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Abstract Spiders amplify their physical capabilities by synthesizing multiple high performing silks. Renowned for its toughness, major ampullate (MA) silk composes the spiderweb frame, providing support and absorbing high-energy impacts. In ecribellate orb-weavers, proline-rich motifs in MaSp2 proteins of MA silk are linked to a range of mechanical properties, including extensibility, elasticity, stiffness, and supercontraction. We show a modification of these motifs outside of this clade in a spider that constructs a spring-loaded web. The triangle weaver spider Hyptiotes cavatus (family Uloboridae) stores energy in the support lines of its triangular web, then rapidly releases the tension to catapult forward, collapsing the web around prey. Hyptiotes has an expanded set of MaSp2 genes which encode proteins with far higher proline contents than typical MaSp2. The predominant GPGPQ motifs present in Hyptiotes spidroins also occur abundantly in MaSp sequences of distantly related spiders that produce the most extensible dragline, implying silk protein convergence. Proline-rich MaSp2 proteins constitute half of all MA gland expression in Hyptiotes, and we show that the resulting fibers are the most proline-rich spider silk measured to date. This unique silk composition suggests a functional importance that may facilitate the spring-loaded prey capture mechanism of this species' web and may inspire the design of novel biomaterials using protein engineering.