For paws

Many insects are capable of rising for walls and gait mouth below on diverse substrata thanks to the adhesive organs that they have in his tarsi (the feet of the insect). In spite of the insects diversity, this skill has been achieved by means of two designs.

There are insects, between which there are the flies, the beetles and the earwigs, which have in the tarsi shaggy ink pads covered by a dense layer of flexible hair of microscopic size. Others, as the ants, the bees, the cockroaches and the insects stick, possess smooth adhesive ink pads, with a relatively soft surface and a soft and specializing cuticle. In both cases, the adherence one achieves thanks to a fluid that they segregate in the surface of contact (for this way saying it, they have the humid feet).

The insect stick Carausius morosus (left) and the beetle Gastrophysa viridula (right). Insects with two different designs to walk on diverse substrata.

The shaggy ink pads have evolved often of independent form in arthropods and other invertebrates, what he suggests that it is an ideal design to stick to surfaces. The insects are continuously exposed to particles of dirt, dust, microorganisms, spores and pollen that they can reduce adhesion to his paws and nevertheless, retain his skill during all his life. They must have, then, some mechanism to eliminate the dirt of his adhesive structures. The smartening up might be a good method because many insects practise the autocleanliness parts of his body being licked. Nevertheless, an insect that was using this system one would have to clean the paws to every few steps and this behavior has never been observed. Another autocleanliness method consists of eliminating the particles for contact with the surface while they walk.

Knowing that the insects stick have smooth feet and the beetles shaggy feet, a team of investigators of the group of biomechanics of insects in the Department of Zoology of the University of Cambridge led by Christofer Clemente, he decided to investigate if these two structures were possessing some autocleanliness mechanism, if it was common to both, if some shears movement was necessary to eliminate the dirt and the influence of the size of the particles.

To simulate the dirt, there used spheres of polystyrene 1, 10 and 45 microns in diameter and measured the forces of friction and adherence of the paws of the insect stick (Carausius morosus) and of the beetle (Gastrophysa viridula) on a surface of crystal covered with spheres of polystyrene.

Shaggy feet and smooth feet. Detail of the shaggy ink pads of the beetle Gastrophysa viridula [To] and the smooth ink pads of the insect stick Carausius morosus [B] revealed across an electronic microscope of sweep.

The dirt provoked a decrease of the adherence of the paws of up to 90 % that was recovering in successive contacts with the surface. This is possible thanks to the fact that the ink pads have a mechanism of autocleanliness that does that the polystyrene balls (or dirt particles) remain beaten to the contact surface together with the adhesive fluid that they segregate. In case of the insects stick, the recovery of the adherence was not depending on the size of the spheres. Nevertheless, in the beetles, the 10 microns particles supposed a problem at the time of recovering the adherence. The reason is that the diameter of these spheres is more or less the same one that the space between the hair of the tarsi and they were remaining caught between the hollows. The 45 microns particles were too big to fit between the hair and they were eliminated by major speed. The one micron particles also were more difficult to eliminate on having got between the hollows of the hair (something similar to what happens when we cross a sandy area or of gravel with a few boots. Some sand particles and piedrecitas they remain fitted into the hollows of the drawing that forms the sole). Nevertheless, the rhythm of elimination of particles for autocleanliness was the double of rapid (excepting the case of the 10 microns sphere, which was similar) in the beetles that in the insects stick. While the first ones had eliminated big part of the particles of dirt to few steps, the second ones were late more and the autocleanliness was finished only when there was a shears movement.

Step to step. One micron spheres deposited in successive steps along Gastrophysa viridula [To] and Carausius morosus [B]. The arrow indicates the direction of the movement. Since it can be verified, the ink pads clean earlier in the beetles that in the insects stick.

Of this it is deduced that the shaggy feet behave better than the smooth ones since they recover more quickly his adhesive properties and do not need to realize any shears movement. Hence this system is so extended in other arthropods and invertebrates.

Studying in depth the autocleanliness mechanism, there might be designed adhesives that were supporting his properties for the continued use.

And of tip...

Merciful Christofer speaks about the sticky feet of the insects, specifically about the ant weaver Oecophylla smaragdina (it is capable of supporting mouth below and and a weight 100 times superior to his does not fall down thanks to the adherence of his paws, which have ink pads with a mechanism to diminish the contact surface with the soil and this way to be able to become alienated) and of the cockroach ash-gray Nauphoeta (it has two types of ink pads, one to rise and other one, in the opposite direction, to go down).

Evidence for self-cleaning in flow - based smooth and hairy adhesive systems of insects Christofer J. Merciful, James M. R. Bullock, Andrew Beale and Walter Federle. 2010. J. Exp. Biol. 213, pp. 635-642. Merciful, C., Bullock, J., Beale, A., and Federle, W. (2010). Evidence for self-cleaning in flow - based smooth and hairy adhesive systems of insects Experimental Journal of Biology, 213 (4), 635-642 DOI: 10.1242/jeb.038232