Deltans were identified by their bald scalps. They had pain-relieving abilities that manifested during tactile contact. This ability did not, however, heal injuries. Because of their high sexuality, Deltans had to swear an oath of celibacy upon entering service in Starfleet, as a promise not to take advantage of any fellow crewmember. (Star Trek: The Motion Picture)
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. On the advice and invitation of his father, he dealt with the feelings they engendered in him by performing exhausting workouts in the gym, a method he returned to (and recommended to Lt. Malcolm Reed) while three Orion slave girls were "guests" aboard Enterprise in December 2154. (ENT: "Bound")
One Deltan, Starfleet Lieutenant Ilia, made direct contact with the V'ger entity when it approached Earth in the 2270s, whereupon she was listed as missing in action. (Star Trek: The Motion Picture) Another Deltan served as an officer aboard the USS Excelsior in 2285. (Star Trek III: The Search for Spock) Four male Deltan ambassadors later served on the Federation Council in 2286. (Star Trek IV: The Voyage Home)
Star Trek IV: The Voyage Home Star Trek Phase II: The Child
Deltans were originally to have been part of the ultimately scrapped series Star Trek: Phase II. The writers/directors guide for that series stated, "The Deltan race is much older than Humans, with brains much more finely evolved in areas of art and mathematics," and postulated that all Deltans were "completely hairless except for the eyes." The guide also suggested that esper abilities were common to the Deltans living on their homeworld and that, in their culture, virtually everything (i.e. all social interaction) was sex-oriented, with sexual foreplay consisting "largely of lovers placing images in each other's minds." (Star Trek Phase II: The Lost Series, pp. 90-91)
In the fan based series Star Trek Phase II episode "The Child" "The Child" we see the strong parent child bond and their above average strength of Lt. Isel.
A character study of Lt. Ilia, written by Gene Roddenberry for Star Trek: The Motion Picture, included more information about the Deltan race, stating, "Deltans, at first impression, may seem to be proudly aloof. However, a quick-eyed observer becomes immediately aware that Deltans actually have a splendidly developed sense of humor and fun lying immediately beneath that poised exterior. Their graceful carriage has lured many a Human into believing them cool intellectuals, only to discover that the Deltan is enjoying a good-humored joke at that Human's expense. Unlike the Vulcan race, Deltans value and delight in emotion – they see emotion as one of the myriad delights of being a life form. They are a sensual race – they enjoy the sensation of feeling hunger and fulfilling appetite in every form from satisfying their palates with exquisite foods, to the caress of a warm breeze or the bite of a bitter wind, the touch of an infant's hand, and especially all the shared communications and physical sensations of acts of love. Along its path to individual awareness, all the five (perhaps six) senses of the Deltan become highly acute and sensitive. Their taste buds, the rods and cones of their inner eyes, even their smallest epidermal nerve ending, all are sensitive far beyond the human norm." The same character study also mentioned historical similarities between the civilizations of Earth and the Deltan homeworld and that the Deltans "long ago lost their interest in space voyaging (concentrating on their own inner-space)," so that, by the time of The Motion Picture's setting, there were only a few Deltans who had joined Starfleet, even though "the highly evolved Deltan intelligence can handle the most complex spherical trigonometric complexities of space navigation as easily as a Human learns simple multiplication tables." (The Making of Star Trek: The Motion Picture, pp. 107-108)
Deltans were additionally mentioned in notes that costume designer Robert Fletcher wrote about the aliens in The Motion Picture. The note specifically concerning the Deltans contained the statement, "Deltans are great jewelry makers. Their jewelry is sold throughout the galaxy and is very popular." Also in that note, Fletcher even made some plans for what a Deltan male would look like, despite the fact that Ilia is the only Deltan seen clearly in that particular film. This section read, "Male Deltan wears traditional caftan with Deltan symbol on sleeve. Made of gabardine." (The Making of Star Trek: The Motion Picture, p. 133)
During a press conference, Ilia actress Persis Khambatta playfully announced, "Sex with a Deltan woman is like nirvana. After that no man can be satisfied with anyone else. The Deltan woman is so sophisticated sexually that any contact with a human man would drive him insane. It's wonderful." After her hair was shaved off by makeup artist Fred Phillips, the Indian actress joked that the new bald appearance made her look like a Hare Krishna. (Star Trek: The Magazine Volume 2, Issue 8, pp. 29 & 30)
The Deltans were the inspiration for Troi's species (Betazoid). This includes the Betazoids deriving their name from the Greek alphabet like the Deltans, the Betazoids' relatively very open attitude toward sexual activities, and Troi along with the rest of the Betazoids' ESP (Extra Sensory Perception) capability. (citation needed • edit) Some non-canon note that the Deltans and Betazoid races may be related in the distant past as part of the seeding done of humanoid races in the galaxy.
In "The Big Goodbye", an illustration of a female Deltan was seen when Data was assimilating the Dixon Hill novels. The illustration was captioned, "Deltan." This illustration was from the FASA RPG module The Federation.
The Deltans' strong sexual presence, even without physical contact, was explained by pheromones and sub-conscious telepathy in Gene Roddenberry's novelization of The Motion Picture. 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.
The novelization of The Wrath of Khan expanded on the suggestion of Jedda being Deltan, 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.