical, Humans (and many other humanoid species) often find Deltans quite attractive. (Decipher sourcebook: Starfleet Operations Manual) Deltans were telepathic and empathic allowing them to sense other peoples emotions and administer medical aid by absorbing and minimising pain, this "Mind-meld anesthesia" meant Deltans made excellent medical personnel.
Deltans were perhaps best known for their high sexuality, both sexes emitted some of the most potent pheromones know to Federation science. As a result Deltans were something of natural aphrodisiacs, most intensely among their own race, but with a strong effect on most other humanoid species. Because of this Deltans swore an "oath of celibacy" when serving in Starfleet, they also developed chemical applications to diminish their pheromonal effect. To limit exposure to Deltan pheromones off-world visitors are not permitted to visit Delta IV itself, instead being restricted to the planet's moons Seyann and Cinera. (TOS movie: The Motion Picture; ST references: The Worlds of the Federation; Star Charts)
The Deltan oath of celibacy served not only to curtail sexual promiscuity with other races but also ensured the safety of non-Deltans; the long evolvement of the Deltan race not only heightened their sensuality but has also resulted in the sex act becoming a complete union in which both body and mind are shared. For Deltans this was natural and pleasant, but the experience of actually becoming part of another persons mind almost always incapacitates a human partner. It was sometimes said that a non-Deltan who entered into a physical relationship with a Deltan risked insanity. (TOS novelization: The Motion Picture)
History
The Enterprise episode "Bound" establishes Human contact with the Deltans as early as 2141. "Star Trek: Star Charts" claim the Deltans became warp capable in 2223 and joined the Federation in the same year.
Following the Deltans becoming full members of the Federation in 2259 Starfleet teams were assigned to Delta IV. The Deltan Ilia worked closely with these teams and formed a strong attachment to the Human Willard Decker. The two later served together briefly on the USS Enterprise before both were reported "missing in action" after the Enterprise encountered the V'Ger when it approached Earth in 2272. (Star Trek: The Motion Picture)
The Project: Genesis research team assigned to Space Station Regula I was made up of two Deltans; Jedda Adzhin-Dall and his mate, Zinaida in 2284. (TOS novelization: Star Trek II: The Wrath of Khan)
In 2141, the ECS Horizon encountered a group of Deltans. Travis Mayweather, who was fifteen at the time, later described them as very attractive and very open. His method of dealing with the feelings they engendered in him was exhaustive workouts in the gym, a method he returned to (and recommended to Lt. Malcolm Reed) while three Orion slave girls were "guests" aboard Enterprise. (ENT: "Bound")
A Deltan served as an officer aboard the USS Excelsior in 2285. (Star Trek III: The Search for Spock) A male Deltan ambassador served on the Federation Council in the same year. (Star Trek IV: The Voyage Home)
Apocrypha
The Deltans' strong sexual presence, even without physical contact, was explained by pheromones and sub-conscious telepathy in Gene Roddenberry's novelization. When Lt. Ilia first appeared in the novelization, James T. Kirk reflected that he seemed to instinctively perceive her as naked, despite the fact she was in uniform, and realized this illusion was due to her baldness. References to some of the Deltans' sexual tendencies and oaths were removed from some versions of the filmed appearance.
In the script of Star Trek II: The Wrath of Khan, stage directions indicated that the Project Genesis scientist, Jedda, was Deltan. The novelization of Star Trek II expanded on this, adding a female Deltan partner. It was also said that sexual activity between a Deltan and a non-Deltan means risking insanity for the non-Deltan.
