Aquatic caddistlies diverged from a silk-spinning ancestor shared with terrestrial moths and butterflies. Caddisfly larva spill adhesive silk underwater to construct protective shelters with adventitiously gathered materials. A repeating (SX)(n) motif conserved in the H-libroin of several caddisfly species is densely phosphorylated. In total, more than half of the serines in cacklisfly silk may be phosphorylated. Major molecular adaptations allowing underwater spinning of an ancestral dry silk appear to have been phosphorylation of serines and the accumulation of basic residues in the silk proteins. The amphoteric nature of the silk proteins could contribute to silk fiber assembly through electrostatic association of phosphorylated blocks with arginine-rich blocks. The presence of Ca2+ in the caddisfly larval silk proteins suggest phosphorylated serines could contribute to silk fiber periodic substructure through Ca2+ crossbridging.