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Spiracle

Spiracles are small openings on the surface of some animals that usually lead to respiratory systems.

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In elasmobranchs (sharks and rays), a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest (Fouts, 2003). A spiracle is also found in primitive bony fishes as the bichir.

Spiracles in insects

In insects and some more advanced spiders, spiracles on their exoskeleton allow air to enter trachea (Solomon et.al., 2002). However, the two groups differ in how the tracheae function; in insects, the tracheal tubes primarily deliver oxygen directly to the animals' tissues. The spiracles can be opened and closed in an efficient manner to reduce water loss. This is done by contracting closer muscles surrounding the spiracle. In order to open, the muscle relaxes. The closer muscle is controlled by the central nervous system but can also react to localized chemical stimuli. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea. In spiders, however, the oxygen diffuses into the hemolymph (Foelix, 1996). A similar diffusion effect also occurs in some insect caterpillars. In these latter groups, then, the respiration is more reminiscent of lungs (in spiders and other arachnids, they have structures called book lungs, in fact).