Mechanisms preventing animals to achieve buoyant flight

In the 300-million-year evolutionary history of animal flight, fliers have universally evolved to exploit the physical principle of drag, whereby an animal uses wings to achieve lift. However, the treehopper <i>Oeda</i> – an Amazonian insect exhibiting a grotesquely large balloon-like thorax – has been described in what may be the only account of an animal engaging in buoyant flight, whereby an animal fills body cavities with low-density air to float, akin to a hot-air balloon. We use <i>Oeda</i> as a study system to conduct a theoretical analysis exploring the biomechanics of buoyant flight and the mechanisms preventing its widespread occurrence in animals. We show that not even an organism with the unique and disproportionate dimensions of <i>Oeda</i> can buoy more than 1% of its body weight. Instead, our results suggest <i>Oeda</i>’s bloated thorax is a hindrance for flight that has selected for larger wings to compensate for the species’ disproportionate dimensions. Our analysis illustrates that animals may only achieve buoyant flight by filling cavities thousands of times larger than the body with heated low-density biogases. Still, the evolution of such traits is likely restricted by a variety of developmental and ecological factors that have prevented animals from evolving buoyant flight. Whether any animal has evolved the means to conquer buoyant flight remains to be found in nature.