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Many terrestrial arthropods have evolved a closed respiratory system composed of spiracles, tracheae, and tracheoles to transport metabolic gasses to and from tissue. Some terrestrial woodlice have evolved pseudotrachea, a system which is also called corpus alatum, and is made up of air tubes that delivers oxygen to their hemolymph; a similar system has been found in some caterpillars. The distribution of spiracles can vary greatly among the many orders of insects, but in general each segment of the body can have no more than one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometers up to 0.8mm. The smallest tubes, tracheoles, penetrate cells and serve as sites of diffusion for water, oxygen, and carbon dioxide. Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their hemolymph; this is one of the factors that may limit their size.
In the head, thorax, or abdomen, tracheae may also be connected to air sacs. Many insects, such as grasshoppers and bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill. Note that despite being internal, the tracheae of arthropods are shed during moulting (ecdysis).
- Wasserthal, Lutz T. (1998). Chapter 25: The Open Hemolymph System of Holometabola and Its Relation to the Tracheal Space. In "Microscopic Anatomy of Invertebrates", Wiley-Liss, Inc..
- Westneat, Mark W.; Betz, Oliver; Blob, Richard W.; Fezzaa, Kamel; Cooper, James W.; Lee, Wah-Keat (January 2003). Tracheal Respiration in Insects Visualized with Synchrotron X-ray Imaging. Science 299: 558–560.
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