In this spider diagram, the book lung is labelled 1.

A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs of modern land-dwelling vertebrates by evolution. Their name describes their structure. Stacks of alternating air pockets and hemolymph-filled tissue gives them an appearance similar to a "folded" book. Their number varies from just one pair in most spiders to four pairs in scorpions. Sometimes the book lungs can be absent and the gas exchange is performed by the thin walls inside the cavity instead, with its surface area increased by branching into the body as thin tubes called tracheae. It is possible that the tracheae have evolved directly from the book lungs, because in some spiders the tracheae have a small number of greatly elongated chambers. Many arachnids, like mites and harvestmen (Opiliones), have no traces of book lungs and breathe through tracheae or through their body surface only.

The unfolded "pages" (plates) of the book lung are filled with hemolymph (the arthropod blood). 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 required to facilitate this kind of respiration.

The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved the 410 million year old Rhynie chert of Scotland. These Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids.

The absence or presence of book lungs divides the Arachnida in two main groups; the pulmonate arachnids (book lungs present, orders Scorpionida, Thelyphonida, Schizomida, Amblypygida, and Araneida), and the apulmonate arachnids (book lungs absent, orders Palpigradi, Opilionida, Acarida, Pseudoscorpionida, Ricinulei and Solifugae).

Book Gills

Book lungs have evolved from book gills. Although they have a similar book-like structure, they are found in different locations. Book gills are found externally while book lungs are found internally. Book gills are still found in Horseshoe crabs which have six pairs of them, though the first pair (called the operculum) serves as a cover for the other five. Book gills are flap-like appendages that are designed for gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage there are attached over 100 thin leaf-like membranes called lamellae which appear as pages in a book, and are the areas of the gill where gas exchange takes place. These appendages move with rhythmic movements to drive blood in and out of the lamellae and to circulate water over them. Respiration being their main purpose, they can also be used for swimming in young individuals. If they are kept moist, the horseshoe crab can live on land for many hours.