When wet, the web of the cribellate spider forms a unique structure that is a combination of dense spindle knots and connecting joints. The web is able to continuously collect water due to the spindle knots serving primarily as drop collection sites and the joints functioning as condensation sights.
The spindle knots are characterized by a random arrangement of tiny fibers that create a rough surface. In contrast, the joints are characterized by a flatter, organized arrangement of fibers that create a relatively smooth surface. This difference in roughness creates a surface energy gradient that forces water toward the less water-resistant area, in this case the spindle knots. The curvature of the spindle knots’ conical shape also creates a pressure difference that further pulls water toward the center of the spindle knots.
This strategy was contributed by Rachel MajorEdit Summary
“…the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the ‘wet-rebuilt’ fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots.” (Zhao et al. 2010:640)
“A rather large water drop finally formed on the spindle-knot through sequential coalescence of smaller drops originating from the joints.” (Zhao et al. 2010:640)
“Magnified images of a spindle-knot…reveal highly random nanofibrils that give a rough surface topography, while comparable images of a joint…show that it is composed of nanofibrils that run relatively parallel to the silk fibre axis and form an anisotropic aligned and relatively smooth topography… The surface energy gradient arising from differences in roughness will thus drive water drops to move from the less hydrophilic region (joint with relative lower surface energy) to the more hydrophilic region (spindle-knot with high surface energy).” (Zhao et al. 2010:641-642)
“The second possible driving force for directional water drop movement arises from the spindle-shaped geometry of the knots, which will generate a difference in Laplace pressure… The overall result is that the surface energy gradient arising from the anisotropic surface structures and the difference in Laplace pressure arising from the conical spindle-knot geometry act cooperatively to drive condensing and growing water drops from the joint to the spindle-knot.” (Zhao et al. 2010:642)