Tracheole (trā'kē-ōl') is a fine respiratory tube of the trachea of an insect or a spider, part of the respiratory system. Tracheoles are about 1 µm in diameter, and they convey oxygen to cells while providing a means for carbon dioxide to escape. Tracheoles branch from the larger tracheae (which can be several mm in diameter) much like capillaries branch from arteries, or twigs from branches of a tree. This increases the surface area for gas exchange in the insect. Areas of intense metabolic activity, such as the digestive tract and flight muscles, have very dense aggregations of tracheoles.