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Spiracle

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

Vertebrates

A bluespotted ribbontail ray (Taeniura lymma); note the spiracle just behind the eye.

The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar to the other gill opening. With the formation of the jaw in the early jawed vertebrates, this gill slit was "caught" between the forward gill-rod (now functioning as the jaw) and the next rod, the hyomandibular bone, supporting the jaw hinge and anchoring the jaw to the skull proper. The gill opening was closed off from below, the remaining opening was small and hole-like, and is termed a "spiracle". The spiracle is still found in all cartilaginous fish except chimaeras, and in bony fishes in the coelacanth, sturgeon, paddlefish and bichirs. It is also seen as an otic notch in the skull of the extinct labyrinthodonts, and is thought to be associated with the ear opening in amniotes.[1]

Arthropods

Indian moon moth (Actias selene) with some of the spiracles identified
Scanning electron micrograph of a cricket spiracle valve

Insects and some more advanced spiders have spiracles on their exoskeletons to allow air to enter the trachea.[2] In insects, the tracheal tubes primarily deliver oxygen directly into 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.[3] A similar diffusion effect also occurs in some insect caterpillars. In these groups the respiration is more reminiscent of lungs. In spiders and other arachnids they have structures called book lungs.

References

  1. ^ Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)
  2. ^ Solomon, Eldra, Linda Berg, Diana Martin (2002): Biology. Brooks/Cole.
  3. ^ Foelix, R. (1996): Biology of Spiders. Oxford University Press
  • Chapman, R. F. The Insects. 1998. Cambridge University Press
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