Like many insects, beetles can walk upside down without falling due to the extremely sticky structures of their foot pads. Scientists James Bullock and Walter Federle from the University of Cambridge recently published a study in the journal Naturwissenschaften (The Nature of Science) that found different hair structures have different levels of stickiness. Their study is the first to measure the adhesive strength of a single seta, the adhesive hairs that are responsible for the “stickiness” of the beetle’s feet.
The researchers found there were three different structures of setae on the foot pads: pointed, flat (spatula-tipped) and disk-like. The three structures have different functions depending on the specific pattern they are arranged in. Each of these structures is made up of thousands of microscopic hairs and prior to this study there was no way to determine the adhesiveness of one individual hair due simply to their microscopic size.
By using an extremely fine glass cantilever and measuring the deflection of the cantilever with a microscope, the exact force needed to detach each hair was calculated. By use of this novel technique the researchers were able to calculate the exact stickiness of each hair, which are only 5 micrometers across.
Of the three different seta structures the disk like hairs had the greatest level of stickiness, followed by the spatula shaped hairs, with the pointed hairs coming in least sticky. The most sticky hairs were also the most stiff, most likely providing stability to the foot-pad. The researchers hypothesize it is these disk-like hairs that are particularly responsible for the strong adhesion the beetles have to smooth surfaces, such as the underside of a leaf.
This adhesion is also important during mating so that males can attach themselves to a female’s back. The other hair structures which aren’t as sticky are probably used for adhesion while running because they are quicker and easier to unstick.
This new understanding of the beetle’s sticky feet may one day lead to the creation of bio-inspired synthetic adhesives, such as extra sticky super glue.