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.

在长达3亿年的动物飞行演化历程中，所有飞行类群均普遍演化出利用阻力（drag）的物理原理：即通过翅膀产生升力。然而，栖息于亚马逊地区的角蝉科<i>Oeda</i>属物种拥有怪异的巨型囊状前胸，被认为是目前唯一有记录的采用浮力飞行（buoyant flight）的动物——即通过体腔填充低密度空气实现漂浮，原理与热气球相仿。本研究以<i>Oeda</i>角蝉为研究对象，开展理论分析，探究浮力飞行的生物力学（biomechanics）机制，以及阻碍该飞行方式在动物中广泛演化的内在机理。研究结果显示，即便拥有<i>Oeda</i>这类独特且比例失调的躯体结构，其可提供的最大浮力也无法支撑超过自身体重1%的重量。反之，本研究结果表明，<i>Oeda</i>膨大的前胸实则对飞行构成阻碍，因此该物种演化出更大的翅膀以弥补躯体比例失调带来的飞行缺陷。本分析进一步证实，动物若要实现浮力飞行，仅能通过填充数千倍于躯体体积的腔体，并注入加热的低密度生物气体方可达成。但这类性状的演化极易受到多种发育与生态因素的限制，这也正是自然界中动物尚未演化出浮力飞行的核心原因。自然界中是否存在真正掌握浮力飞行的动物，仍有待进一步探索发现。