Sea butterfly at different stages of wing beat. Photo: David Murphy.
In a remarkable example of convergent evolution, we show that the zooplanktonic sea butterfly Limacina helicina ‘flies’ underwater in the same way that very small insects fly in the air. Both sea butterflies and flying insects stroke their wings in a characteristic figure-of-eight pattern to produce lift, and both generate extra lift by peeling their wings apart at the beginning of the power stroke (the well-known Weis-Fogh ‘clap-and-fling’ mechanism). It is highly surprising to find a zooplankter ‘mimicking’ insect flight as almost all zooplankton swim in this intermediate Reynolds number range (Re=10–100) by using their appendages as paddles rather than wings. The sea butterfly is also unique in that it accomplishes its insect-like figure-of-eight wing stroke by extreme rotation of its body (what we call ‘hyper-pitching’), a paradigm that has implications for micro aerial vehicle (MAV) design. No other animal, to our knowledge, pitches to this extent under normal locomotion.
To make the discovery, Dr Murphy and his colleagues used a system they call tomographic particle image velocimetry: four high-speed cameras trained on a tiny volume of fluid, which is illuminated with laser beams and seeded with shiny particles to trace flow movements.
"Using our four cameras, we make a 3D measurement of the flow that the animal produces as it's swimming," he explained.
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