TAXONOMIC VARIATION AMONG SPIDERS IN THE ABILITY TO REPEL WATER: SURFACE ADHESION AND HAIR DENSITY

A variety of arthropods forage and avoid predators via locomotion on the surfaces of ponds and streams. For these animals, cuticular hydrophobicity functions to keep them dry and well supported by the water's surface tension, and also allows them to move easily between wet and dry habitats. Among spiders, members of the family Pisauridae exemplify this semi-aquatic lifestyle and, not surprisingly, these spiders remain entirely dry even when submerged. In the current study, we sought to quantify the degree to which spiders in a variety of families resist wetting by liquid water. Two properties of a spider's cuticular hairs are predominant in determining this resistance: adhesion energy (a consequence of molecular interactions between the hair surface and water) and hair density. When hair density is low, the adhesion energy of the cuticle itself also plays a role. Among the ten families we studied, pisaurids and pholcids defined the ends of the spectrum of resistance, with the pisaurids nearly 50 times more resistant to wetting than the pholcids. We discuss both the impact of this variation on spiders' potential for aquatic locomotion and the variety of selective forces that may have contributed to this impressive variation in capabilities.