Editing Book lung (section)

==Structure and function==
{{further information|Spider anatomy}}
Book lungs are not related to the [[lungs]] of modern land-dwelling [[vertebrate]]s. Their name instead describes their structure and purpose as a case of [[convergent evolution]]. Stacks of alternating air pockets and tissue filled with [[hemolymph]]{{efn|[[Hemolymph]] is the [[arthropod]] equivalent of blood.}} give them an appearance similar to a "folded" book.<ref name="rain">{{cite book |title=Biology of Spiders |last=Foelix |first=Rainer F. |url=https://archive.org/details/biologyofspiders00foel_0 |url-access=registration |quote=book lung. |year=1996 |publisher=Oxford University Press US |isbn=0-19-509594-4 |pages=[https://archive.org/details/biologyofspiders00foel_0/page/61 61]–64 }}</ref>

Their number varies from just one pair in most spiders to four pairs in scorpions. The unfolded "pages" (plates) of the book lung are filled with hemolymph. The folds maximize the surface exposed to [[air]], and thereby maximize the amount of [[gas]] exchanged with the environment. In most species, no motion of the plates is needed to facilitate this kind of respiration.

===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}}