What respiratory organs in many arachnids have thin, page-like lamellae?

The respiratory organs with thin, page-like lamellae in many arachnids are book lungs.

A book lung is an internal respiratory structure made of stacked plates, or lamellae, separated by air spaces. The arrangement resembles the pages of a book and creates a large surface area for diffusion. Hemolymph circulates through the lamellae while air enters through a small opening called a spiracle.

Book lungs occur in scorpions and several groups of spiders and other tetrapulmonate arachnids. Most spiders have one pair, whereas scorpions typically have four pairs. Some spiders have supplemented or replaced book lungs with tracheae, so not every arachnid relies on the same respiratory system.

The name can cause two common misunderstandings. Book lungs are not lungs like those of vertebrates; they are modified appendage-derived structures and are an example of convergent evolution. They are also different from book gills, the aquatic respiratory organs of horseshoe crabs. Fossil book lungs from Scotland’s Rhynie chert show that this respiratory design existed roughly 410 million years ago.