Book lung
type of lung commonly found in arachnids
A book lung is a type of respiration organ used for atmospheric gas exchange present in some arachnids such as scorpions and tetrapulmonates (spiders and whip scorpions). It is a terrestrial adaptation of the book gills seen in aquatic chelicerates such as horseshoe crabs, where the gills are ventral appendages under the abdomen (opisthosoma) arranged into a series of page-like lamellae in order to maximize surface area for diffusion. In arachnids, each of these organs is housed inside an air-filled cavity (atrium) that opens to the surrounding atmosphere through a small opening (spiracle), which provides a controlled environment for respiration.
Structure and function
Book lungs are not related to the balloon-like lungs of terrestrial vertebrates (tetrapods), the latter not evolved from any gills but rather a homolog of swim bladders. The name "lung" instead merely describes the structure's function as an aerial respiratory organ, which is a case of convergent evolution producing an analog of vertebrate lungs. Stacks of alternating air pockets and lamellar tissue filled with hemolymph give them an appearance similar to a "folded" book.
The number of book lungs varies from just one pair in most spiders to four pairs in scorpions. The unfolded "pages" (plates) of the book lung maximize the surface area exposed to environmental air, and thereby maximize the amount of diffusional gas exchange. In most arachnid species, passive ventilation alone is sufficient to facilitate respiration needed for metabolism.
Occasionally absent
Many arachnids, such as mites and harvestmen, have no traces of book lungs and breathe through their body-surfaces only or through tracheae.
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