by Peishu Li

Flying is hard. It takes master control over the production, maintenance and redirection of forces, as well as a keen sense of the surrounding environment. Yet the physical challenges of flying have not stopped animals from taking to the skies. If anything, animals have evolved a myriad of flying behaviors to suit their individual needs. Some of the most agile fliers in nature are found in hawkmoths (Sphingiidae) and silkmoths (Saturniidae), two sister groups that differ considerably in life history and flight behavior. Hawkmoths are highly agile fliers known for long periods of hovering flight while feeding from flowers (Below, or watch on youtube HERE). In contrast, silkmoths pitch and bob across the air, an erratic yet useful strategy to out-maneuver predators like birds and bats.

How did hawkmoths and silkmoths evolve these distinct flight behaviors? Dr. Brett R. Aiello, a postdoctoral researcher in the lab of Dr. Simon Sponberg at the Georgia Institute of Technology, and a diverse team of scientists and engineers addressed this question in their latest work funded by a National Science Foundation Postdoctoral Research Fellowship in Biology. The researchers presented their findings in a talk titled “The evolution of two flight strategies in bombycoid moths” at the 2021 Society for Integrative and Comparative Biology virtual annual meeting. 

“We are finding a way to connect physics and museum collections to study the evolution of neuromechanical systems and reveal general principles of locomotion across a diverse group of animals,” Aiello said.

Aiello and colleagues started by asking if wing shape varied with different flight behaviors in hawkmoths and silkmoths. Through a collaboration with Dr. Akito Kawahara at the Florida Museum of Natural History and Dr. Chris Hamilton at the University of Idaho, the team took advantage of the incredible collections at the McGuire Center for Lepidoptera and Biodiversity. By mapping features of wing shape and size on a new moth family tree generated by Dr. Hamilton, the team found differences in both wing shape and size between the sister families. Silkmoths sport wings advantageous for maneuverability, while hawkmoths evolved wings that aid in power reduction and efficient force production.  

Yet wing shape is not all to flight. Unlike fixed-wing aircrafts, moths flap their wings to fly, so wing movement could also make a big impact on aerodynamic performance. Using high-speed video cameras, Aiello and colleagues saw that silkmoths mostly flap their wings up and down vertically, whereas hawkmoths move their wings more horizontally like hovering hummingbirds. 

More importantly, the moths could use wing movement to compensate for performance metrics they could not achieve with wing shape. For instance, while silkmoths did not evolve wings to help reduce power like hawkmoths’, silkmoths save energy through slow and large-amplitude wing strokes. Likewise, hawkmoths beat their wings faster than silkmoths to help with flight control, which they cannot achieve with their wing shape alone.

With observations on both wing shape and movement, Aiello and colleagues went further to investigate how much aerodynamic force moths experience during flight. To do this, Usama Bin Sikandar, an engineer and one of the co-authors on the study, used a mathematical model to calculate the total aerodynamic force acting on a flying moth based on empirical theories in physics. They found that in silkmoths, the direction of the aerodynamic force switches from pointing upward during one half of the wingstroke to pointing forward during the other half. This shift in force direction shows why silkmoths often pitch and bob in midair with frequent ups and downs.

Using the same modeling approach, Aiello and colleagues were also able to tweak input parameters in the model to directly examine how each parameter is related to flight performance, just like automobile engineers customizing different mechanical parts to see how they affect racecar speed. For example, by changing only wing movement patterns in the model, Aiello and colleagues found that the evolution of large and slow wings or the evolution of small and fast wings are two equally effective ways to generate sufficient vertical force for moths flight.

“The model allows us to more easily demonstrate the different knobs natural selection can act on to change the performance of an animal,” Dr. Aiello said, “now we can systematically go in and start changing the models, and we can turn these knobs that natural selection is also acting on.”

Through this modeling approach, Aiello and colleagues think animals are flexible to tune their aerodynamic performance by changing either wing shape, size, or movement patterns. This flexibility could remove potential restrictions on the range of wing shapes and sizes available to flying animals, and contribute to the diversity of wings we see today.

“When you increase the number of variables that natural selection can act on, the number of knobs you could turn, you could potentially unlock performance spaces or have combinations of morphology and movement that were previously unobtainable because of different kinds of constraints,” Aiello said.

 

Peishu Li is a second-year PhD student at the University of Chicago. He is broadly interested in the biomechanics and evolution of the mammalian feeding complex. Outside the lab, Peishu enjoys reading, roadtrips, and a whole lot of tennis.