In an experiment, scientists shook living earthworms at high frequencies to see how their flexible bodies responded to vibration. What they found was unexpected: the worms developed waves that oscillated at half the frequency of the force driving them. According to the 2019 study published in Nature, titled ‘Excitation of Faraday-like body waves in vibrated living earthworms’, there were similarities between the worms’ behaviour and nonlinear waves found in vibrating liquids. By studying several earthworm species, researchers found that the animals’ water-filled bodies and elastic skin could support these unusual subharmonic oscillations. The experiment provided a new way to explore complex physical phenomena in living organisms and their biological systems.
Scientists tested earthworms on a vibrating Teflon plate
The experimental set-up was relatively simple but closely monitored. The worms were immobilised before being placed on a thin Teflon plate, which was then vibrated vertically. A red laser and photodetector measured the vibrations, while a digital camera continuously monitored the worm. The researchers gradually increased the vibration amplitude until they observed the onset of the instability they were looking for.One of the clearest results appeared when the Teflon plate was vibrated at 40 Hz. The researchers detected subharmonic peaks in the earthworms’ vibration spectrum, including a response at half the driving frequency. Video analysis confirmed that the entire body of the worm was involved: the paper reported that the worm oscillated at both the imposed frequency and its subharmonic frequency.
Earthworm’s liquid-filled body explained the strange waves
The researchers found that the phenomenon could be explained by treating the worm as an elongated, liquid-filled body covered by a thin elastic skin. Its water-filled body cavity and flexible outer layer allowed the worm to respond to vibration in a way that bore a strong resemblance to parametrically excited waves in vibrating liquid drops.The study also compared the real animals with an earthworm-mimicking phantom made from a thin latex glove filled with water. Its vibration spectrum was found to be in good qualitative agreement with those of the actual worms. This gave the researchers further support for their approach of viewing the worm’s body as an elastic structure containing a liquid-like interior.
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Researchers found possible uses beyond earthworms
Their findings suggested that nonlinear subharmonic waves could potentially be used to investigate biological processes. The paper said that the ability to excite such waves in a living organism could help researchers probe, and potentially control, processes such as the propagation of nerve impulses.The authors also suggested that the phenomenon might not be limited to earthworms. Because excitation of Faraday-like waves in living organisms had received relatively little attention, they argued that their findings could help expand understanding of nonlinear phenomena in biological systems.
