Sectoids are an advanced humanoid species which developed space travel. Sectoid Leaders and Sectoid Commanders are known to have psionic abilities. They are descending from the Aquatroids,with who they share webbed hand and feet.
The webbings on their feet and legs. They have cybernetic implants throughout the body, enhancing the strength of its atrophied limbs and the function of its organs. All race members of this race are identical which leads to beleive that the whole species is madeo of clones.Source: X-COM series