What is a book lung?

Book lungs are respiratory organs found in some species of arachnids, such as spiders, scorpions, and harvestmen. They consist of a series of leaflets or lamellae that are arranged like the pages of a book. These lamellae are lined with a network of blood vessels, and air is drawn in between the lamellae by muscular action. The oxygen in the air diffuses across the thin walls of the blood vessels and is carried throughout the body, while the carbon dioxide diffuses out of the blood vessels and is expelled through the spiracles.

Book lungs are more efficient at extracting oxygen from the air than the tracheal system found in insects, but they are not as effective at transporting oxygen throughout the body. As a result, book lungs are typically found in slow-moving animals that do not require a high metabolic rate.

Scorpions have four pairs of book lungs located in the mesosoma, while spiders have two pairs of book lungs located in the abdomen. Harvestmen have only one pair of book lungs, which are located in the opisthosoma.

Book lungs are an important adaptation for arachnids that live on land, as they allow them to breathe air. Without book lungs, these animals would not be able to survive out of the water.