Owing to their exquisite geometric structures and excellent mechanical properties, spider orb webs possess an outstanding ability to capture flying prey. In this work, we report a mechanism that enhances the energy absorption ability of spider webs. Through systematic measurements of the mechanical properties of both spiral and radial silks, we find that the spiral silks feature a distinct gradient variation in the diameter and tensile stiffness along the radial direction of the web, while the radial silks have a much higher but approximately uniform stiffness. A mechanical model is proposed to reveal the functional gradient effects on the energy absorption of the web. The results show that due to the gradient variation in the mechanical properties of spiral silks, the web exhibits a nearly uniform energy absorption ability regardless of the position where a flying prey impacts the web. This optimal structural feature of the web greatly enhances its efficiency and robustness in prey capture. This work not only helps understand the optimal mechanisms of spider webs but also provides clues for designing anti-impact structures.

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