Quaddies were a genetically altered species of humans, in this case engineered to live in zero-gravity environments on spaceships and on space stations. They had four arms and no legs; they had no tendencies towards motion-sickness; their muscles maintained tone more easily than downsiders' (legged humans) muscles; their bones didn't deteriorate in null-gee; and they were highly radiation-resistant. Their lower arms served as power-grippers, being much stronger than the upper ones. In addtion, they were made fertile, male and female, so that they could reproduce and maintain themselves indefinitely as a slave labor force.