External power amplification drives prey capture in a spider web

Abstract

Power amplification allows animals to produce movements that exceed the physiological limits of muscle power and speed, such as the mantis shrimp’s ultrafast predatory strike and the flea’s jump. However, all known examples of nonhuman, muscle-driven power amplification involve anatomical structures that store energy from a single cycle of muscular contraction. Here, we describe a nonhuman example of external power amplification using a constructed device: the web of the triangle-weaver spider, Hyptiotes cavatus, which uses energy stored in the silk threads to actively tangle prey from afar. Hyptiotes stretches its web by tightening a separate anchor line over multiple cycles of limb motion, and then releases its hold on the anchor line when insects strike the web. Both spider and web spring forward 2 to 3 cm with a peak acceleration of up to 772.85 m/s2 so that up to four additional adhesive capture threads contact the prey while jerking caused by the spider’s sudden stop subsequently wraps silk around the prey from all directions. Using webs as external “tools” to store energy offers substantial mechanical advantages over internal tissue-based power amplification due to the ability of Hyptiotes to load the web over multiple cycles of muscular contraction and thus release more stored energy during prey capture than would be possible with muscle-driven anatomical elastic-energy systems. Elastic power amplification is an underappreciated component of silk’s function in webs and shows remarkable convergence to the fundamental mechanical advantages that led humans to engineer power-amplifying devices such as catapults and ballistae.

Thermal robustness of biomechanical processes

“…Other movements hypothesized to be spring actuated have not been studied in the context of temperature, but may yet be revealed as thermally robust. For example, recoiling elastic structures power prey capture and processing in numerous aquatic invertebrates and vertebrates (Longo et al, 2018;Patek et al, 2004;Van Wassenbergh et al, 2008) as well as terrestrial animals (Gibson et al, 2018;Han et al, 2019;Kaji et al, 2018;Patek et al, 2006;Wood, 2020;Wood et al, 2016). Moreover, they enable jumping in several insect species (Burrows, 2003;Sutton and Burrows, 2018) and sound production in some insects (Bennet-Clark and Daws, 1999;Davranoglou et al, 2019).…”

How spiders hunt heavy prey: the tangle web as a pulley and spider's lifting mechanics observed and quantified in the laboratory

“…In this context, the use of silk as an external tool to store elastic energy is not limited to Theridiid spiders. Hyptiotes cavatus, for example, uses its web as a power amplification to capture flying prey, which offers many advantages over the muscles limitations [11].…”

“…Interestingly, spider silks and webs can also act as external power amplifiers because of the elastic energy stored in the material and structure. For example, the spider Hyptiotes cavatus stretches its web by tightening an anchor line over multiple cycles of limb motion, and then releases its hold on the anchor line when an insect strikes the web, which rapidly tangles it [11]. This is a quite rare feature in animals that commonly store the elastic energy in the organisms' own anatomical structures [12][13][14][15][16][17][18].…”

Uncoiling springs promote mechanical functionality of spider cribellate silk

“…The coiled fiber may have been lost as a result of high metabolic costs involved in production coupled with prey capture not necessitating its presence. For instance, Hyptiotes spiders (Uloboridae) employ power amplification to rapidly collapse their spring-loaded triangular webs to entrap prey (Han et al, 2019), a strategy that may not need the high extensibility of individual cribellate threads. Additionally, high extensibility could have been disadvantageous for prey capture in certain instances.…”