Insects are the most successful group of animals on our planet by almost any measure, and the ability to fly is a major key to their success. However, flight in the natural world is also one of the most physiologically demanding forms of locomotion, requiring versatile, robust mechanical systems and adaptable control behaviors. And despite their modest appearance, insect wings are in fact highly adapted structures that serve multiple biomechanical and behavioral functions, all of which are critical for survival and reproduction.
My research is focused on understanding the form and function of features related to insect flight, including wing material properties and structural designs, within a broad ecological context that accounts for the real-world conditions, interactions, and performance tradeoffs that have shaped their evolution. I use a combination of experimental methods, comparative approaches and computational modeling to answer fundamental questions about the design and performance of flying insects.
Insect wings are flexible structures that passively bend and twist during flight. Click the image below (photo: Armin Hinterwirth) to see a high-speed movie of a hovering Manduca sexta hawkmoth, which exhibits pronounced wing deformations, especially at ventral stroke reversal when the wings transition from downstroke to upstroke.
Although our understanding of the mechanisms by which insects produce aerodynamic force has increased dramatically over the last several decades, nearly all of this research has ignored flexible wing deformations by modeling wings as simplified rigid plates. Are wing deformations an inherent liability of lightweight structures subject to large forces during flapping flight, or have insects evolved wing designs that promote dending dynamics that are advantageous for aerodynamic performance? I am addressing this question using both experimental and computational approaches.
To explore the aerodynamic consequences of wing flexibility, I robotically flapped real moth wings at natural wingbeat frequencies and measured their induced flows. I tested both naturally flexible and experimentally stiffened wings of Manduca sexta moths, and quantified induced flows using Digital Particle Image Velocimetry (DPIV). Below is a schematic of my DPIV setup - click to see a movie of a robotically-flapped wing and induced flows in action. (And head on over to the B-side to learn more about PIV!)
I found that flexible wings with biologically realistic deformations induced substantially greater airflows than stiffened wings, at orientations that are more favorable for lift production, suggesting that wing flexibility enhances aerodynamic force production (Mountcastle, A.M. and Daniel, T.L., Exp. Fluids, 2009).
However, other recent physical modeling studies have shown that rigid wings can produce greater lift than flexible ones, raising uncertainty about the adaptive significance of wing flexibility. Thus, I decided to approach this problem at the level of the whole organism for the first time by experimentally manipulating wing stiffness in live bumblebees and testing their resulting flight performance.
Wing flexibility in many insects may be enhanced by embedded resilin structures. Resilin is a flexible, rubberlike protein with an extremely high elastic efficiency that appears in the locomotory structures of many insects. The few studies that have mapped the distribution of resilin in insect wings report that the protein is typically found in joints between veins, where wings either fold at rest or are likely to flex during flight. One of the ongoing projects in our lab is to map the distribution of resilin in the wings of various insects, and try to understand the functional implications of individual resilin joints. I recently identified a resilin vein joint in the bumblebee wing, Bombus impatiens, that appears to play a particularly important role in chordwise wing flexibility, and developed a technique to artificially increase wing stiffness by applying a single piece of extra-fine glitter across the joint to splint it. Click the image below to see a demonstration of wing flexing before and after glitter application.
Motivated by the discovery that bumblebee wing stiffness can be altered by splinting just one vein joint with minimal addition of mass, I applied this technique to live bees, and used a load-lifting test to directly measure the contribution of wing flexibility to maximum vertical aerodynamic force production and load carrying capacity. Load lifting tests involved attaching a string of beads to a bee, and recording how much weight the bee was able to lift as it tried to ascend. Click below to see a sample load-lifting trial.
I found that bees with artificially stiffened wings showed a significant reduction in maximum vertical aerodynamic force production and load-lifting capacity, demonstrating for the first time in a live insect that wing flexibility enhances aerodynamic force production (Mountcastle, A.M. and Combes, S.A., Proc. R. Soc. B, 2013). These results also indicate that resilin plays an important role in determining overall wing stiffness and shaping wing deformations during flapping flight.
