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Spiracles are breathing openings found on the surface of insects, certain cartilaginous fish such as certain species of sharks, and stingrays. Hammerheads and chimeras don't have spiracles. In fish, spiracles are composed of a pair of openings just behind the fish's eyes that allow it to draw oxygenated water in from above without having to bring it in through the gills. The spiracles open into the fish's mouth, where water is passed over its gills for gas exchange and out of the body. Spiracles aid fish in breathing even when they are lying on the ocean bottom or when they're buried in the sand.
Evolution of Spiracles
Spiracles likely evolved from gill openings. In primitive jawless fish, spiracles were simply the first gill openings behind the mouth. This gill opening eventually separated as the jaw evolved out of the structures between it and the other gill openings. The spiracle remained as a small, hole-like opening in most cartilaginous fish. Spiracles are useful for the types of rays that bury themselves in the ocean bottom because they allow them to breathe without the aid of exposed gills.
Primitive bony fish with spiracles include the sturgeon, paddlefish, bichirs, and coelacanth. Scientists also believe that spiracles are associated with the hearing organs of frogs and some other amphibians.
Examples of Spiracles
Southern stingrays are sand-dwelling sea animals that use their spiracles to breathe when they are lying on the ocean bottom. Spiracles behind the ray's eyes draw in water, which is passed over the gills and expelled from its gills on its underside. Skates, cartilaginous fish that have a flat body and wing-like pectoral fins attached to their head, and stingrays sometimes use spiracles as their primary method of breathing, bringing oxygenated water into the gill chamber where it is exchanged for carbon dioxide.
Angel sharks are large, flat-bodied sharks that bury themselves in the sand and breathe through their spiracles. They lie in wait, camouflaged, for fish, crustaceans, and mollusks and then lunge to strike and kill them with their jaws. By pumping water in through their spiracles and out through their gills, these sharks can absorb oxygen and eliminate carbon dioxide without constantly swimming, as more mobile sharks must do.
Insects and Animals With Spiracles
Insects have spiracles, which allow air to move into their tracheal system. Since insects don't have lungs, they use spiracles to exchange oxygen and carbon dioxide with the outside air. Insects open and close their spiracles through muscle contractions. Oxygen molecules then travel via the insect's tracheal system. Each tracheal tube ends with a tracheole, where the oxygen dissolves into the tracheole fluid. The O2 then diffuses into the cells.
The blowhole of the whale is also sometimes called a spiracle in older texts. Whales use their blowholes to take in the air and dispel carbon dioxide when they surface. Whales have lungs like other mammals rather than gills like fish. They have to breathe air, not water.