Caddisflies are aquatic relatives of silk-spinning terrestrial moths and butterflies. Casemaker larvae spin adhesive silk fibers for underwater construction of protective composite cases. The central region of Hesperophylax sp. H-fibroin contains a repeating pattern of three conserved subrepeats, all of which contain one or more (SX)(n) motifs with extensively phosphorylated serines. Native silk fibers were highly extensible and displayed a distinct yield point, force plateau, and load cycle hysteresis. FTIR spectroscopy of native silk showed a conformational mix of random coil, beta-sheet, and turns. Exchanging multivalent ions with Na+ EDTA disrupted fiber mechanics, shifted the secondary structure ratios from antiparallel beta-sheet toward random coil and turns, and caused the fibers to shorten, swell in diameter, and disrupted fiber birefringence. The EDTA effects were reversed by restoring Ca2+. Molecular dynamic simulations provided theoretical support for a hypothetical structure in which the (pSX)(n) motifs may assemble into two- and three-stranded, Ca2+-stabilized beta-sheets.