Insect limbs strike a balance between strength and mobility: their cuticle exoskeletons withstand the stresses of motion without being so heavy that they impede it. This raises the question of whether insect legs are engineered for normal activities such as walking and running, or for emergency activities that could save their lives, such as jumping, righting when overturned, and wedging into tight spaces.
According to new research published today in PLOS ONE, the answer is the latter for the tibias of desert locusts, American cockroaches, and death’s head cockroaches. This discovery and others like it could ultimately help optimize the design of manmade structures.
Emergency maneuvers can put extreme stress on insect legs. When a cockroach thrusts its hind legs to wedge itself into a crevice at great speed, for example, the force generated by a single leg can be four times that of its entire body weight. In addition, the force generated by a single leg while righting is up to eight times higher than while running.
To find out whether insect legs are engineered for everyday or emergency movements, Eoin Parle from Trinity College, Ireland, and colleagues investigated the biomechanics of tibias from three insects – desert locusts, American cockroaches, and death’s head cockroaches – with different sizes, shapes, and ways of moving. The researchers measured the force it took to bend the tibias and used existing data to determine the stresses of activities such as walking, running and jumping. For each tibia during each activity, they then calculated the safety factor: the ratio of the tibia’s failure strength to its maximum applied stress during the activity.
The researchers found that safety factors were lower for emergency than for everyday motion, showing that these insect tibias are engineered for the former. Moreover, the tibias’ lowest safety factors were comparable to those of structures engineered by people. For example, the safety factor of a locust tibia while jumping is 1.7, which is close to the 1.5 safety factor of an aircraft fuselage. At the other extreme for manmade structures, power station components can have safety factors of more than 100.
Because the tibias operate at stresses that approach their limits, the researchers conclude that insect limbs have evolved to the point that they are nearly optimal. Understanding how insect exoskeletons balance lightness with resilience could help people solve engineering problems.
“Natural materials have been evolving and developing over millennia – what can we learn from them?” says Parle, who has studied the mechanical properties of insect exoskeletons since his final year as an undergraduate in mechanical engineering. “Lightweight durable structures and materials seen in the insect kingdom could prove useful in the fields of aerospace design and engineering.”
Research Article: Parle E, Larmon H, Taylor D (2016) Biomechanical Factors in the Adaptations of Insect Tibia Cuticle. PLoS ONE 11(8): e0159262. doi:10.1371/journal.pone.0159262
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