Deltan

The Deltans were a humanoid species which originated from the planet Delta IV in the Alpha Quadrant. The Deltan government was the Deltan Union and they were members of the United Federation of Planets. (ST reference: Star Charts)

Overview
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
In 2182 the Deltans made first contact with the Andorians, the Deltan sage Niro guided the planet through its tumultuous decades from first contact to Federation membership, developed the Oath of Celibacy and writing much of Deltans' modernized legal code. (Last Unicorn Games RPG: Starfleet Operations Manual)
 * The Enterprise episode "Bound" establishes Human contact witht he 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 (NCC-1701) before both were reported "missing in action" after the Enterprise encountered the V'Ger probe in 2273. (TOS novelization: The Motion Picture)

In 2370 a pair of Deltan females visited Deep Space 9. Whilst enjoying drinks in Quark's they attracted the attention of a dozen or more Argelian men. 

Gaila had a Deltan female working for him in 2372. She dealt with Quark when he called Gaila to ask for help. 

Culture
Delta IV's unpredictable tides and otherwise hospitable ecology combined to create a culture at once polyglot and unified; although many microcultures evolved on the planet's myriad islands, they could never remain isolated long enough to develop either xenophobia or devastating cultural advantages. To avoid inbreeding, the island-bound Deltans also developed a very open sexual culture; stranded mariners were often adopted into large group amours to blend genes more effectively. Brought together by ties of sex and geography, Deltans learned to get along with each other by necessity; with a nurturing planet and a low species birthrate, wars over resources were almost unknown. (Decipher sourcebook: Starfleet Operations Manual)

Deltans were also pushed in the direction of cultural unity by their genetic predisposition toward empathy and other psionic abilities; their ability to feel each others' pain soon made war, and even serious emotionally-scarring argument, unthinkably impolite. Unlike other psionic races, Deltans' strong pheromones made them physically unable to hide their emotions; the smell gave them away every time. Their culture evolved toward a global ideal of supportive, nurturing behavior based on time-honored Deltan sexual openness and a deeprooted sense of species and personal security that not even their encounters with the Andorians (and eventually the other starfaring cultures of the Beta Quadrant) could shake. Their sole concession to a dangerous galaxy was the adoption of an Oath of Celibacy out of necessity (and simple politeness) when dealing with less sexually-mature species and cultures. (Decipher sourcebook: Starfleet Operations Manual)