Multi-length scale mechanical investigation of the flying insect thorax

Abstract

Flying insects are small, efficient, and agile- all traits that engineers want to incorporate into designs for small flapping wing drones. Therefore, engineers study flying insects' adaptations to understand what makes them successful flyers. One such adaptation is indirect actuation. During indirect actuation, the flight muscles deform the thorax exoskeletal. Thorax deformations are translated into wing rotation via the wing hinge, where the wings attach to the exoskeleton. Indirect actuation may reduce the energetic cost of flight by allowing energy to be stored in the thorax during one part of the wing cycle and then used later. Researchers can model indirect actuation as a two-degree-of-freedom mechanical model where a parallel spring represents the combined stiffness of the thorax exoskeleton and indirect flight muscles, and a series spring represents the wing hinge stiffness. However, these stiffnesses have not been evaluated experimentally. Evaluating the thorax stiffness will help us better understand insect flight. I hypothesize that thorax stiffness depends on the flight muscle activation type. Insects with synchronous flight muscles convert one action potential into one wing flap, while those with asynchronous flight muscles can convert a single action potential into many wing flaps. In this thesis I compared the thorax stiffness of insects with synchronous and asynchronous muscle on multiple scales. On a microscale, I measured the elastic modulus of the thorax exoskeleton using nanoindentation. I found differences in the modulus gradient through the cuticle thickness and between thorax regions through between insects with synchronous and asynchronous muscle. On a macroscale, I first qualitatively compared the thorax stiffness for insects with synchronous and asynchronous. I found that insects with asynchronous muscle may rely more on their wing hinge for wing rotation. Next, I created a frequency response function to quantify the role of wing hinge resonance in flight. I found that both insects are flapping post-resonance. These studies improve our understanding of insect flight evolution by elucidating the connections between muscle activation, flight control, and flight energetics. With this knowledge, engineers can make informed decisions about which species they should mimic in their designs.