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Scaling and function of spider safety line Ortlepp, Christine Silke

Abstract

Major ampullate silk, also known as dragline, is one of the most common types of silk used by spiders. Most spiders spool dragline out as they move, tacking it down periodically, so that the silk will act as a safety line in the event of a fall and brings the spider to a stop. While many spiders are small enough not to be damaged by a fall, gravid female orb weavers have such large, heavy abdomen that a hard impact can be fatal. The worst case scenario is a fall with a preformed length of silk and no additional silk spooled by the spider. Tests of the material properties show that the dragline produced by the adult Araneus diadematus, an orb weaver, and Salticus scenicus, a jumping spider, would break during such a fall. Where this scenario fails, is that spiders spool out more silk as they fall. By activating a friction brake, gravitational energy can be converted to heat which prevents the silk from being overloaded. The possible sources of friction available to a falling spider, namely the rear leg grabbing the silk, and an internal friction brake, were characterized by forcibly silking A. diadematus spiders. Results show that each rear leg can apply up to 3 body weights of force, and depending on the mass of the spider, the internal brake can exert up to 6 body weight of force. To determine to what extent spiders apply brakes during normal behaviour, falling A. diadematus and S. scenicus were filmed with a high speed camera. This showed that these spiders apply no more than 2 and 1.6 body weights of force respectively. The resulting dynamic safety factor was between 2.5 and 6 for A. diadematus, and 1 for S. scenicus. Therefore, by spooling out additional silk while applying friction brakes, the silk is prevented from failing while bringing the spider safely to a stop.

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