Close up of the abdomen of an Annam stick insect (Medauroidea extradentata) featuring spiracles, each a fraction of a millimeter in diameter, located near the joint of each abdominal segment. Spiracles are holes which connect to an insect's tracheal, or 'breathing,' system that consists of tubes that carry oxygen to all parts of an insect's body. The spiracles open and close by way of valves that are controlled by muscles. Spiracles are lined with hairs that filter out dust as air enters. In this view, the abdomen throbs as the insect pumps fluid and air through its body.