Editing Book lung (section)

===Occasionally absent===
Many [[arachnid]]s, such as [[mite]]s and [[Opiliones|harvestmen]], have no traces of book lungs and breathe through their body-surfaces only or through [[invertebrate trachea|tracheae]]. Gas exchange is performed by the thin walls inside the cavity instead, or with their surface area increased by branching into the body as thin tubes, the tracheae. These tracheae may possibly have evolved directly from book lungs because the tracheae in some spiders have a small number of greatly elongated chambers.{{citation needed|date=April 2025}}