How insects spring to attention

Modelling reveals the trade-offs involved when animals use spring-like structures to get around. Fiona McMillan reports.

A western green tree frog spring into action.
A western green tree frog spring into action.
David A. Northcott

What do archers have in common with fleas? A new study now has identified the common engineering principles that enable animals, plants, fungi and machines to maximise kinetic energy.

The findings, published in the journal Science, explain why some animals use stored energy and spring-like systems to generate movement, while others primarily rely on muscles.

When it comes motion, three major factors allow power to be generated and amplified: a motor that puts energy into the system; a spring-like component that stores that energy; and a latch that can trigger its release.

Archery is a good example of how these work together: muscles drawing the bow act as the motor; the bow string stores the elastic energy; and the fingers act as a latch. On release, the arrow flies a lot further than if you had just thrown it.

Many organisms – such as frogs, mantis shrimps, and fleas – amplify power by using a combination of these factors, wherein energy is loaded into and stored by a spring-like mechanism that is ultimately released by a latch.

But this alone doesn’t quite explain why smaller organisms are better at jumping than larger ones, or why, at even smaller sizes, jumping performance actually starts to decline. Fleas aren’t as quick off the mark as slightly bigger leaf-hoppers, arthropods from the family Cicadellidae.

To figure out the core design principles that both maximise and limit power generation in small organisms, researchers led by engineer Mark Ilton, from the University of Massachusetts Amherst in the US, analysed more than 100 species of animals, plants and fungi, then used the data to develop models of motion. They were able to identify when motors, springs, and latches work really well together – and when they don’t.

The research revealed that muscles function best in organisms above a certain size – where initial inertia presents a challenge – but as size and mass decrease, springs become better at enhancing power and motion.

But there’s a trade-off.

As an organism gets smaller still, so does the spring, and recoil begins to suffer. Smaller insects are able to compensate for this by increasing the stiffness of the spring. Thus, frogs store energy in collagen tendons, while grasshoppers use a stiffer combination of the protein resilin and tough chitinous cuticle, and small insects, such as frog-hoppers (from the family Cercopoidea), just use chitinous cuticle.

Keep going even smaller, though, and the benefit vanishes. Teensy insects can’t load stiff springs very well, and performance declines. Thus, leaf-hoppers outperform fleas.

The study also found that the properties of the latch are important, too. In particular, a more curved latch releases energy more slowly, reducing a spring’s ability to amplify power. Conversely, a shallow, quick-releasing latch allows for more powerful movement.

Understanding how motors, springs, latches and mass intersect to amplify power in biological organisms could help us better understand evolutionary trade-offs that have taken place in different species and could aid the design of new artificial motors.

F mcmillan headshot.jpg?ixlib=rails 2.1
Fiona McMillan a science communicator with a background in in physics, biophysics, and structural biology. She was awarded runner up for the 2016 Bragg UNSW Press Prize for Science Writing.
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