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  1. ITV Report

Cambridge scientists getting to grips with what makes stick insects stick.

It's long been a matter of some conjecture - how the stick insect is able to stick. Well, scientists in Cambridge may have discovered the reason.

An ant’s foot showing a fluid trail. Credit: Walter Federle

They've found that geckos, tree frogs, spiders and insects all share a special skill – they can walk up vertical surfaces and even upside down using adhesive pads on their feet.

But geckos have ‘dry’ feet, while insects have ‘wet’ feet. Scientists have assumed that the two groups use different mechanisms to keep their feet firmly attached to a surface, but new research from David Labonte and Dr Walter Federle in the University of Cambridge’s Department of Zoology provides evidence that this isn’t actually the case.

“It has generally been assumed that the fluid on their feet must be involved in helping insects like stick insects adhere to a surface by capillary and viscous forces – in the same way that a beer glass will stick to a glass table if it’s wet on the bottom.

"Our research shows that the fluid is likely used for something else entirely – it may even help insects unstick their feet.”

– David Labone, report author

By measuring how much force was required to detach the foot of a stick insect from a glass plate at different speeds and applying the theory of fracture mechanics, Labonte and Federle found that only a ‘dry’ contact model could explain the data.

They also carried out a comparison of the sticking performance of wet and dry adhesive pads, which revealed that there is a striking lack of differences between the two, contrary to previous opinion.

Insects and geckos need to walk up vertical surfaces and even upside down in order to get to the places where they feed and to escape from predators. As smooth surfaces don’t allow them to grip with their claws, they need soft adhesive pads on their feet and legs.

This means they need to have excellent control over adhesion – to ensure their feet stick when they want them to, but can also unstick easily to allow them to walk around or run away from predators.

“The fluid that insects have on their adhesive pads doesn’t seem to increase the pads' stickiness by means of capillary or viscous forces, and the same may hold for the fluid on the feet of spiders and tree frogs.”

– David Labone

So what is this fluid for?

“If you think of commercial adhesives, like Scotch tape, there are often bits of tape or residue left behind when you remove it quickly. But a stick insect needs to be able to unstick its feet without expending a lot of energy or leaving bits of its foot still stuck to a leaf.

“The fluid may act as a lubricant to make detachment easier, giving insects greater control over adhesion at very short timescales.”

– Dr Walter Federle, report author

But it’s not just an age-old question that this research is helping to answer. The researchers say there may be lessons to learn for modern manmade devices.

This research was enabled by funding from the Biotechnology and Biological Sciences Research Council and the Human Frontier Science Programme.