̽��ֱ�� of Cambridge - entomology

How stick insects honed friction to grip without sticking

fpjl2 — Wed, 19 Feb 2014 12:38:20 +0000

When they’re not hanging upside down, stick insects don’t need to stick. In fact, when moving upright, sticking would be a hindrance: so much extra effort required to ‘unstick’ again with every step.

Latest research from Cambridge’s Department of Zoology shows that stick insects have specialised pads on their legs designed to produce large amounts of friction with very little pressure. When upright, stick insects aren’t sticking at all, but harnessing powerful friction to ensure they grip firmly without the need to unglue themselves from the ground when they move.

In a previous study last year, the team discovered that stick insects have two distinct types of ‘attachment footpads’ - the adhesive ‘toe pads’ at the end of the legs, which are sticky, and the ‘heel pads’, which are not sticky at all. ̽��ֱ��insect uses different pads depending on direction and terrain.

By studying the ‘heel pads’ in more detail, researchers discovered the insects have developed a way to generate massive friction when walking upright. They do this through a system of tiny hairs that use combinations of height and curvature to create a ‘hierarchy’ of grip, with the slightest pressure generating very strong friction - allowing stick insects to grip but not stick.

̽��ֱ��researchers say the study - published today in the Journal of the Royal Society Interface - reveals yet another example of natural engineering successfully combining “desirable but seemingly contradictory properties of man-made materials” - namely, the best of both hard and soft materials - simply through clever structural design.

“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.

̽��ֱ��power of friction relies on ‘contact area’, the amount of close contact between surfaces. In rigid materials, such as steel, even the tiniest amount of surface roughness means there is actually relatively little ‘contact area’ when pressed against other surfaces - so any amount of friction is very small.

On the other hand, soft materials achieve a lot of contact with surfaces, but - due to the larger amount of contact area - there is also a certain amount of adhesion or ‘stick’ not there with hard materials.

To solve this, stick insect’s hairy friction pads employ three main tricks to allow contact area to increase quickly under pressure, creating a scale or ‘hierarchy’ of grip with absolutely no stick:

• Both the pad itself and the tips of the hairs are rounded. This means that, when pressure is applied, more contact area is generated - like pushing down on a rubber ball.
• Some hairs are shorter than others, so the more pressure, the more hairs come into contact with the surface.
• When even more pressure is applied, some of the hairs bend over and make side contact - greatly increasing contact area with very little extra force.

These design features work in harmony to generate large amounts of friction with comparatively tiny amounts of pressure from the insect. Importantly, there is hardly any contact area without some tiny amount of pressure - which means that the specialised ‘frictional hairs’ don't stick.

Arrays of tiny hairs have been found before, for example on the feet of geckos, beetles and flies. However, these hairs are designed to stick, and are used when creatures are vertical or hanging upside down.

Sticky hairs are completely aligned and have flat tips - meaning that they immediately make full contact that hardly changes with additional weight - as opposed to friction hairs, with their higgledy-piggledy height ranges and rounded tips.

“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,” said Labonte.

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.”

Adds Labonte: “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.”

Inset image: Scanning electron microscopy image of conical, micrometre-sized outgrowths that cover the tarsal ‘heel pads’ of some stick insects (false colours). Image by David Labonte & Adam Robinson.

Scientists have discovered that, when upright, stick insects don’t stick. Instead, they deploy special hairy pads designed to create huge amounts of friction from the tiniest of pressure increases - ensuring that the insects grip but don’t stick.

Lessons from nature such as this might also be useful for inspiring new approaches in man-made devices

David Labonte

Thomas Endlein

Stick insect

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Functioning ‘mechanical gears’ seen in nature for the first time

fpjl2 — Thu, 12 Sep 2013 18:05:28 +0000

̽��ֱ��juvenile Issus - a plant-hopping insect found in gardens across Europe - has hind-leg joints with curved cog-like strips of opposing ‘teeth’ that intermesh, rotating like mechanical gears to synchronise the animal’s legs when it launches into a jump.

