||The aquatic caddisfly larvae (insect order Trichoptera) use their adhesive silk in diverse species-specific ways to construct shelters and procure food. Structural H-fibroin protein comprising the bulk of aquatic caddisworm silk is extensively phosphorylated (10-15 mol%), an attribute atypical of the terrestrial silks from the caddisfly sister order Lepidoptera. Mechanochemical characterization of caddisworm silk from a local genus (Hesperophylax) shows a definitive and central role for the peptidyl phosphates in the silk structure, mechanical properties, and during silk spinning. Phosphates, in the form of phosphoserines (pS), are in blocky serial alternating repeats throughout the H-fibroin. As the silk fibers are drawn from the contents of the silk gland, environmental sources of multivalent metal ions like Ca2+ diffuse into the nascent silk fiber and together with the pSs form periodic structured complexes that stabilize insolubilizing Î²-secondary structures. Complexation with multivalent metal ions decreases the symmetry of the pS â€"PO32- moiety, as observed by changes in the number and frequency of IR-absorption bands. Under mechanical load, Ca2+-pS domains rupture to reveal hidden length and dissipate mechanical energy. The fibers are therefore tough, requiring nearly twice the mechanical energy of tendon collagen to fracture: ~17.3 Â± 6.2 MJ/m3. Remarkably, silk fibers cycled to 20% elongation completely recover mechanically within 120 min because an elastic entropic network guides the post-yield refolding of the Ca2+-pS domains. Various techniques were used to mechanochemically probe the Ca2+-stabilized Î²-domains. Silk fibers in solutions of descending pHs showed a drastic decrease in the force required to rupture the Ca2+-pS networks coinciding with the pKa of the silk pSs (pH 4.5). Exchanging the intrinsic Ca2+ ions in the silk with other species of multivalent metal ions modulates the initial stiffness and pseudo-yield stress of the silk fibers increasing in the order Na+ << Mg2+ < Ca2+ < La3+ < Zn2+ â‰ˆ Fe2+. The integrated intensities of the â€"PO32- symmetric stretching IR-absorption bands, indicative of the metal ionâ€™s effect on transition dipole moment of the P-O bonds and thereby the strength of the phosphate metal complex, increase in the same order, establishing a link between P-O transition dipole moments and the silk fiber strength.