A) Tree Frog Adhesion
Collaborators: Werner Baumgartner (Aachen), Walter Federle (Cambridge), Stas Gorb (Stuttgart/Kiel), Daniel Matz (student, Bremen), Julia Platter (student, Bremen), Mathis Riehle (Glasgow), Jo Smith (Glasgow), Stephanie Wuttke (student, Bremen)
Our studies of the adhesive forces generated by tree frogs (Figure 1) indicate that they adhere by wet adhesion, the main component being capillarity (surface tension forces generated at the meniscus around the edge of each toe pad). These forces scale with toe pad area. The epithelium of the toe pads (studied using SEM, TEM and AFM) shows that it is made up of hexagonal, columnar epithelial cells, separated from each other at their tips (Figure 2). Cells are thus separated from each other by mucous-filled channels. Using a laser tweezer technique, we have shown that the viscosity is low, only about 1.5x greater than water. The thickness of the fluid layer that separates the pads from the substratum is extremely thin over the flat surfaces of the epithelial cells (<20 nm as shown by interference reflection microscopy). This aids good adhesion. Pads detach by peeling, while a combination of friction and adhesion is responsible for preventing frogs falling as they are tilted from the vertical towards an upside-down position. Nanofabrication of tree frog toe pads to investigate possible biomimetic applications will form an important part of future research.
B) Vision, eye movements and the sensory guidance of behaviour in crabs
Collaborators: Martin Macauley (Glasgow), John Layne (Cincinnati) Jonathan Neidhardt (student, Bremen)
Freely moving crabs carry out an elementary form of flow-field analysis in that they use eye movements to compensate for the rotatory but not the translatory components of their visual flow field. This simple procedure makes available all the information contained within their flow fields, including distance travelled, velocity, a 3-dimensional view of the world, and deviations from an intended course. Previous work with Geoff Horseman has examined the properties of neurones in the crab visual tract that detect the translational component of optic flow, while work with John Layne on homing in fiddler crabs demonstrates a role for optomotor responses in course control. Our current work examines the integration of different sensory inputs in the production of both compensatory eye movements and body turns in the land crab, Cardisoma, (Figure 3) using the techniques of a control systems engineer. Visual, proprioceptive (from legs) and vestibular inputs interact to produce high gain optomotor and compensatory eye movements responses, which can be analysed in detail since our experimental set-up (Figure 4) allows us to control both the sign and gain of visual feedback.