Stiffness gradients in spider webs

The familiar spoke-and-spiral pattern of spiders’ orb webs incorporates a remarkable degree of engineering. For starters, the spiders produce different silks for each part: Radial threads are strong and stiff, while glue-coated spiral threads are soft and stretchable. The two sets of threads work synergistically to absorb the impact of a flying insect that gets trapped. Yang Guo and colleagues at Tsinghua University and Brown University now report that whereas the intrinsic mechanical properties of the radial threads in a web are roughly uniform, those of the spiral threads vary with distance from the center. Through systematic measurements of multiple webs spun by Araneus ventricosus spiders, the researchers found that although the diameters of the spiral silks are fairly constant, spiral silks at the periphery have up to 10 times the Young’s modulus—a measure of intrinsic stiffness—and 6 times the tensile strength of spiral silks near the center. To understand the implications of the stiffness and strength gradients, the team turned to a model web with 10 radial and 10 spiral threads; the impact of a flying insect was represented as a force concentrated at the midpoint of a given spiral segment. The spiral threads’ stiffness was either uniform or varied linearly with distance from the center. The researchers found that the stiffness gradient enhanced the web’s structural integrity and ability to efficiently absorb the impact energy regardless of how far out the impact occurred. (Y. Guo et al., Appl. Phys. Lett. 113, 103701, 2018, doi:10.1063/1.5039710 .)