Most true spiders have one pair of book lungs. These are respiratory organs built from stacks of thin, blood-filled plates, or lamellae, that resemble the pages of a book and allow oxygen and carbon dioxide to diffuse through the body.
Book lungs are found in several arachnid groups, including spiders, whip scorpions, and scorpions. Their evolutionary story reaches back to ancient book gills, the aquatic respiratory structures of horseshoe-crab relatives. Fossil book lungs from Scotland’s 410-million-year-old Rhynie chert show that this system was already adapted for life on land during the Devonian Period.
The number of pairs varies among arachnids. Most spiders have one pair, while scorpions have four pairs. Some spiders also use tracheae, narrow tubes that carry air more directly into the body. This is why “one pair” describes the typical arrangement, rather than every spider species without exception.