̽��ֱ��finding demonstrates that gear mechanisms previously thought to be solely man-made have an evolutionary precedent. Scientists say this is the “first observation of mechanical gearing in a biological structure”.

Through a combination of anatomical analysis and high-speed video capture of normal Issus movements, scientists from the ̽��ֱ�� of Cambridge have been able to reveal these functioning natural gears for the first time. ̽��ֱ��findings are reported in the latest issue of the journal Science.

̽��ֱ��gears in the Issus hind-leg bear remarkable engineering resemblance to those found on every bicycle and inside every car gear-box. Each gear tooth has a rounded corner at the point it connects to the gear strip; a feature identical to man-made gears such as bike gears – essentially a shock-absorbing mechanism to stop teeth from shearing off.

̽��ֱ��gear teeth on the opposing hind-legs lock together like those in a car gear-box, ensuring almost complete synchronicity in leg movement - the legs always move within 30 ‘microseconds’ of each other, with one microsecond equal to a millionth of a second.

This is critical for the powerful jumps that are this insect’s primary mode of transport, as even miniscule discrepancies in synchronisation between the velocities of its legs at the point of propulsion would result in “yaw rotation” - causing the Issus to spin hopelessly out of control.

“This precise synchronisation would be impossible to achieve through a nervous system, as neural impulses would take far too long for the extraordinarily tight coordination required,” said lead author Professor Malcolm Burrows, from Cambridge’s Department of Zoology.

“By developing mechanical gears, the Issus can just send nerve signals to its muscles to produce roughly the same amount of force - then if one leg starts to propel the jump the gears will interlock, creating absolute synchrony.

“In Issus, the skeleton is used to solve a complex problem that the brain and nervous system can’t,” said Burrows. “This emphasises the importance of considering the properties of the skeleton in how movement is produced.”

"We usually think of gears as something that we see in human designed machinery, but we've found that that is only because we didn't look hard enough,” added co-author Gregory Sutton, now at the ̽��ֱ�� of Bristol.

“These gears are not designed; they are evolved - representing high speed and precision machinery evolved for synchronisation in the animal world.”

Interestingly, the mechanistic gears are only found in the insect’s juvenile – or ‘nymph’ – stages, and are lost in the final transition to adulthood. These transitions, called ‘molts’, are when animals cast off rigid skin at key points in their development in order to grow.

It’s not yet known why the Issus loses its hind-leg gears on reaching adulthood. ̽��ֱ��scientists point out that a problem with any gear system is that if one tooth on the gear breaks, the effectiveness of the whole mechanism is damaged. While gear-teeth breakage in nymphs could be repaired in the next molt, any damage in adulthood remains permanent.

It may also be down to the larger size of adults and consequently their ‘trochantera’ – the insect equivalent of the femur or thigh bones. ̽��ֱ��bigger adult trochantera might allow them to create enough friction to power the enormous leaps from leaf to leaf without the need for intermeshing gear teeth to drive it, say the scientists.

Each gear strip in the juvenile Issus was around 400 micrometres long and had between 10 to 12 teeth, with both sides of the gear in each leg containing the same number – giving a gearing ratio of 1:1.

Unlike man-made gears, each gear tooth is asymmetrical and curved towards the point where the cogs interlock – as man-made gears need a symmetric shape to work in both rotational directions, whereas the Issus gears are only powering one way to launch the animal forward.

While there are examples of apparently ornamental cogs in the animal kingdom - such as on the shell of the cog wheel turtle or the back of the wheel bug - gears with a functional role either remain elusive or have been rendered defunct by evolution.

̽��ֱ��Issus is the first example of a natural cog mechanism with an observable function, say the scientists.

Inset image: an Issus nymph

For more information, please contact fred.lewsey@admin.cam.ac.uk

Previously believed to be only man-made, a natural example of a functioning gear mechanism has been discovered in a common insect - showing that evolution developed interlocking cogs long before we did.

In Issus, the skeleton is used to solve a complex problem that the brain and nervous system can’t

Malcolm Burrows

Mechanical gears in jumping insects

Burrows/Sutton

Cog wheels connecting the hind legs of the plant hopper, Issus

This work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page.

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