Mechanical description of Drosphila wing expansion

During its final transformation into its adult form, just after hatching from its pupal shell, an insect unfolds its wings within minutes. The wings expand rapidly from a compact, pleated structure to a plane that then solidifies to generate rigidity. We study wing expansion in Drosophila melanogaster. Expansion is regulated by increasing internal pressure and injecting a non-Newtonian viscous fluid (hemolymph) into a folded deployable structure under hormonal control (Bursicon). We first characterize the unfolding kinematics through macroscopic observations. Using optical microscopy imaging, we describe the shape of the initial origami-like folded wing and its relationship to the final network of veins. We then image fly wing sections using transmitted electron microscopy (TEM) to study the morphological evolution of the wing cross section at different stages of expansion. We use micro-tomography (micro-CT) to gain insight into the 3D structure of the folded wings as well as their internal structure. Next, we quantify the pressure and fluid flow in vivo in the insect during wing expansion. We inject fluorescent particles to follow the hemolymph flow and study the global and local characteristics of the flow in the wings. Mechanical traction tests of the wing allow us to determine its elastic properties. Finally, we combine scaling analysis, numerical simulations and experiments to build a fundamental understanding of the wing expansion dynamic.