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Biophysics

Spring and latch mechanism flings click beetles into the air

29 Jan 2021
Photo of a click beetle held in an X-ray apparatus
Click and release: Experimental set-up used to constrain click beetles during X-ray synchrotron imaging at the Advanced Photon Source. (Courtesy: John J Socha)

Click beetles use the elastic recoil of a spring-like mechanism in their exoskeleton to throw themselves into the air. The mechanism of the insects’ motion was discovered using high-speed synchrotron X-ray imaging, and researchers in the multidisciplinary team responsible say their techniques could further our understanding of energy storage and release strategies in other animals that perform ultrafast movements.

Small creatures like fleas, mantis shrimp and trap-jaw ants produce some of the fastest movements in the animal kingdom. These extreme accelerations are powered not by muscles, but by complex energy storage and amplification structures. One such animal is the click beetle, Elater abruptus, which can jump more than 20 body lengths without using its legs. It does this using a latch to bend its body at a hinge between its thorax and abdomen. When the latch releases, the beetle unbends extremely quickly, makes a loud clicking sound and – if unconstrained – leaps into the air.

“There have been many speculations about why beetles evolved this manoeuvre, including to escape predators or scare them away, or to free itself from tight places such as wooded areas or the soil,” Aimy Wissa, a mechanical engineer at the University of Illinois Urbana-Champaign, says. “However, the jump itself is almost vertical, making it a bad escape strategy.”

Latch, load, release

While the click beetle’s jump has been studied before, there has been little analysis of its clicking motion or energy storage and release mechanism. To record the rapid movement of this mechanism, Wissa and her colleagues took four beetle specimens to the Advanced Photon Source at Argonne National Laboratory. Using a high-speed synchrotron X-ray imaging system, they captured the beetle’s internal movements at rates of 30 000 frames per second.

The beetle’s latch is on the underside of its exoskeleton and is composed of a peg on the thorax and a lip on the abdomen. The high-speed X-ray recordings show that the click has three phases: latching, loading and energy release. To “latch”, the beetle moves its head and thorax away from the underside of its body, causing the peg to slide over and latch onto the lip.

During the loading phase, the beetle maintains this bend in its body while the soft cuticle behind the peg deforms and stores elastic energy, acting like a spring. After this contraction – which takes around 240 milliseconds – the peg slips, the latch unlocks, and the energy stored in the cuticle is rapidly released. As a result, the beetle’s thorax is flung in the opposite direction and the insect fires itself into the air.

“The keys for the extreme accelerations are both the latch and the spring,” Wissa tells Physics World. “The spring allows for storing elastic energy and releasing it quickly, while the latch prevents the spring from recoiling prematurely.”

Damage limitation

The X-ray images show that the peg reaches a maximum velocity of 1.8 metres per second during the energy release. This velocity corresponds to about 1000 peg lengths per second and a maximum acceleration of more than 500 times the acceleration due to Earth’s gravity.

The images also show that the deformation in the soft cuticle is released in less than 1 millisecond. This is much faster than the loading time, which supports the idea that click beetles can amplify mechanical power during the clicking motion and is characteristic of an elastic recoil.

Despite the power they generate, Wissa says that the beetles can click repeatedly without apparent injury. “We found out that energy is dissipated using a nonlinear damping force,” she says, adding that “the exact mechanism that prevents damage is an active area of research”.

Understanding the physical mechanisms such small animals use to achieve extreme accelerations could have biomimetic applications. “The most obvious might be insect-scale robotic systems,” Wissa says. “However, what we discovered in the paper can also help us design new actuation strategies for small mechanical systems where power might be limited.”

The research is described in PNAS.

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