  In this letter, we adopt a new approach combining theoretical modeling with
silk stretching measurements to explore the mystery of the structures between
silkworm and spider silks, leading to the differences in mechanical response
against stretching. Hereby the typical stress-strain profiles are reproduced by
implementing the newly discovered and verified "$\beta$-sheet splitting"
mechanism, which primarily varies the secondary structure of protein
macromolecules; our modeling and simulation results show good accordance with
the experimental measurements. Hence, it can be concluded that the
post-yielding mechanical behaviors of both kinds of silks are resulted from the
splitting of crystallines while the high extensibility of spider dragline is
attributed to the tiny $\beta$-sheets solely existed in spider silk fibrils.
This research reveals for the first time the structural factors leading to the
significant difference between spider and silkworm silks in mechanical response
to the stretching force. Additionally, the combination of theoretical modeling
with experiments opens up a completely new approach in resolving conformation
of various biomacromolecules.
