Contaminants decrease adhesive strength by interfering with substrate contact. Spider webs adhering to moths present an ideal model to investigate how natural adhesives overcome contamination because moths' sacrificial layer of scales rubs off on sticky silk, facilitating escape. However, Cyrtarachninae spiders have evolved gluey capture threads that adhere well to moths. Cyrtarachne capture threads contain large glue droplets oversaturated with water, readily flowing but also prone to drying out. Here, we compare the spreading and adhesion of Cyrtarachne akirai glue on intact mothwings, denuded cuticle and glass to the glue of a common orb-weaving spider, Larinioides cornutus, to understand how C. akirai glue overcomes dirty surfaces. Videos show that C. akirai 's glue spreading accelerates along the underlying moth cuticle after the glue seeps beneath the moth scales—not seen on denuded cuticle or hydrophilic glass. Larinioides cornutus glue droplets failed to penetrate the moth scales, their force of adhesion thus limited by the strength of attachment of scales to the cuticle. The large size and low viscosity of C. akirai glue droplets function together to use the three-dimensional topography of the moth's scales against itself via capillary forces. Infrared spectroscopy shows C. akirai glue droplets readily lose free-flowing water. We hypothesize that this loss of water leads to increased viscosity during spreading, increasing cohesive forces during pull-off. This glue's two-phase behaviour shows how natural selection can leverage a defensive specialization of prey against themselves and highlights a new design principle for synthetic adhesives for adhering to troublesome surfaces.

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.

Friction is one of the leading causes of energy loss in moving parts, and understanding how roughness affects friction is of utmost importance. From creating surfaces with high friction to...

No abstract