Saturday 20 July 2019

How are insects' legs stronger than ours?

Professor David Taylor and Dr Jan-Henning Dirks: Every day, and most of the time without even noticing it, we are surrounded by one of the most successful groups of animals -- insects.

They have been running, crawling and flying around the earth since before the dinosaurs, they can live in rainforests, deserts and even in Antarctica and their size varies from less than a hair's breadth up to an arm's length.

One of the insects' secrets of evolutionary success is their exoskeleton. Like a knight's suit of armour it protects their softer parts; and like our bones, it is also responsible for body posture and locomotion.

All parts of the insect exoskeleton are made from a material called "cuticle" -- the soft joints, hard mouthpieces, protective back-plates and even the transparent eyes.

After wood, cuticle is the most common biological material in the world, but we know very little about its fundamental mechanical properties.

Funded by the Irish Research Council for Science, Engineering and Technology (IRCSET), a collaborative project was set up involving researchers in the Department of Mechanical Engineering and the Department of Zoology at Trinity College Dublin.

Our results show that locust cuticle is one of the toughest of all natural materials. Interestingly, it takes much more energy to crack a piece of cuticle from a locust than it does to crack a piece of bone from a human. All the more surprising, considering that cuticle is a lighter material than bone, having a lower density.

Not only is this result interesting from the biologist's point of view, knowing more about cuticle properties could also be potentially very useful for engineers and scientists creating new materials and new structures, which are stronger, lighter and tougher and maybe even self-healing.

We also looked at the "design" of legs and wings (their wall thickness and diameter) and found that they can resist forces better than our own leg bones.

Engineers who make structures such as buildings and aircraft try to optimise the design, which means making the structure both light and strong at the same time.

Our research shows that insects' legs are very close to the optimum strength-to-weight ratio, whilst human bones are not. Some of the tricks used by nature, such as the use of veins to stiffen a wing and stop cracks growing in it, can help human engineers to improve their designs, creating structures which are strong, but which use less material, making them lighter and so saving energy and resources in their construction, something that will be very important in the future.

So, can insects get broken legs? Of course, but our research shows that their legs are much better designed than ours, and made from better material.

uProfessor David Taylor is the Professor and Head of Department of Mechanical & Manufacturing Engineering in Trinity College Dublin. His specialisation is strength of materials and how they fracture and he is often called an expert witness in court proceedings dealing with public injury cases.

uDr Jan-Henning Dirks is a research fellow within Fracture and Fatigue of Materials research group in Trinity College Dublin. He previously worked in Cambridge University and he specialises in insect adhesion, and why for instance, a fly can walk up a wall and not fall off. He has developed some special coatings to which insects cannot stick.

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