Spider viscid silk sticks better to superhydrophobic surfaces

Abstract

In a likely coevolutionary arms race, insects evolved a variety of counter strategies to avoid capture by spider webs, while spiders evolved innovations web structure and especially their adhesive silks. For instance, insects cuticles employ a variety of potential anti-adhesion mechanisms such as the detachable scales of moths and surface waxes and superhydrophobic structures that might resist spreading of glues. In contrast, the viscid capture threads of most spider orb webs are covered with aggregate glue droplets that absorb atmospheric water, tuning glue viscosity to balance the competing demands of surface spreading versus maintaining strong bulk cohesion. Here, we test the hypothesis that superhydrophobicity in insects acts as an anti-adhesion defense against spider silk. We used lotus leaves as a model substrate because its superhydrophobicity outperforms most known insect surfaces. The adhesion of spider capture silk from the web of Larinioides cornutus was studied against three substrates: raw lotus leaves, oxygen plasma treated lotus leaves (hydrophilic lotus), and smooth glass, differing in roughness and chemistry. We found that spider capture silk sticks better to the superhydrophobic lotus than to other surfaces. Both chemistry and physical properties of the leaves contribute to higher adhesion, as raw lotus showed a mean increase in adhesion of 74 % compared to glass, while the similar surface roughness of the hydrophilic lotus increased adhesion by 64 % compared to glass. Thus, evolving a hydrophobic cuticle is unlikely to be a defensive trait used to mitigate the effectiveness of spider webs.