If you are referring to tracheal tubes, they are located along the length of the abdomen in 8 pairs and there are also two pairs in the thorax region. They have spiracles or openings to the outside environment. They are used by members of class Insecta for respiration. The oxygen enters the tubules, which branch into very fine tubules called tracheoles, and then, it diffuses into cells inside the body cavity. The interior cells of Insects are never far from a tracheole. Likewise, carbon dioxide diffuses from the cells of the body, into the tracheal tubes and out of the spiracles back to the environment.