1. (windpipe) The tube in air-breathing vertebrates that conducts air from the throat to the bronchi. It is strengthened with incomplete rings of cartilage.

2. An air channel in insects and most other terrestrial arthropods. Tracheae occur as ingrowths of the body wall. They open to the exterior by spiracles and branch into finer channels (tracheoles) that terminate in the tissues (see also air sac). In some larger and more active insects, pumping movements of the abdominal muscles cause air to be drawn into and out of the tracheae. Also, by opening and closing different spiracles during certain phases of ventilation, the insect causes the directional flow of air through the tracheae, as in the locust.