Fig. 1. Force–extension behavior of representative spring and fall threads. (A) The thread produced in spring extended beyond the yield point and stiffened again before failing at the plaque (F_b=0.21 N). (B) The thread produced in fall broke at the plaque during the first tensile test (solid line, F_b=0.05 N). When retested without the plaque (broken line), the thread extended beyond the yield point and stiffened before failing in the proximal region (F_b=0.14 N). Threads produced in both summer and winter were similar to fall threads, breaking before reaching the second (yield) phase of the force–extension curve (data not shown).