Transcript Slide 1
Lecture # 6: Spider Silk Silk ‘worm’ Bombyx mori Spider webs are complex Red (final part of) exploration phase Yellow construction of radii White construction of auxiliary spiral Blue construction of sticky spiral Pink (in some species) construction of stabilimentum What is silk? Crystaline Portion b sheet with glycine and alanine residues b sheet crystals in amorphous network amorphous portion Spiders make many kinds of silk. primary amino acid structure spinning silk Mechanical properties of silk varies. material strength safety factor = maximum stress experienced worst-case jump bungee jump Safety factor in body weights: S BW Fmax / mg Energy absorbed stretching thread to breaking point: Es ,max 12 Fmaxxmax Gravitational energy released during fall to failure point: EG ,max Mgx0 Mgxmax If energy from fall equals energy absorbed by thread: Mgx0 Mgxmax 12 Fmax xmax Combine: Mgx0 Mgxmax 12 Fmax xmax S BW Fmax / mg max xmax / x0 For ‘bungee jump’: For ‘worst case’: S BW (2 / max ) 2 S BW (4 / max ) 2 S BW (2 / max ) 2 FAILURE ZONE yield point scaling of breaking strain and strength: Scaling of breaking strain and strength do not vary with body mass, nor do they vary between species. scaling of cross sectional area M0.739 M0.77 A. diadematus silk thicker than S. scenicus silk. Area scales with mass0.739 and mass0.77. scaling of breaking force: A. diadematus Fmax 11.2M 0.786 S. scenicus Fmax 0.363M 0.66 Fmax 11.2M 0.786 Fmax 0.363M 0.66 S BW Fmax / mg S BW 1.14M 0.214 SBW 0.037M 0.340 Large spiders use drag silk that is not sufficient to save them from a Bungee jump, even though falls for large spiders could be fatal. Small spiders use drag silk that is sufficient to save them from a Bungee jump, even though falls for small spiders would not be fatal. ……Why? 1) Developmental constraints. Spiders can’t scale silk area to M1. 2) Wind drag. drag D (4 / v ) /[ln( 1/ r ) 0.193] tension =~2xdrag Largest forces encountered by juvenile spiders are produced by the wind.