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Insects show diverse flight kinematics and morphologies reflecting their evolutionary histories and ecological adaptations. Many silk moths use low wingbeat frequencies and large wings to fly and display body oscillations. Their bodies pitch and bob periodically, synchronized with their wingbeat cycle. Similar oscillations in butterflies improve weight support and forward thrust while reducing flight power requirements. However, how instantaneous body and wing kinematics interact for these beneficial aerodynamic and power consequences is not well understood. We hypothesized that body oscillations affect aerodynamic power requirements by influencing wing rotation relative to the airflow. Using three-dimensional forward flight video recordings of four silk moth species and a quasi-steady blade-element aerodynamic model, we analysed the aerodynamic effects of body and wing kinematics. We find that the body pitch and wing sweep angles maintain a narrow range of phase differences, which enhances the angle of attack variation between each half-stroke due to increased wing rotation relative to the airflow. This redirects the aerodynamic force to increase the upward and forward components during the downstroke and upstroke, respectively, thus lowering overall drag without compromising weight support and forward thrust. Reducing energy expenditure is beneficial because many adult silk moths do not feed and rely on limited energy budgets.
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The evolution of two distinct strategies of moth flight
Across insects, wing shape and size have undergone dramatic divergence even in closely related sister groups. However, we do not know how morphology changes in tandem with kinematics to support body weight within available power and how the specific force production patterns are linked to differences in behaviour. Hawkmoths and wild silkmoths are diverse sister families with divergent wing morphology. Using three-dimensional kinematics and quasi-steady aerodynamic modelling, we compare the aerodynamics and the contributions of wing shape, size and kinematics in 10 moth species. We find that wing movement also diverges between the clades and underlies two distinct strategies for flight. Hawkmoths use wing kinematics, especially high frequencies, to enhance force and wing morphologies that reduce power. Silkmoths use wing morphology to enhance force, and slow, high-amplitude wingstrokes to reduce power. Both strategies converge on similar aerodynamic power and can support similar body weight ranges. However, inter-clade within-wingstroke force profiles are quite different and linked to the hovering flight of hawkmoths and the bobbing flight of silkmoths. These two moth groups fly more like other, distantly related insects than they do each other, demonstrating the diversity of flapping flight evolution and a rich bioinspired design space for robotic flappers.
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- Award ID(s):
- 1612862
- PAR ID:
- 10555710
- Publisher / Repository:
- Journal of the Royal Society Interface
- Date Published:
- Journal Name:
- Journal of The Royal Society Interface
- Volume:
- 18
- Issue:
- 185
- ISSN:
- 1742-5662
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1098/rsif.2021.0632
