in arthropods, the small external opening of a trachea (respiratory tube) or a book lung (breathing organ with thin folds of membrane resembling book leaves). Spiracles are usually found on certain thoracic and abdominal segments. In elasmobranch and ganoid fishes a pair of spiracles, derived from the gills, is used as a water passageway during respiration. The nasal opening of whales (whale) and other cetaceans (cetacean) is called a spiracle, as is the respiratory opening behind the eyes of rays (ray) and skates (skate). In tadpoles (tadpole) the spiracle is the excurrent opening from the gill chamber.