Behavioral tuning of spider silk thread stiffness circumvents biomaterial trade-offs.

Biological polymers often face a trade-off between stiffness, strength, and extensibility: Materials that are strong and stiff tend to be brittle, while those that are elastic and extensible usually lack strength. Here, we show that netcasting spiders (Deinopidae) overcome this trade-off by forming mixed-silk metastructures, which enable both high elastic deformation and load resistance. These spiders have evolved a unique predatory strategy, casting a sticky silk web over prey, which subjects the web radii to extreme strains far exceeding those sustained by typical spider silk fibers. The radii consist of a compound filament with an elastomeric core surrounded by looped bundles of thin fibers. This architecture results in an unusual mechanical profile: The threads are initially compliant and highly extensible, but they stiffen as the fiber loops straighten, enhancing load-bearing capacity. Notably, spiders control this compound architecture through a reel-spinning technique, controlling loop formation and fiber mixture to establish an elasticity gradient across the web-stiff and strong in the main frame lines, yet soft and hyperelastic in the lower radii that undergo the greatest deformation during prey capture. These findings represent a unique case of behavioral modulation of silk processing to circumvent biomaterial trade-offs, enabling extraordinary dynamics and specialization of web architecture. The herein described principle of looped fiber-reinforced elastomers may also be transferred to the design of artificial materials for applications that require both high elasticity and strength.