Unless hanging directly upside down stick insects don’t actually stick themselves onto a surface. Doing so can even be a hindrance, using vital energy to un-stick each foot when they move. But how do they hang on if they aren’t stuck down? Scientists at Cambridge University have discovered that they harness the power of friction, using a design honed by evolution that could be highly valuable to humans…

Stick insects have two distinct types of foot pad – the classic sticky ‘toe pads’ on the end of each leg and a second ‘heel pad’, which is not sticky at all. On closer inspection, you can see that these ‘heel pads’ are covered in a plethora of tiny hairs.

This system of hairs is complex, using combinations of height and curvature to create what the researchers describe as a “hierarchy” of grip. Even the smallest pressure on these hairs generates huge friction allowing them to grip seamlessly onto almost any surface.

SEM image of frictional hairs

SEM image of frictional hairs

There are three special qualities of these heel pads that allow the insects to clamber around…

  • The pad and the tips of the hairs are rounded, meaning when more pressure is applied, more contact is made with the surface, like pushing down on a rubber ball.
  • Some hairs are shorter than others, so when more pressure is applied, more hairs make contact.
  • And with even more pressure, the hairs bend over and make side contact – greatly increasing contact area with very little extra force.

The researchers from the Department of Zoology at Cambridge say the study reveals an incredible example of natural engineering, successfully combining “desirable but seemingly contradictory properties of man-made materials”.

“Just by arrangement and morphology, nature teaches us that good design means we can combine the properties of hard and soft materials, making elemental forces like friction go a very long way with just a small amount of pressure,” said David Labonte, lead researcher from the Department of Zoology.

“We investigate these insects to try and understand biological systems, but lessons from nature such as this might also be useful for inspiring new approaches in man-made devices.”

He uses the example of a running shoe as a possible man-made item that could be enhanced by stick insect engineering: “If you run, you don’t want your feet to stick to the ground, but you also want to make sure you don’t slip.”

Labonte adds: “Stickiness is the force that is needed to overcome when trying to detach one thing from another. If the soles of your feet were made of Scotch tape, it may be helpful when you are walking up walls or hanging upside down, but the rest of the time it would be incredibly frustrating.”

“Stick insects have developed an ingenious way of overcoming the conflict between attachment and locomotion, with a dual pad system that alternates between stick and grip depending on the situation.”

Similar hairs have been found before on the feet of other species, such as geckos, beetles and flies. However, these hairs are designed to stick, and are used when creatures are hanging upside down. They are completely aligned and have flat tips – meaning that they immediately make full contact that hardly changes with additional weight.

Stick insects have given us the first known example of friction hairs. With their unique higgledy-piggledy, rounded feet hairs, moving up and down is easy, making them the masters of graceful climbing.