We report the honeybee’s propulsion at the air–water interface. Honeybees trapped on a water surface use their wings as hydrofoils, which means their wings generate hydrodynamic thrust. The surface wave and flow patterns generated around the bee are the first indication that the wings are used as hydrofoils. Furthermore, the water flow measured under a mechanical wing model showed that both net and oscillatory thrust contribute to their locomotion. Hydrofoiling highlights the versatility of their flapping-wing systems that are capable of generating propulsion with fluids whose densities span 3 orders of magnitude. This discovery inspires an aerial–aquatic hybrid vehicle. Moreover, the findings may have biological implications on the survival of water foragers and preflight locomotion mechanisms.

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.

It is difficult for insects to retain the aerodynamic function of their wings when they contact a water surface. The large-amplitude wing motion required for producing thrust demands hydrophobic wing surface or enough clearance between the wing and the water to prevent wetting. Insects that satisfy 1 of these 2 conditions can perform nonflying aerodynamic locomotion on the water surface, known as surface skimming (1⇓–3) (e.g., stoneflies, mayflies, and water lily beetles). Other insects whose wings touch the water surface and continue to be bounded, lose their ability to generate aerodynamic thrust. In this condition, it is unclear whether the insects are still capable of propelling with their wetted wings.

Water-collecting honeybees fly close to a water surface. When they fall on the water, they lose their aerodynamic ability. While their buoyant bodies provide flotation, the ventral side of the body and wings get wetted. They are not able to free their wings from the water surface, likely due to the relatively high wettability of their wings requiring large energy input for detachment (4). The contact angle (a quantitative measure of wettability) of the honeybee’s wing is 85° to 102° (5, 6) compared to 118° to 125° for the stonefly’s wing (5), which can be detached from the surface (1, 7). However, upon beating their wetted wings, honeybees display forward locomotion while producing a distinct wave and flow pattern on the water surface (Fig. 1 and Movies S1–S3). Here, we report that honeybees fallen on a water surface use their wetted wings as hydrofoils, generating positive thrust by transporting momentum to the water underneath the wing. Hydrofoiling highlights the versatility of their flapping-wing systems, which are capable of generating propulsion with fluids whose densities span 3 orders of magnitude.

Surface wave and flow visualization. (A) Honeybee’s locomotion on a water surface. The ventral side of the wings and body are attached to the water surface ( Movies S1 and S2 and SI Appendix, Fig. S1 ). (B) Wave pattern visualized using shadowgraph. The light and dark fringes indicate the wave crests and troughs, respectively. Wing-beat frequency, 69 Hz (Scale bar, 1 cm.) ( SI Appendix, Fig. S4 and Movie S3 ). (C) Surface streaming flow pattern generated by a horizontally tethered bee. (D) Water flow at 2.0 mm below the water surface generated by a constrained bee. Flow directions are schematically shown in the Bottom Left corner for C and D.

Results

Our data were obtained with honeybees (Apis mellifera) collected from a garden in Pasadena, California. To simulate its accidental fall, a honeybee placed in a plastic tube (1-inch-diameter opening) was gently tapped with the opening facing the water surface. The bees were flight capable before contacting the water surface. The temperature of the water was kept above 20 °C. Note that another factor that may contribute to a honeybee’s inability to detach its wings from the water surface could be the lowered temperature of its flight muscles, which operate optimally at a temperature much higher than the experimentally set water temperature (8). The depth of the water was maintained at 2.5 to 5 cm, which is much longer than the width of their wing (∼4 mm) and the wavelength of the water wave (∼5 mm) generated by the bee. The honeybees that fell dorsal side facing up were used to collect data. The bees that fell ventral side facing up were excluded from data collection. These bees were not able to upright themselves but still showed the capability to propel.