Supersaturation with water explains the unusual adhesion of aggregate glue in the webs of the moth-specialist spider,Cyrtarachne akirai

Abstract

Orb webs produced by araneoid spiders depend upon aggregate glue-coated capture threads to retain their prey. Moths are challenging prey for most spiders because their scales detach and contaminate the glue droplets, significantly decreasing adhesion. Cyrtarachne are moth-specialist orb-weaving spiders whose capture threads adhere well to moths. We compare the adhesive properties and chemistry of Cyrtarachne aggregate glue to other orb-weaving spiders to test hypotheses about their structure, chemistry and performance that could explain the strength of Cyrtarachne glue. We show that the unusually large glue droplets on Cyrtarachne capture threads make them approximately 8 times more adhesive on glass substrate than capture threads from typical orb-weaving species, but Cyrtarachne adhesion is similar to that of other species after normalization by glue volume. Glue viscosity reversibly changes over 1000-fold in response to atmospheric humidity, and the adhesive strength of many species of orb spiders is maximized at a viscosity of approximately 105–106 cst where the contributions of spreading and bulk cohesion are optimized. By contrast, viscosity of Cyrtarachne aggregate glue droplets is approximately 1000 times lower at maximum adhesive humidity, likely facilitating rapid spreading across moth scales. Water uptake by glue droplets is controlled, in part, by hygroscopic low molecular weight compounds. NMR showed evidence that Cyrtarachne glue contains a variety of unknown low molecular weight compounds. These compounds may help explain how Cyrtarachne produces such exceptionally large and low viscosity glue droplets, and also why these glue droplets rapidly lose water volume after brief ageing or exposure to even slightly dry (e.g. < 80% RH) conditions, permanently reducing their adhesion. We hypothesize that the combination of large glue droplet size and low viscosity helps Cyrtarachne glue to penetrate the gaps between moth scales.

Spidroin profiling of cribellate spiders provides insight into the evolution of spider prey capture strategies

“…The aggregate gland glue is composed of adhesive glycoproteins surrounded by an aqueous solution of low molecular mass compounds (LMMCs) 14 . Therefore, the difference in droplet size changes the proportion of water and achieves different viscosity 15 . Spiders that use a dry capture thread with mechanical stickiness instead of the viscid capture silk are called cribellate spiders.…”

Toward Spider Glue: Long Read Scaffolding for Extreme Length and Repetitious Silk Family Genes AgSp1 and AgSp2 with Insights into Functional Adaptation

“…Bolas spiders mimic female moth pheromones to attract their male counterparts, which they capture by swinging a single large glue droplet that hangs at the end of a silk strand (Figure 1C) (Eberhard 1977). The remarkable glue of these spiders is able to adhere in spite of a moth’s scales, which easily detach allowing them to escape other forms of predation (Sahni et al 2011b; Diaz et al 2018).…”

Analysis the water in aggregate glue droplets of spider orb web by TOF‐SIMS

“…However, K(H 2 O) + was not detected in the suspended pristine aggregate glue droplet (Figure 2). Diaz et al reported that water will lose from just made “fresh” aggregate glue droplets even exposure to >90% RH conditions 28 . The “fresh” aggregate glue droplet volume decreased by as much as 90% within 30–60 min, and it is irreversible 28 .…”

“…Diaz et al reported that water will lose from just made “fresh” aggregate glue droplets even exposure to >90% RH conditions 28 . The “fresh” aggregate glue droplet volume decreased by as much as 90% within 30–60 min, and it is irreversible 28 . However, water was not completely lost 28 .…”