Neosconas

Reconstructing web evolution and spider diversification in the molecular era
¨ Todd A. Blackledgea,1, Nikolaj Scharffb, Jonathan A. Coddingtonc, Tamas Szutsb, John W. Wenzeld, Cheryl Y. Hayashie, and Ingi Agnarssona,f
of Biology and Integrated Bioscience Program, University of Akron, Akron, OH 44325-3908; bZoological Museum, University of Copenhagen, Department of Entomology, Universitetsparken15, DK-2100 Copenhagen, Denmark; cDepartment of Entomology, NHB 105, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013-7012; dDepartment of Entomology, 1315 Kinnear Road, Ohio State University, Columbus, OH 43212; eDepartment of Biology, University of California, Riverside, CA 92521; and fDepartment of Biology, University of Puerto Rico, P.O. Box 23360, San Juan,Puerto Rico 00931-3360 Communicated by Thomas W. Schoener, University of California, Davis, CA, February 9, 2009 (received for review October 6, 2008)
aDepartment

The evolutionary diversification of spiders is attributed to spectacular innovations in silk. Spiders are unique in synthesizing many different kinds of silk, and using silk for a variety of ecological functions throughout their lives,particularly to make preycatching webs. Here, we construct a broad higher-level phylogeny of spiders combining molecular data with traditional morphological and behavioral characters. We use this phylogeny to test the hypothesis that the spider orb web evolved only once. We then examine spider diversification in relation to different web architectures and silk use. We find strong support for asingle origin of orb webs, implying a major shift in the spinning of capture silk and repeated loss or transformation of orb webs. We show that abandonment of costly cribellate capture silk correlates with the 2 major diversification events in spiders (1). Replacement of cribellate silk by aqueous silk glue may explain the greater diversity of modern orb-weaving spiders (Araneoidea) compared withcribellate orb-weaving spiders (Deinopoidea) (2). Within the ‘‘RTA clade,’’ which is the sister group to orb-weaving spiders and contains half of all spider diversity, >90% of species richness is associated with repeated loss of cribellate silk and abandonment of prey capture webs. Accompanying cribellum loss in both groups is a release from substrate-constrained webs, whether by aerially suspendedwebs, or by abandoning webs altogether. These behavioral shifts in silk and web production by spiders thus likely played a key role in the dramatic evolutionary success and ecological dominance of spiders as predators of insects.
Araneidae behavioral evolution cribellate silk orb web speciation

piders are exceptionally diverse and abundant in terrestrial ecosystems. In contrast to megadiverseorders of insects, evolutionary diversification of spiders is not coupled with major trophic shifts. All spiders are predators of arthropods, and spiders are dominant consumers at intermediate trophic levels (1, 2). Spider diversification is instead linked to key innovations in silk use (3–7). For instance, the araneoid orb web (Fig. 1) with stretchy capture spirals, coated by adhesive viscid silksecretions, provides access to abundant flying insects (3, 8). However, many spiders produce cribellate silk, a radically different dry adhesive that adheres to prey, using van der Waals interactions and hygroscopic forces (9). Some cribellate spiders also construct aerial orb webs, whereas most spin sheet-like webs on the substrate (Fig. S1) or have abandoned capture webs altogether. Furthermore,the most diverse families within ‘‘orb-weavers’’ (Orbiculariae) no longer build orb webs, but instead spin aerial sheet webs (Linyphiidae) or cobwebs (Theridiidae) (Fig. S2). Thus, discovering the pattern of evolution of web spinning behaviors is essential for understanding spider diversification. Orb webs possessing dry cribellate capture spirals are architecturally similar to those spun from...