Bactrican Third

A Bactrician Third is a genderless member of the Bactrician species. Thirds are necessary in order for male and female Bactricians to react to one another and complete the act of procreation. Thirds are housed and trained in special "nunneries," kept separate from the general population. When a Third joins in a Triad with two others, a lifetime bond is created. As such, when the other members of the Triad die, the Third is expected also to take its life. (DS9 short story: "The Music Between the Notes")