The moth specialist spider Cyrtarachne akirai uses prey scales to increase adhesion

Abstract

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.

Connecting materials, performance and evolution: a case study of the glue of moth-catching spiders (Cyrtarachninae)

“…We also conducted ecologically relevant biomechanics tests. When non-moth-specialist orb-weaver capture threads were brought into contact with moth substrates, the adhesion strength dropped relative to glass, measuring nearly zero (Diaz et al, 2018b(Diaz et al, , 2020. This decrease was independent of glue volume or size: the adhesive toughness was the same, limited by the number of prey scales contacted (Diaz et al, 2018b(Diaz et al, , 2020.…”

“…The connection between these scales and the moth's body is relatively weak and when they come into contact with a web, the scales stick to the glue on the capture threads and flake off; this system is akin to dirt contamination on a wall when attempting to put up a poster. The combination of this weakened adhesion force and moth thrashing generally leads to escape (Stowe, 1986;Diaz et al, 2020).…”

“…However, this supposition was also not supported, as C. akirai glue was found to be proportionally weaker, when normalized for glue volume, on glass. The measured adhesive strength and tensile strength would imply that the web of C. akirai should be incapable of retaining moth prey (Cartan and Miyashita, 2000;Diaz et al, 2018aDiaz et al, , 2020.…”

Adhesive Droplets of Glowworm Snares (Keroplatidae: Arachnocampa spp.) Are a Complex Mix of Organic Compounds

“…Many invertebrates use viscous fluids or soft solids as reversible adhesives to capture prey, such as in spider capture silk, velvetworm slime, and harvestmen glue (Betz and Kölsch, 2004;Suter and Stratton, 2009;Haritos et al, 2010;Sahni et al, 2010;Wolff et al, 2014;. Such adhesives have recently come into focus in ecological and biodiversity research (Agnarsson and Blackledge, 2009;Zhang and Weirauch, 2013;Blamires et al, 2014;Opell et al, 2018;Diaz et al, 2020). Biological adhesives are often adapted toward special requirements, such as generating adhesion to contaminated substrates or at variable humidity, and show a remarkable performance under conditions that are challenging for artificial adhesives (Wolff et al, 2014;Opell et al, 2018;Diaz et al, 2020).…”

“…Such adhesives have recently come into focus in ecological and biodiversity research (Agnarsson and Blackledge, 2009;Zhang and Weirauch, 2013;Blamires et al, 2014;Opell et al, 2018;Diaz et al, 2020). Biological adhesives are often adapted toward special requirements, such as generating adhesion to contaminated substrates or at variable humidity, and show a remarkable performance under conditions that are challenging for artificial adhesives (Wolff et al, 2014;Opell et al, 2018;Diaz et al, 2020). Therefore, they have also been proposed as promising biomimetic models for the design of novel artificial adhesives (von Byern and Grunwald, 2010;Sahni et al, 2011).…”