Abstract
Spiders are unique in having a dual respiratory system with book lungs and tracheae, and most araneomorph spiders breathe simultaneously via book lungs and tracheae, or tracheae alone. The respiratory organs of spiders are diverse but relatively conserved within families. The small araneoid spiders of the symphytognathoid clade exhibit a remarkably high diversity of respiratory organs and arrangements, unparalleled by any other group of ecribellate orb weavers. In the present study, we explore and review the diversity of symphytognathoid respiratory organs. Using a phylogenetic comparative approach, we reconstruct the evolution of the respiratory system of symphytognathoids based on the most comprehensive phylogenetic frameworks to date. There are no less than 22 different respiratory system configurations in symphytognathoids. The phylogenetic reconstructions suggest that the anterior tracheal system evolved from fully developed book lungs and, conversely, reduced book lungs have originated independently at least twice from its homologous tracheal conformation. Our hypothesis suggests that structurally similar book lungs might have originated through different processes of tracheal transformation in different families. In symphytognathoids, the posterior tracheal system has either evolved into a highly branched and complex system or it is completely lost. No evident morphological or behavioral features satisfactorily explains the exceptional variation of the symphytognathoid respiratory organs.
Highlights
The study of the respiratory organs of Arachnida has a long trajectory, with the first anatomical account published more than two centuries ago (Meckel, 1809:109)
The ancestral mysmenid respiratory system based on the total evidence phylogenetic hypothesis of Lopardo et al (2011) is as follows: anterior tracheal system extending into the prosoma and connected by a transverse duct; posterior tracheal system comprising a single narrow spiracle adjacent to the spinnerets and internally consisting of a small atrium, median entapophyses and lateral tracheae restricted to the opisthosoma, and arranged separately, most likely resulting in four trunks usually arising independently from the posterior spiracle, via a small atrium (Figs. 5 and 6)
The reconstruction of the ancestral respiratory system of the ANTS (Anterior Tracheal System) clade on both the phylogenetic hypotheses of Lopardo et al (2011) and Kulkarni et al (2021) results in a similar arrangement to that reported as the mysmenid ancestral reconstruction
Summary
The study of the respiratory organs of Arachnida has a long trajectory, with the first anatomical account published more than two centuries ago (Meckel, 1809:109). Respiratory organs within Arachnida are diverse and include book lungs, tracheae, or can be completely absent (see, e.g., Levi, 1967; Dunlop, 2019; Küntzel et al, 2019). Book lungs seem to be structurally homologous within pulmonated Arachnida (Scholtz & Kamenz, 2006), and their presence is a synapomorphy of Arachnopulmonata (Sharma et al, 2014; Scorpiones, Araneae, Pedipalpi, and most likely Schizomida). The morphology of book lungs is quite conservative across all pulmonate arachnids and these organs are considered as homologous, comprehensive comparative studies have found an interesting diversity of fine structures of the book lungs of Scorpiones, Amblypygi, Uropygi, and Araneae (Kamenz et al, 2005; Scholtz & Kamenz, 2006; Kamenz & Prendini, 2008; Küntzel et al, 2019). While the respiratory system within most arachnid orders remains constant, spiders are unique in that breathing
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