Propulsion mechanism at intermediate Reynolds numbers : swimming of midge larva

Organisms with all sizes live in the water. For different sizes, the water as an environment is very different due to the change in Reynolds number. Studies on the locomotion at intermediate Reynolds number are scare. Previous study on the mosquito larva and our own observation suggest that while the elongated larva body resembles eels and snakes, the millimeter-sized larva uses a different way for locomotion.

We found that, in contrast to undulatory swimmers who try to reduce rotation all the time to align their propulsion with the direction of motion, the mosquito larva purposely generates a significant rotation and timely generate the propulsion forces when the propulsion forces align with the direction of motion. The key of body rotation is the asymmetry of body deformation. The asymmetry causes the torque of fluid on body to not completely cancel outThe mosquito larva also modulates the speed of body deformation to increase the propulsion force since force scales greater than linear with velocity. As a result, the swimming speed is greater than the speed (~0.3 body length/cycle) of the undulatory gait in the same condition (Reynolds number). Besides, wavelength of curvature wave and Reynolds number could influence body rotation angle. It is found that there is an optimal angle range which make the swimming path straighter and swim faster. Therefore, we can control larva swimming under different Reynolds number by adjusting wavelength and body deformation frequency. It is worthy noting that larva can increase angular velocity of body rotation by shrink body like figure skating. Although ‘figure-of -8’ swimming style could swim faster than some creatures, such as eel, it is a kind of energy-wasting swimming pattern.