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First published online August 22, 2008
Journal of Experimental Biology 211, 2832-2840 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014191
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The scaling of safety factor in spider draglines

Christine Ortlepp and John M. Gosline*

Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver BC, Canada V6T 1Z4

* Author for correspondence (e-mail: gosline{at}zoology.ubc.ca)

Accepted 24 May 2008

This study documents the effect of body mass on the size and strength of draglines produced by the orb-weaving spider Araneus diadematus and the jumping spider Salticus scenicus. Silk samples obtained from individuals spanning the range from first-instar juveniles to gravid adults were tested to determine both the properties of the silk material and the strength and static safety factor of the draglines produced by each individual spider. Analysis of material properties indicates that the tensile strength and extensibility of the silks employed by each species are identical over the entire size range of the species. Analysis of the breaking forces for individual draglines, however, indicates that the draglines scale allometrically with the spider's body mass. For Araneus, breaking force (N) scales with body mass (kg) as Fmax=11.2M0.786, and the static safety factor (SBW=Fmax/Mg) scales as SBW=1.14M–0.214. For Salticus, Fmax=0.363M0.66 and SBW=0.037M–0.34. Thus, static safety factors decrease as these spiders grow, with values falling to 4–6 for adult Araneus and to 1–2 for adult Salticus. Analysis of these results suggests that the safety lines produced by these two species are not able to absorb the impact energy of most falls with a fixed length of pre-existing silk, except in the smallest of the Araneus spiders. It is therefore likely that both spiders must draw new silk from their spinnerets during falls to keep the dynamic loads on their safety-lines below failure levels.

Key words: dragline, safefy factor, spider silk, Araneus diadematus, Salticus scenicus


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© The Company of Biologists Ltd 2008