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Fig. 3. Comparison of the biomechanical performance of the capture threads (in
black) and the supporting major ampullate threads (in gray) for three genera
of cribellate (deinopoid) spiders and one ecribellate (araneoid) spider. Major
ampullate silk performs similarly across all four genera. In contrast, the
strength, stiffness and extensibility of capture threads can vary greatly
among taxa with diverse web architectures. (A) Uloborus diversus
spins a complete orb web. (B) Hyptiotes cavatus spins a reduced
triangle web that is held under tension by the spider. (C) Deinopis
spinosa spins a specialized net that is held between the front six legs
and pressed on top of prey, thereby stretching the web greatly during prey
capture. (D) Argiope argentata (Fabricius 1775) spins a complete orb
web but uses a capture spiral that is composed of glue-coated flagelliform
fibers (see Blackledge and Hayashi,
2006). The capture threads in A-C exhibit behavior typical of
cribellate silk where most of the stress is generated within a pair of core
axial fibers that fail at high peak stresses and moderate strains. The thread
then continues to strain and absorb force through the extension and failure of
hundreds of surrounding fibrils until final failure. Because we could not
measure the total cross-sectional area of these fibrils, stress values after
failure of the axial fibers should be interpreted only as a relative
indication of qualitative changes in force generated by the fibrils as the
entire structure is strained.
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