Book lungs are the main respiratory organ in most arachnids (spiders and scorpions). Book lungs are within small openings in the abdomen of the arachnid. The book lungs themselves consist of a series of haemolymph filled plate-like structures. Between the plates there is an air space and this allows air to circulate around the plates. Gaseous exchange then occurs through the surface of the plates.
Book lungs get their name from the fact that the stacked plates have the appearance of the closed pages of a book.
A photograph of the underside of a spider illustrating the book lungs (white patches) at the top of the abdomen.
Photograph by Kaldari.
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