Hold on tight!
Beetles have thousands of slender and flexible hair-like structures located under their feet. These structures enable them to adhere to any surface thanks to a miniscule quantity of liquid present on their tips. This is what the Microfluidics Lab of the University of Liege has been able to demonstrate by studying the dock beetle. The scales at play are so tiny that no consensus had been reached up to the present, either in terms of the quantity or the role played by this systematically present liquid. Using a method of interference reflection microscopy, the researchers were able to observe the deformations of these microstructures in vivo. They then verified that a simple theory based on the influence of capillary forces could predict the level of adhesion reached by the insects. They calculated the quantity of liquid necessary: it is of the order of a femolitre (the equivalent of a cube with one micrometre long sides) per structure! These results were obtained with the help of researchers from ULB and the University of Cambridge (UK) and have just been published in the Journal of the Royal Society Interface (1)
The study of adhesion mechanisms developed by animals is not new. The adhesion methods of mussels or other molluscs have been abundantly studied, so were those of the gecko, a small lizard which is characterised by an astonishing ability to rapidly climb a wide variety of surface. The gecko uses dry adhesion, a phenomenon involving the Van der Waals forces (low-intensity electric interactions which occur at short distances between atoms or molecules). This mode of adhesion has been studied for several years and has resulted in the production of glues and even small robots whose functioning is based on this principle.
The mechanism developed by insects is very different because it is based on capillarity. It is implemented in two different ways. Firstly, smooth capillary adhesion is used by insects such as ants or stick insects which adhere to surfaces by means of pads which are a few tenths of a millimetre in width and are entirely covered in liquid. Secondly, other insects developed a system known as hairy capillary adhesion. In this case the adhesion relies on very fine structures with a diameter in the order of a micron coupled with many liquid menisci instead of only one per feet such as described above. This applies to flies but also to beetles such as the ladybird for example.
The dock beetle
“There are of course many ways of studying insects”, explains Sophie Gernay. “In our case, we try to create physical models. We do not only observe the structures or materials but also try to examine how these correspond to physical laws and mathematical models. It allows us to simplify and get some idea of how they work with a view to reproducing in the laboratory what we have observed about the insect”.
The insect chosen by the researchers from Liege is the dock beetle Gastrophysa viridula, thus named because this particular plant is its only food source! About five mm in length, it shows great levels of adhesion. But more importantly, it is easily bred in a laboratory, does not need to hibernate, does not fly and is correctly documented by biologists including Professor Walter Federle’s team from Cambridge, who also participated in the study. In a nutshell, this little animal is absolutely ideal for the laboratory!
(1) Elastocapillarity in insect fibrillar adhesion, Sophie Gernay et al. Journal of the Royal Society Interface.