The ultrastructure of the book lungs of the Italian trap-door spider Cteniza sp. (Araneae, Mygalomorphae, Ctenizidae) Elvira Brunelli * , Pierluigi Rizzo, Antonello Guardia, Francesca Coscarelli, Settimio Sesti, Sandro Tripepi Department of Biology, Ecology and Earth Science, University of Calabria, Via P. Bucci, I-87036 Rende, Cosenza, Italy article info Article history: Received 18 December 2014 Accepted 4 March 2015 Available online xxx Keywords: Spider respiration Book lungs Cteniza Ultrastructure Araneae Mygalomorphae abstract The fine structure of book lungs is not homogeneous across Arachnids and is considered phylogenetically informative, however few reports on the ultrastructural features of this organ have been published. In this study, we examined the general morphology and ultrastructure of adult spiders of the genus Cteniza. The respiratory system of Cteniza sp. consists of two pairs of well-developed book lungs, which is considered indicative of primitive spiders. The general organization of the book lungs is similar to that described for other arachnids and consists of leaves of alternating air and hemolymph channels. The air channels are lined with cuticle and open to an atrium that leads to a slit-like spiracle. The air channels are held open by cuticular trabeculae. The space holders in the hemolymph channels are pillar trabeculae formed by two cells from the opposed walls. The pillar cells have a complex ultrastructure that includes an interdigitating connection, gap junctions, microtubules and hemidesmosomes. These features apparently help strengthen the pillar cells and their interconnections with each other and the underlying cuticle. The cytoskeleton resembles that of arthropod tendon cells where substantial structural support is needed. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Arthropod terrestrialization depended on respiratory organ adaptations (Opell, 1998; Damen et al., 2002), and book lungs are widely accepted as the most primitive mode of terrestrial respira- tion (Dunlop, 1998). Book lungs are presumed to have evolved as a consequence of terrestrialization in the first arthropods to appear on land (the extinct trigonotarbids), and are believed to resemble the book lungs of modern arachnids (Hsia et al., 2013; Scholtz and Kamenz, 2006). The results of comparative anatomical studies have long sug- gested a common origin for air breathing organs in Chelicerata (Shultz, 1987; Foelix, 2011; Anderson, 1998; Dunlop, 1998; Scholtz and Kamenz, 2006). More recently, molecular approaches have provided new evidence suggesting the same dorsal epipod/gill ancestral structure led to different organs including book lungs, tracheal tubes, and spinnerets (Damen et al., 2002). Primitive spiders such as tarantulas and trap-door spiders possess two pairs of opisthosomal book lungs situated on the second and third abdominal segments. In most modern spiders, the third abdominal segment has a tracheal system rather than a pair of book lungs. The tracheal system is usually bilateral and has different degrees of development in different species (Foelix, 2011; Opell, 1998). Tracheae in spiders can function as tracheal lungs or may be used for terminal diffusion. As tracheal lungs, they ex- change gases using hemocyanin within the hemolymph, and gaseous exchange occurs across the walls of the entire tracheal system. In terminal diffusion, the terminal branches of tracheae (tracheoles) penetrate into the epithelia to allow oxygen to diffuse directly into the tissues (Hsia et al., 2013; Schmitz, 2005). Among extant Arachnids, book lungs are present in scorpions and Tetrapulmonata (Uropygi, Amblypygi and Araneae), and are homologous and apomorphic, as described by Scholtz and Kamenz (2006) and references therein. In those species where book lungs have been studied, they exhibit a similar general organization, an assemblage of alternating air and hemolymph channels (lamellae) that resemble the pages of a book, thus giving these organs their name. Book lungs communicate * Corresponding author. Tel.: þ39 0984492996; fax: þ39 0984492986. E-mail address: elvira.brunelli@unical.it (E. Brunelli). Contents lists available at ScienceDirect Arthropod Structure & Development journal homepage: www.elsevier.com/locate/asd http://dx.doi.org/10.1016/j.asd.2015.03.001 1467-8039/© 2015 Elsevier Ltd. All rights reserved. Arthropod Structure & Development xxx (2015) 1e9 Please cite this article in press as: Brunelli, E., et al., The ultrastructure of the book lungs of the Italian trap-door spider Cteniza sp. (Araneae, Mygalomorphae, Ctenizidae), Arthropod Structure & Development (2015), http://dx.doi.org/10.1016/j.asd.2015.03.001