Why are a spiders lungs called book lungs?

Spider lungs are called "book lungs" because their structure resembles the pages of a book. These specialized respiratory organs consist of stacked, thin membranes that create a large surface area for gas exchange, similar to how pages in a book are stacked. This design allows Spiders to efficiently extract oxygen from the air while minimizing water loss, making them well-suited for their environments. The term reflects both the anatomical appearance and the function of these lungs.