1 Department of Biology, University of Vermont, Burlington VT, ; 2 National Museum of Natural History, Smithsonian Institution, Washington DC, USA; 3 Institute of Biology, Scientific Research Centre, Slovenian Academy of Sciences and Arts, Ljubljana Slovenia; 4 Department of Biology and Integrated Bioscience Program, University of Akron, Akron OH, USA; 5 College of Life Sciences, Hubei University, Wuhan Hubei, China The phylogenetic placement of Psechridae within Entelegynae and the convergent origin of orb-like spider webs I NGI AGNARSSON 1,2 *, MATJAŽ GREGORI Č 3 ,TODD A. BLACKLEDGE 4 and MATJAŽ KUNTNER 2,3,5 Abstract Evolutionary convergence of phenotypic traits provides evidence for their functional success. The origin of the orb web was a critical event in the diversification of spiders that facilitated a spectacular radiation of approximately 12 000 species and promoted the evolution of novel web types. How the orb web evolved from ancestral web types, and how many times orb-like architectures evolved in spiders, has been debated for a long time. The little known spider genus Fecenia (Psechridae) constructs a web that resembles the archetypical orb web, but morphological data suggest that Psechri- dae (Psechrus + Fecenia) does not belong in Orbiculariae, the ‘true orb weavers’, but to the ‘retrolateral tibial apophysis (RTA) clade’ consisting mostly of wandering spiders, but also including spiders building less regular webs. Yet, the data are sparse and no molecular phylogenetic study has estimated Fecenia’s exact position in the tree of life. Adding new data to sequences pulled from GenBank, we reconstruct a phylogeny of Entelegynae and phylogenetically test the monophyly and placement of Psechridae, and in doing so, the alternative hypotheses of monophyletic origin of the orb web and the pseudo-orb versus their independent origins, a potentially spectacular case of behavioural convergence. We also discuss the implications of our results for Entelegynae systematics. Our results firmly place a monophyletic Psechridae within the RTA clade, phylogenetically distant from true orb weavers. The architectural similarities of the orb and the pseudo-orb are therefore clearly convergent, as also suggested by detailed comparisons of these two web types, as well as the spiders’ web-building behaviours and ontogenetic development. The convergence of Fecenia webs with true orbs provides a remarkable opportunity to investigate how these complex sets of traits may have interacted during the evolution of the orb. Key words: Fecenia – Psechrus – orb web – evolution – convergence Introduction Spiders are a model system for the phylogenetic study of adapta- tion; for instance, orb-weaving spiders provide a test case for investigating the importance of homology versus convergence in adaptive evolution (Coddington 1994). They also illustrate how phylogeny elucidates the origins of sexual size dimorphism (Coddington et al. 1997; Hormiga et al. 2000). Finally, silk pro- duction and web-spinning behaviours provide a powerful exam- ple of the role of key innovations for species diversification (Bond and Opell 1998). In particular, innovations associated with the evolution of the araneoid orb web helped make these spiders dominant predators of insects in most terrestrial ecosystems. The elegant architecture of the spider orb web (Fig. 1) repre- sents a highly efficient snare for flying insect prey (Eberhard 1986; Blackledge et al. 2011). The orb is a derived web architec- ture that contrasts with most other types of spider webs in being suspended in the air column by a discrete framework of silk threads. The orb’s size and shape are therefore predetermined by this framework, and the web targets a unique set of prey – flying insects – in contrast to ancestral web types (Blackledge et al. 2009). The orb web depends upon two distinct types of silk to capture flying insects – dry stiff dragline silk dissipates prey energy as the insects impact webs while highly extensible adhe- sive capture silk then retains the insects long enough to be sub- dued by the spider (Blackledge et al. 2011; Blackledge 2012). The dragline silk produced by orb spiders is notably stronger and tougher than in other taxa (Swanson et al. 2006), while the viscid capture silk is a unique homology in orb spiders and their derived kin (Eberhard 1982; Opell 1997; Blackledge et al. 2011). The evolution of orb webs therefore required significant innovations in both web-spinning behaviours and the production of silk (Blackledge et al. 2009, 2011; Harmer et al. 2011). In addition to making orb spiders dominant predators of flying insects in many terrestrial ecosystems, the innovations necessary to spin orb webs also facilitated the origin of new types of webs, in particular major radiations of cobweb and sheet web-spinning spiders (Blackledge et al. 2009). More than 95% of all extant orb spiders coat their capture threads with viscid glue that consists of adhesive glycoproteins surrounded by an aqueous cocktail of low-molecular weight mol- ecules. However, a few species employ dry cribellate adhesive silk that adheres through van der Waals forces, depending on the thin diameters and high surface areas of the numerous dry fibrils (Blackledge et al. 2011). Orb spiders are therefore categorized as ecribellate if they coat their capture spiral with viscid glue or cri- bellate if they use dry adhesive silk. Viscid capture silk requires its aqueous coating for adhesion while the cribellate fibrils must remain dry to maintain high surface area, suggesting that the two types of orb webs might have evolved independently of one another. Thus, the origin of the spider orb web, and especially whether the orb web evolved more than once, has been the sub- ject of a long debate (Kaston 1964; Kullmann 1972; Eberhard 1982; Coddington 1986a,b; Garb et al. 2006; Blackledge et al. 2009). However, recent advances in both morphological and molecular phylogenetics more or less settled the issue in favour of Coddington’s (1986a) hypothesis of a single origin of the orb web, where cribellate capture threads transitioned to viscid cap- ture threads, followed by various modifications of the orb leading to aerial sheet webs, cobwebs and other architectures (Griswold et al. 1998; Eberhard et al. 2008; Kuntner et al. 2008, 2010; Blackledge et al. 2009). Orbs, therefore, are monophyletic despite substantial diversity in the details of their architectures among various taxa. Determining how the orb derived from ancestral webs, how- ever, remains a challenging question, as the sister group of Orbiculariae (cribellate + viscid orb weavers) has not been Corresponding author: Ingi Agnarsson (iagnarsson@gmail.com) Contributing authors: Matjaž Gregorič (matjaz.gregoric@gmail.com), Todd A. Blackledge (blackledge@uakron.edu), Matjaž Kuntner (kuntner@gmail.com). J Zoolog Syst Evol Res (2013) 51(2), 100--106 Accepted on 22 September 2012 © 2012 Blackwell Verlag GmbH J Zoolog Syst Evol Res doi: 10.1111/jzs.12007