To explore in closer detail how wing flexural stiffness and actuation patterns affect flight forces, I used a combination of computational approaches. I first used a finite element method (FEM) to predict emergent kinematics of model moth wings subject to periodic forcing of the leading edge in elevation and pitch. I simulated a range of flapping wings, systematically varying both flexural stiffness and elevation/pitch phase. I then used a computational fluid dynamics (CFD) model to predict the flows and forces generated by the simulated wing motions. This model uses a distribution of point sources of vorticity (vortexlets). Click below for a brief video that introduces the vortexlet model and how it works.
Results from this study show that subtle spatial redistribution of flexural stiffness, or small shifts in the phase of actuation, can lead to dramatic changes in aerodynamic force production (Mountcastle, A.M. and Daniel, T.L., Bioinspir. Biomim., 2010). In general, wings with flexural stiffness distributions that decline logarithmically from leading to trailing edge produce greater lift and thrust forces than wings with uniform flexural stiffness distributions, providing evidence for the adaptive significance of logarithmically varying flexural stiffness distributions found in real insect wings. Moreover, I found that thrust, lift and their controllability are optimized at different phases of elevation/pitch driving kinematics, suggesting that flight forces in insects could be tuned by simply modulating the phase of activation of the thoracic flight muscles that drive flexible wings.
It's tempting to assume that evolution must have optimized wing designs for aerodynamic performance, but in fact there are myriad other selective pressures operating on wing morphology. The wings of many insects exhibit cumulative wear and tear over the course of their lifespan. In bumblebees (and likely many other species), wing wear is caused primarily by repeated collisions with vegetation during foraging activity, and area loss associated with wing wear has been shown to alter foraging behavior and increase mortality in both bumblebees and honeybees. I am interested in understanding the role that collisions play in wing evolution, as well as how wing wear affects flight behavior and performance.
In light of the significant effects of wing wear on survival and the high rate at which collisions occur in the wild, I wondered whether insect wings display biomechanical adaptations to help mitigate the damage associated with collisions? For example, the wings of wasps, as well as a number of other related insects, feature a flexible resilin joint or “costal break” along the distal leading edge of the wing, which could help mitigate wing damage by allowing the wing tip to crumple reversibly when it hits an obstacle. Curiously, however, the costal break is absent from the wings of bumblebees, a genus that shares a common ancestor with wasps and displays similar life history traits.
To examine the role of wing morphology in mitigating cumulative wing damage, I built a novel device to artificially induce wing wear in bumblebees and yellowjacket wasps. Since isolated wings dry out quickly, I attached live insects to a rotational motor and spun them at high frequencies, forcing their wing tip to repeatedly collide with the surface of a leaf. Click below to see a video of my wingwacking device in action, and the characteristic flexion of the costal break in a yellowjacket wing.
To test the role of the costal break in mitigating collision damage in wasp wings, I used a glitter splint to immobilize the joint. I found that the costal break does indeed play a critical role in mitigating collision damage, as wings with a stiffened costal break displayed far more damage after the same number of collisions (Mountcastle, A.M. and Combes, S.A., J. Exp. Biol., 2014). Click below to see a time lapse video contrasting the typical wing wear pattern for splinted v. unsplinted wasp wings.
Interestingly, bumblebee wings did not experience as much damage as would be expected based on their lack of a costal break, suggesting that these two species rely on alternative biomechanical strategies for mitigating wing damage. Yellowjacket wings have supportive veins that extend all the way to the wing tip, and therefore require a costal break to allow an otherwise rigid wing to buckle upon contact with an obstacle. In contrast, bumblebee wing veins are withdrawn to more proximal regions of the wing, leaving the entire distal region unreinforced and more continuously flexible than the yellowjacket wing tip, thereby reducing their need for a costal break. We're currently working on a phylogeny that maps the gain and loss of the costal break across bees and wasps, to improve our understanding of the factors and constraints acting on this unique morphological feature.