Quantifying the aerodynamic power required for flight and testing for adaptive wind drift in passion-vine butterflies Heliconius sara (Lepidoptera: Nymphalidae)

Authors: Srygley, Robert; DUDLEY, ROBERT - Smithsonian Tropical Research; HERNANDEZ, EDGAR - Universidad Del Rosario, Columbia; KAINZ, FRANZ - University Of Utah; RIVEROS, ANDRE - Universidad Del Rosario, Columbia; ELLINGTON, C. - University Of Cambridge

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2023
Publication Date: 1/21/2023
Citation: Srygley, R.B., Dudley, R., Hernandez, E.J., Kainz, F., Riveros, A.J., Ellington, C.P. 2023. Quantifying the aerodynamic power required for flight and testing for adaptive wind drift in passion-vine butterflies Heliconius sara (Lepidoptera: Nymphalidae). Insects. 14(2). Article 112. https://doi.org/10.3390/insects14020112.
DOI: https://doi.org/10.3390/insects14020112

Interpretive Summary: Theory predicts that flying animals maximizing their migratory distance should adjust their airspeeds for headwinds and tailwinds. This energy-conserving adjustment in airspeed has been qualitatively demonstrated in migratory dragonflies and butterflies. However it is based on the theory that as an insect increases its airspeed, the power required to fly increases exponentially above a minimum power velocity. Here we sought to quantify the aerodynamic power required to fly at different speeds in a migratory insect Heliconius sara, and predict the adjustment in airspeed necessary to minimize energy consumption in headwinds or tailwinds. We filmed butterflies as they migrated naturally using two high-speed video cameras. Simultaneously we measured the butterflies’ airspeeds across the Panama Canal by pacing them with a boat and using an anemometer to measure the speed of the boat through the air at the same height as the insects. We captured the same butterflies to measure weight distribution and the shape of the body and wings. We found that the power required to fly increased exponentially from a minimum power velocity of 0.9 m/s. When no wind was present, the insects were predicted to maximize their flight distance by adopting a velocity of 2.25 m/s. Faced with a headwind, the butterflies increased their airspeed in accordance with that predicted, and given a tailwind, the butterflies decreased their airspeed. This is the first quantitative demonstration that migratory butterflies adjust their airspeed even while flying over water, demonstrating sophisticated long-distance optimization of their fuel expenditure for the distance flown. By coupling environmental and aerodynamic analyses of migratory insects, we have demonstrated behavioral adjustments to maximize their capacity to migrate long-distances. This approach can be used to improve our ability to predict insect movement between agricultural patches.

Technical Abstract: Although theoretical work on optimal migration has been largely restricted to birds, relevant free-flight data are now becoming available for migratory insects. Here we report, for the first time in passion-vine butterflies, that Heliconius sara migrate directionally. To test optimal migration models for insects, we quantified the aerodynamic power curve for free-flying H. sara as they migrated across the Panama Canal. Using synchronized stereo-images from high-speed video cameras, we reconstructed three-dimensional flight kinematics of H. sara migrating naturally across the Panama Canal. We also reconstructed flight kinematics from a single-camera view of butterflies flying through a flight tunnel. We calculated the power requirements for flight for H. sara over a range of flight velocities. The relationship between aerodynamic power and velocity was “J”-shaped across the measured velocities with a minimum power velocity of 0.9 m/s and a maximum range velocity of 2.25 m/s. Migrating H. sara did not compensate for crosswind drift. Changes in airspeed with tailwind drift were not significantly different from those predicted to maximize the migratory range of the insects.