How spiders anchor their webs to water.

A common orb web is designed to capture prey with precariously slender elements of extremely strong material. Its function is likewise dependent on the quality of its anchoring the radial dragline ends to external objects, which take the form of "attachement disks" of slenderer threads painted with glue. The performance of attachment disks can be characterized in terms common to other adhesive systems, with adhesive and cohesive failure modes. In the unusual case of the aquatic hunter, Wendilgarda clara, certain draglines are anchored to the surface of water. Here, we will probe the mechanical logic of adhering a thread to a fluid interface: revisiting the meaning of adhesive and cohesive failure, and identifying the origin in capillarity of its strength. We propose that flexibility of the attachment disk is tuned to manage a trade-off between delaying cohesive pinch-off of the meniscus and dewetting of the disk from the water, and present results from experiments with artificial analogs.