One of the two kinds of respiratory organ (compare tracheal spiracles) found in spiders. They are located in the abdomen, just below the pedicel, and are composed of many fine leaves. Blood is passed over a large surface area to absorb oxygen. The openings of the book lungs (branchial opercula) are situated on the ventral surface of the abdomen and may be closed to prevent water loss. Book lungs are believed to be the ancestral type of respiratory organ in arachnids.
Subjects: Zoology and Animal Sciences.