First published online May 5, 2005
Journal of Experimental Biology 208, 1937-1949 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01597
Quasistatic and continuous dynamic characterization of the mechanical properties of silk from the cobweb of the black widow spider Latrodectus hesperus
Todd A. Blackledge1,*,
John E. Swindeman2 and
Cheryl Y. Hayashi1
1 Department of Biology, University of California, Riverside, CA 92521,
USA
2 MTS Systems Corporation, 1001 Larson Drive, Oak Ridge, TN 37830,
USA
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Fig. 1. Western black widows (Latrodectus hesperus) construct cobwebs that
consist of three primary regions. A supporting structure (SSt) of dragline
threads is used to suspend a non-sticky sheet (SH) that the spider uses to
move around the web. Gumfooted lines (GF) extend downward from the sheet to
the substrate and are held under tension by the sheet.
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Fig. 3. Mean (±S.E.M.) diameter of single
fibers from the sheet and supporting structure regions (SH/SSt;
N=54), gumfooted lines (GF; N=36), and forcibly silked major
ampullate fibers (FS; N=52). Diameters for the thicker and thinner
pair of fibers at the base of gumfooted lines are shown separately.
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Fig. 4. Exemplar stress-strain curves for each type of silk. The gray and black
lines denote test data for two different spiders (N=6 for each
spider).
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Fig. 5. Mean (±S.E.M.) material properties
of L. hesperus silk from the sheet/supporting thread region (SH/SSt)
of webs (N=54), gumfooted capture threads (GF; N=47), and
single strands of major ampullate silk forcibly pulled from spinnerets of
anesthetized spiders (FS; N=52). Horizontal bars denote significant
pairwise differences between means using post hoc Tukey's HSD tests
for unequal sample sizes. (A) Young's modulus; (B) yield strain; (C)
extensibility; (D) ultimate strength; and (E) toughness.
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Fig. 6. Relationship between extensibility and ultimate strength for sheet and
supporting thread silk (N=54), gumfooted capture threads
(N=47), and forcibly silked major ampullate threads (N=52).
Correlation coefficients are from linear regression (P<0.05 for
sheet/supporting threads and P<0.001 for gumfooted lines).
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Fig. 7. Relationship between cross-sectional areas of silk threads and mechanical
properties. Data are the means for each spider (N=7). Regressions of
mechanical characteristics against cross-sectional area with significant
correlation coefficients and P-values are indicated. (A) Young's
modulus; (B) yield strain; (C) extensibility; (D) ultimate strength; and (E)
toughness.
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Fig. 8. Dynamic mechanical properties for black widow silk. Exemplar curves are
shown for sheet/supporting threads (black), gumfooted lines (dotted), and
forcibly silked major ampullate fibers (gray).
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Fig. 9. Mean (±S.E.M.) dynamic properties
for sheet/supporting threads (SH/SSt; N=12), gumfooted lines (GF;
N=21), and forcibly silked major ampullate threads (FS;
N=14). All properties differed among silks, except strain at max.
loss tangent (one-way ANOVAs, F2,35=5-13, P at
least <0.005). Post hoc comparsions of means using Tukey's HSD
tests for unequal sample sizes indicated that the performance of the forcibly
silked major ampullate fibers was significantly different from
sheet/supporting threads and gumfooted lines for all properties except strain
at max. loss tangent (P at least <0.05).
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Fig. 10. Gumfooted lines consist of two pairs of silk fibers that are coated with an
aggregate silk glue along their lower 5-15 mm. One pair of fibers has a larger
diameter and runs continuously from the sheet to the substrate. The second
pair of smaller diameter silk fibers is cut by the spiders just above an
attachment point to the larger fibers. Thus, the `foot' of the line consists
of four fibers that are viscid (glue covered) along their base and are then
dry until the attachment point, while the bulk of the gumfoot is two dry,
large diameter fibers.
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Fig. 11. Mean (±S.E.M.) mechanical
properties of adjacent regions of dry and viscid gumfooted lines
(N=21).
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Fig. 12. Dynamic mechanical properties of viscid (black) and dry (gray) portions of
the gumfooted lines. Two pairs of samples are shown.
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© The Company of Biologists Ltd 2005
) is the displacement of the
sinusoidal stress response (gray) to a tiny applied strain oscillation
(black). The degree to which the peak amplitude of the stress response curve
is in phase with that of the applied strain oscillation measures elasticity
while the viscosity is measured by the degree to which the stress response
90° out of phase with strain. (B) Vector relationship between dynamic
material properties are illustrated by the solid line vectors. Dynamic
stiffness (E*) is determined by the storage modulus
(E') and the loss modulus (E''). Tan