Jumping Legs Resist Failure

Biological Strategy

Desert locust

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Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The jumping legs of the locust avoid failure due to high and frequent loading through viscoelasticity and plasticity of the chitin protein matrix.

“Insect cuticle is one of the most common biological materials, yet very little is known about its mechanical properties. Many parts of the insect exoskeleton, such as the jumping legs of locusts, have to withstand high and repeated loading without failure.* This paper presents the first measurements of fracture toughness for insect cuticle using a standard engineering approach. Our results show that the fracture toughness of cuticle in locust hind legs is 4.12 MPa m^1/2 and decreases with desiccation of the cuticle. Stiffness and strength of the tibia cuticle were measured using buckling and cantilever bending and increased with desiccation. A combination of the cuticle’s high toughness with a relatively low stiffness of 3.05 GPa results in a work of fracture of 5.56 kJ m^–2, which is amongst the highest of any biological material, giving the insect leg an exceptional ability to tolerate defects such as cracks and damage. Interestingly, insect cuticle achieves these unique properties without using reinforcement by a mineral phase, which is often found in other biological composite materials.” (Dirks and Taylor 2012:1502)

* When jumping, the two tube-like metathoracic tibiae not only have to withstand repeated high bending forces of up to 20 times the locust’s body mass, they also temporarily store and release up to 10% of the jumping energy.

“The mechanical properties of cuticle, like those of most biological composite materials, are determined by the volume fraction and the mechanical properties of its components. In particular, the viscoelasticity and plasticity of the matrix play an important role, allowing the material to dissipate energy, trap cracks and redistribute stress (Ji and Gao, 2010).” (Dirks and Taylor 2012:1507)

“We analysed natural jumping when a hindleg slips, and kicking when a target is missed, and show that buckling can occur during both movements. Buckling did not occur when the tibia was flexed in preparation for a jump, unless flexion was impeded. We show that buckling is capable of dissipating energy, and thereby reduces the energy that would have to be absorbed by structures such as the joints. We also show that this buckling region contains a band of resilin that may help in energy storage and enable the original shape of the tibia to be restored after buckling. Finally, of the sense organs close to this region, we show that a group of campaniform sensilla responds to buckling movements.” (Bayley et al 2012:1152)

Last Updated August 18, 2016

References

Journal article

Fracture toughness of locust cuticle

Journal of Experimental Biology | 11/04/2012 | J.-H. Dirks, D. Taylor

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Reference

Journal article

A buckling region in locust hindlegs contains resilin and absorbs energy when jumping or kicking goes wrong

Journal of Experimental Biology | 07/03/2012 | T. G. Bayley, G. P. Sutton, M. Burrows

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