Asymmetric spatula heads combined with lateral forces provide a mechanism for controlling the adhesive attachment of a range of spider species

Dry adhesion is a common strategy utilized throughout nature, allowing attachment of animals to a large range of surfaces. Crucially, it enables the animal to switch rapidly and safely between attachment and detachment in order to facilitate motion. Here we investigate the magnitude and directionality of the dry adhesion systems of a number of spider species. Atomic force microscopy (AFM) was used to measure the normal adhesion strength of a single setule. Secondly, scanning electron microscopy (SEM) was used to directly observe the interaction of a spider seta with an AFM cantilever tip and then, based on the known stiffness of the cantilever and the deflection distance, to estimate the adhesion force between them. The AFM-measured values for normal adhesion of single spider setules to a surface (9–16 nN) were similar to values predicted by the Johnson–Kendall–Roberts (JKR) model. In contrast, the force of adhesion, estimated from the SEM images, between the seta and the cantilever reached values as high as 10 μN, much higher than predicted by the JKR theory alone. With spiders, as with geckos, the principle of contact splitting leads to an increase in adhesion force. In addition, it was observed that significantly higher adhesion occurred when the setules were dragged laterally across a spherical surface in the proximal direction (i.e. towards the spider's tarsus). When pushed in the opposite direction, adhesion was greatly reduced. This confirms that the spider dry adhesion system is another example of directional adhesion.