Book lung

Book lung is a type of respiratory system found in many arachnids, such as scorpions and spiders. It is named for its structure, which resembles a book with many pages or leaves.

Structure and Function

A book lung consists of a series of thin, flat plates, or "leaves," of respiratory tissue. These leaves are arranged in a stack, like the pages of a book, hence the name "book lung." The leaves are enclosed in an internal cavity, the atrium, which opens to the outside through a small hole, the spiracle.

Air enters the book lung through the spiracle and flows between the leaves, where oxygen is absorbed into the blood and carbon dioxide is released. This process of gas exchange is facilitated by a thin layer of moisture on the surface of the leaves, which dissolves the gases and allows them to diffuse across the cell membranes.

Evolution and Diversity

Book lungs are believed to have evolved from the gills of aquatic arachnids. The transition from gills to book lungs would have allowed these animals to colonize terrestrial environments, where oxygen is more abundant and easier to extract from the air than from water.

Not all arachnids have book lungs. Some, like the harvestman and the mite, have evolved other types of respiratory systems, such as tracheae or lungs. However, book lungs are a defining characteristic of the order Araneae, which includes all true spiders.

See Also

• Respiratory system
• Arachnid
• Spider
• Scorpion
• Gas exchange