The Deltans themselves are closely related to the human norm. The primary differences are neurochemical, including a complex pheromone system which effects other humanoids. In appearance, they have only a small range of pigmentation, which tends to have a pale blue tinge. They have very little hair, with little to no facial or body hair, and baldness being quite common (although not universal). By Terran standards, Deltans tend to an overwhelming beauty.
The Deltan pheromone system causes many problems in contact with other humanoids, because they uncontrollably release a erotic attraction pheromone. Chemical applications can reduce that effect, which is virtually neccessary for interactions in enclosed spaces. At great intensities, such as during intercourse, exposure can be damaging or even fatal to other humanoids.
Deltans also have an extra pheremone control system in thier bodies which allows them to produce pheremonal chemicals naturally. It is assumed that this system is used during normal courtship rituals between Deltans, but they can also control other pheremones besides the erotic enhancing ones. Among the pheremonal abilities of a trained Deltan is the ability to produce a pheremonal chemical which seems to reduce pain in most humanoid races.
The sexual attractiveness of Deltans generally makes for bad psychological effects on members of a mixed crew ship. Hence, few Deltans serve on mixed crew vessels in Star Fleet, and those that do are required to take an Oath of Celebacy. This is meant to prevent any unfortunate incidents while on board any Starship.
The Deians had established colonies on three planets within their system and were actively seeking contact with other races when GSC Magellan entered the orbit of their homeworld in Y150.