In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Quantitative Biosciences
in the School of Biological Sciences

Ethan Wold

Defends his thesis:
Evolution of rapid wingbeats in insects through supra-resonant elasticity and actuation

Wednesday, April 16, 2025
12:00pm Eastern
Location: Howey Physics Building N201-202
Zoom: https://gatech.zoom.us/j/99358246173

Advisor: 
Dr. Simon Sponberg
School of Physics
Georgia Institute of Technology

Committee: 
Dr. Nicholas Gravish
Department of Mechanical and Aerospace Engineering
University of California – San Diego

Dr. Gregory Sawicki
Woodruff School of Mechanical Engineering
Georgia Institute of Technology

Dr. Saad Bhamla
School of Chemical and Biomolecular Engineering
Georgia Institute of Technology

Dr. Flavio Fenton 
School of Physics
Georgia Institute of Technology

Abstract:
     Nature’s fastest fliers, swimmers, and runners have evolved to generate and control
mechanical power over very short timescales to move around the world. Insects push this
form of locomotion to the extreme, generating wingbeats across three orders of magnitude
in wingbeat frequency. The presence of elasticity in the thorax of insects gives them
resonant mechanics, suggesting that insects may flap at their resonant frequency to fly more
efficiently. For fast-flapping insects like bees and flies, specialized stretch-activated flight
muscles self-excite to limit cycle oscillations at a frequency that is influenced by resonance.
However, a lack of direct, comparative studies of insect exoskeleton and muscle limits our
understanding of whether and how insect flight systems are broadly tuned to resonance.
This work explores how insect wingbeat frequencies are influenced by and emerge
from morphology, elasticity, and actuation. In Aim 1, we explore how the material
properties of insect exoskeleton behave under non-sinusoidal conditions typical of flight.
In Aim 2, we measured resonant properties comparatively across moths that vary in
wingbeat frequency, illuminating which features of the flight system change to enable
favorable resonant mechanics. In Aim 3, we use materials testing and a biophysical
model of stretch-activated muscle to show that fast-flapping insects’ wingbeat frequencies
are dictated by interplay between muscle and mechanical timescales. Finally, in Aim 4,
we develop an experimental paradigm for giving muscle novel physiological properties in
closed-loop called the physiological dynamic clamp. We use it to uncover the minimal
physiology needed to elicit transitions in flight actuation mode in real flight muscle, which
have enabled wingbeat frequency diversification in insects over evolutionary time.