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.
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