I examined the structure of the cryofractured book lung of Phidippus audax with a scanning electron microscope. Each book lung is essentially a stack of flattened air-sacs, or lamellae, which project anteriorly into the lateral hemolymph space of the anterior opisthosoma. Each lamella is roughly triangular in shape. Hemolymph flows across each lamella from the medial to the lateral side (Fig. 1). Air enters the lamellae from the third, posterior side, after passing through a network of irregular cuticular struts (air filter) which lines the atrium of the book lung. The thin walls of each lamella are joined by.
