A click beetle rolling around on its back may look pitiful — but do not be deceived. Beyond the hard exterior of their bodies lies a jack-in-the-box-like mechanism that, once snapped, springs them high into the air with a signature click!
Scientists have known of this click-trick, but what they've struggled to understand is precisely how and why these beetles spring into the air. However, using high-speed x-ray technology and fundamental equations of motion, a team of mechanical engineers recently examined how this action takes place and what its limits are.
A better understanding of this elegant, biological mechanism not only reveals insight into a natural phenomenon — it may help roboticists super-charge the strength of future machines without increasing their "muscular" strength.
The findings were published Monday in the journal Proceedings of the National Academy of Sciences.
They might look helpless, but these beetles are poised to pop at any moment.
Here's the background — We typically imagine springs and latches are something human-designed — doodads used to close doors or accelerate pinballs into obstacle courses. But in fact, click beetles are far from outliers when it comes to natural examples of these mechanisms.
Venus flytraps, for example, use a similar biological spring to close their leafy lips around their prey. Meanwhile, mantis shrimp can snap their claws so quickly they create tiny sonic booms in the ocean.
When it comes to click beetles (which encompass a family of beetle called Elateridae discovered by an English zoologist in 1815), scientists have studied its kinematics (the beetles' resultant acceleration) as well as the geometry of the beetles' biological latch. However, the researchers behind this new study argue the question of how this motion arises from the beetles' biology is still largely unexplored.
"Previous studies have focused on the [click beetle] jump, in this paper, we are focused on the bending maneuver referred to as the click because of the audible click it produces," Aimy Wissa, assistant professor of mechanical science and engineering at the University of Illinois Urbana-Champaign, tells Inverse.
"The bending movement happens in a hinge in the thorax and only takes a few milliseconds. In the paper, we identify and characterize the different phases of the click and we identify the forces governing the energy release phase of the click."
With four beetles on loan for the University of Illinois Urbana-Champaign's permanent research sites, the team began the process of learning exactly what makes these beetles tick (or click).
What they did — Because only a number of milliseconds separate a still click beetle from a click beetle pirouetting through the air, the researchers used a high-speed x-ray imager to slow down the process for analysis.
The team zeroed in on a small cavity in the beetle's thorax that transforms into a latch when they begin to twist back and forth.
With this visual data, the team had three big questions they wanted to answer:
1) What are the phases of the clicking motion?
2) What are the elastic energy storage and release mechanisms in click beetles?
3) Can the energy release mechanism be inferred from the latch dynamics during the energy release phase?
What they discovered — Examining the motion of these four beetles, the researchers identified three distinct phases of behavior that predicate the clicking spring: latching, loading, and energy release.
Based on this behavior, the researchers were able to classify this mechanical movement as "elastic recoil," which is a fast release of stored elastic energy. Essentially, as the beetle twists its body, its latch creates more and more elastic energy. This energy is eventually released when the latch breaks free, creating the springing movement.
Studying this motion, the researchers observed that the loading phase (where a peg tip was wedged into the cavity) took up to 235 milliseconds while the energy release took only 17.4 milliseconds at most.
This means that after a relatively slow build-up of elastic energy, the beetle expells it all at once for maximum recoil.
Because the beetle's writhing dance appears to be the main determinant of when how and this movement takes place, the researchers were able to closely model the oscillatory movement of the beetle post-jump as a one-degree-of-freedom system.
Being able to match the biological motion to an existing model will make it easier to replicate this motion in non-living systems as well.
"Learning about how small organisms can generate extreme acceleration will lead to breakthroughs in the field of insect-scale robotics and other small devices," Wissa tells Inverse. "Microrobots currently suffer from severe actuation limitations and cannot move nearly as fast as even the slowest [spring-equipped] biological organism"
What's next — While the researchers were able to answer a number of important questions about this mechanism, there are still outstanding questions yet to be explored.
For example, the external latch that they identified is likely only part of an interior spring mechanism. What exactly that mechanism is and how it works is something they hope to explore in future studies.
In the future, the researchers write that they hope this understanding can help build a symbiotic relationship between advancing robotics and advancing the study of biological life as well. Eventually, this understanding could "enable the creation of a new generation of insect-inspired robots capable of generating and sustaining high-acceleration movements," the study team writes.
"Such robots will also serve as research platforms to answer critical questions about their biological counterparts."
Abstract: Many small animals use springs and latches to overcome the mechanical power output limitations of their muscles. Click beetles use springs and latches to bend their bodies at the thoracic hinge and then unbend extremely quickly, resulting in a clicking motion. When unconstrained, this quick clicking motion results in a jump. While the jumping motion has been studied in depth, the physical mechanisms enabling fast unbending have not. Here, we first identify and quantify the phases of the clicking motion: latching, loading, and energy release. We detail the motion kinematics and investigate the governing dynamics (forces) of the energy release. We use high-speed synchrotron X-ray imaging to observe and analyze the motion of the hinge’s internal structures of four Elater abruptus specimens. We show evidence that soft cuticle in the hinge contributes to the spring mechanism through rapid recoil. Using spectral analysis and nonlinear system identification, we determine the equation of motion and model the beetle as a nonlinear single-degree-of-freedom oscillator. Quadratic damping and snap-through buckling are identified to be the dominant damping and elastic forces, respectively, driving the angular position during the energy release phase. The methods used in this study provide experimental and analytical guidelines for the analysis of extreme motion, starting from motion observation to identifying the forces causing the movement. The tools demonstrated here can be applied to other organisms to enhance our understanding of the energy storage and release strategies small animals use to achieve extreme accelerations repeatedly.