Book lungs are an iconic character for arachnids, yet previous accounts of their morphology in spiders (Arachnida: Araneae) are based on sporadic reports from a few species using a diverse suite of terminologies. Here, we focus on the fine structure of spider book lungs using standardized terminology and a wider taxon sampling of seven species studied as histological sections and forty species studied with scanning electron microscopy. All spider lungs share a similar basic morphology, which also matches the ground pattern for arachnids in general. This includes a spiracle opening into an atrium with a folded wall from which stacked lamellae containing pillar cells project into a haemolymph sinus. The air spaces are separated by proximal trabeculae, which span the space completely, and distal trabeculae which originate from the lamellar dorsal surface. Within this framework, several differences amongst spider lungs could be identified. Distal trabeculae can be pilate or reticulate. Lamellar margins pointing into the atrium can be echinate, arbuscular-reticulate or arbuscular-reticulate-echinate. The atrium wall can be psilate, verrucate or arbuscular-reticulate. The character states identified here offer new perspectives for apomorphies of major spider clades. Reticulate distal trabeculae are only seen in the Mygalomorphae investigated here. The arbuscular-reticulate condition on the lamellar margin is only seen in representatives of Opisthothelae- with a further modification to arbuscular-reticulate-echinate in the Ctenidae. With one notable exception, an arbuscular-reticulate atrium wall is seen in the Araneomorphae sampled. These data are further compared to other pulmonate arachnids. Book lung fine structure in the earliest branching spider clade - the Mesothelae - matches the condition observed for the closely related Amblypygi (whip spiders).

Arachnida, Araneae, book lung, morphology, Scorpiones, Amblypygi, Uropygi.