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 micrometres up to 0.8 mm. 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 haemolymph. This is one of the factors that may limit their size.
A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. 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).
Some terrestrial woodlice have evolved pseudotrachea, also called corpora allata (singular: corpus allatum), which are made up of air tubes that deliver oxygen to their haemolymph; a similar system has been found in some caterpillars.
- 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. ISBN 0-471-15955-7.
- 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 (5606): 558–560. doi:10.1126/science.1078008. PMID 12543973.