Spider webs that serve as snares are one of the most fascinating and abundant type of animal architectures. In many cases they include an adhesive coating of silk lines-so-called viscid silk-for prey capture. The evolutionary switch from silk secretions forming solid fibres to soft aqueous adhesives remains an open question in the understanding of spider silk evolution. Here we functionally and chemically characterized the secretions of two types of silk glands and their behavioural use in the cellar spider, Pholcus phalangioides. Both being derived from the same ancestral gland type that produces fibres with a solidifying glue coat, the two types produce respectively a quickly solidifying glue applied in thread anchorages and prey wraps, or a permanently tacky glue deployed in snares. We found that the latter is characterized by a high concentration of organic salts and reduced spidroin content, showing up a possible pathway for the evolution of viscid properties by hygroscopic-salt-mediated hydration of solidifying adhesives. Understanding the underlying molecular basis for such radical switches in material properties not only helps to better understand the evolutionary origins and versatility of ecologically impactful spider web architectures, but also informs the bioengineering of spider silk-based products with tailored properties.

ARC Centre of Excellence for Nanoscale BioPhotonics Department of Physics and Astronomy Macquarie University Sydney New South Wales 2109 Australia

Department of Biological Sciences University of Massachusetts Lowell Lowell MA 01854 USA

Department of Biomaterials Max Planck Institute of Colloids and Interfaces Am Mühlenberg 1 Potsdam 14476 Germany

Evolution and Ecology Research Centre School of Biology Earth and Environmental Sciences University of New South Wales UNSW Sydney NSW 2052 Australia

Evolutionary Biomechanics Zoological Institute and Museum University of Greifswald Loitzer Str 26 Greifswald 17489 Germany

Functional Biodiversity Team Crop Research Institute Drnovská 507 CZ 16106 Prague 6 Ruzyně Czechia

Functional Morphology and Biomechanics Zoological Institute University of Kiel Am Botanischen Garten 1 9 Kiel 24098 Germany

ICFO Institut de Ciencies Fotoniques The Barcelona Institute of Science and Technology Castelldefels 08860 Spain

Katharina Gaus Light Microscopy Facility Mark Wainwright Analytical Centre University of New South Wales UNSW Sydney NSW 2052 Australia

School of Life Sciences University of Nottingham University Park Nottingham NG7 2RD UK

School of Natural Sciences Macquarie University Sydney New South Wales 2109 Australia

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From fibres to adhesives: evolution of spider capture threads from web anchors by radical changes in silk gland function