Unique 'figure-of-8' swimming style provides a new idea for robot design: rotation is important for swimming.

Studying the swimming strategy of creatures could help to design bio-robots. Here, we display a unique swimming style of larva of Chironomus plumosus. The larva with a length about 14mm uses an interesting ‘figure-of-eight’ swimming style during the escape movement, that is, the body periodically bends into a circle and then fully unfolds for propulsion. We use a 3D model combining with computational fluid dynamics to study the kinematics and dynamics of larval locomotion and found that larva not only moves forward but also rotates its body around the center of mass. Specially, body rotation is important for swimming faster, which influences the net force orientation. Wavelength(λ) of curvature wave, body deformation frequency(ω) and Reynolds number(Re) could influence body rotation angle. It is found that there is an optimal angle range which makes the travel path straighter and makes larva move faster. Therefore we can control larval swimming by adjusting λ, ω and Re to make body rotation angle be in the optimal range. Our study provides a better understanding on the hydrodynamics of the larva and guidance for the design of underwater robots at millimeter scale.