Tracheae are to be found only in members of two invertebrate phyla, the Arthropoda and the Onychophora, although similar respiratory tubules do occur in a few molluscs and crustaceans. They are primarily an adaptation to the terrestrial environment, but a number of species possessing them have become secondarily aquatic. Among the arthropods they are typically found in most insects, chilopods (centipedes), diplopods (millipedes) and symphylans, as well as in a number of arachnid orders.
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