Honeybees use their wings for water surface locomotion
Honeybees display a unique biolocomotion strategy at the air–water interface. When water's adhesive force traps them on the surface, their wetted wings lose ability to generate aerodynamic thrust. However, they adequately locomote, reaching a speed up to 3 body lengths·s−1. Honeybees use their wetted wings as hydrofoils for their water surface propulsion. Their locomotion imparts hydrodynamic momentum to the surrounding water in the form of asymmetric waves and a deeper water jet stream, generating ∼20-μN average thrust. The wing kinematics show that the wing's stroke plane is skewed, and the wing supinates and pronates during its power and recovery strokes, respectively. The flow under a mechanical model wing mimicking the motion of a bee's wing further shows that nonzero net horizontal momentum is imparted to the water, demonstrating net thrust. Moreover, a periodic acceleration and deceleration of water are observed, which provides additional forward movement by "recoil locomotion." Their water surface locomotion by hydrofoiling is kinematically and dynamically distinct from surface skimming [J. H. Marden, M. G. Kramer, Science 266, 427–430 (1994)], water walking [J. W. M. Bush, D. L. Hu, Annu. Rev. Fluid Mech. 38, 339–369 (2006)], and drag-based propulsion [J. Voise, J. Casas, J. R. Soc. Interface 7, 343–352 (2010)]. It is postulated that the ability to self-propel on a water surface may increase the water-foraging honeybee's survival chances when they fall on the water.
© 2019 The Authors. Published under the PNAS license. Edited by Howard A. Stone, Princeton University, Princeton, NJ, and approved October 11, 2019 (received for review June 4, 2019). We thank davidkremers, Cong Wang, and Jennifer Han for their reviews and comments on the manuscript. We also thank anonymous reviewers for their insights and comments. An IDT-OS3-S3 camera was provided by IDT, for which we are grateful. We thank Editage for their language-editing service. This material is based upon work supported by the National Science Foundation under Grant CBET-1511414; additional support was provided to C.R. by a National Science Foundation Graduate Research Fellowship under Grant DGE-1144469. This work was partially supported by Charyk Bio-inspired Laboratory at California Institute of Technology. Data Availability Statement. All data discussed in the paper will be made available to readers upon request. The flow field data in Fig. 5 are available at https://data.caltech.edu/records/1292. MATLAB code developed during the current study are available from the corresponding author upon reasonable request. Author contributions: C.R. and M.G. designed research, performed research, analyzed data, and wrote the paper. The authors declare no competing interest. This article is a PNAS Direct Submission. Data deposition: All data discussed in the paper will be made available to readers upon request. The flow field data in Fig. 5 have been deposited at CaltechDATA, https://data.caltech.edu/records/1292. MATLAB code developed during the current study are available from the corresponding author upon reasonable request. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1908857116/-/DCSupplemental.
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