Insect wing design: evolution and biomechanics

昆虫是地球上种类最多的动物，飞行可能是这一成功的关键。然而，尽管经历了数亿年的进化，昆虫的翅膀并没有收敛到单一的最佳形状。取而代之的是，当今世界上有一系列非同寻常的翅膀形态(甚至更多的化石)，但从根本上说，它们都执行着相同的任务--使飞行成为可能。这让我不禁要问，为什么没有最适合拍打飞行的单一机翼形状？答案很可能在于各种局部最优解决方案，特别是针对每种昆虫一生中所承担的任务进行调整。飞行任务对每一种昆虫来说都是独一无二的，因此对机翼形态和运动学的选择压力也因物种而异。蜻蜓要用机翼捕捉猎物，并与竞争对手展开空战，必须速度快、机动性强。相比之下，死头鹰蛾在欧洲迁徙，袭击蜂巢。它们的效率必须很高，因为在迁徙过程中能源非常宝贵，但在蜂房里，它们也必须足够强壮，能够抵御蜜蜂的攻击。了解100多万种描述的飞行昆虫的形态是不可行的，但贯穿其中的趋势对空气动力学工程来说是令人兴奋的，因为它们展示了针对特定要求的解决方案，这些解决方案已经被尝试、测试并被证明是成功的。我的研究试图理解昆虫翅膀形状如何以及为什么在空气动力学性能的巨大选择压力下会有这样的变化，以及为什么在生态位之间转换时形态会发生变化。检验这一点的最好方法是观察趋同进化的例子，这些物种具有相似的生态和形态，但起源于不同的分类分支。选择彼此完全不相关的物种可以区分机翼形状的各个方面，这些方面是设计优化的一部分，而不是仅仅因为它们的历史起点而造成的。因此，我的实验利用比较的方法来评估整个班级的代表性物种。在我的研究计划的第一个轨道上，博士后将直接测量飞行昆虫的空气动力学输出，因为了解空气在机翼周围和尾流中移动的速度和方向是至关重要的。通过在空气中播撒轻雾，并用脉冲激光照射颗粒，将计算出被拴在风洞中的昆虫周围的气流速度。这项技术被称为数字粒子图像测速，是工程师研究复杂流动的首选技术。最近，我成功地将这项技术应用于飞行昆虫，尽管它们体型小，翅膀拍打的频率很高。昆虫的翅膀上没有肌肉。扑翼周期的所有变形复杂性要么由机翼铰链上的肌肉主动控制，要么由机翼结构上的惯性力和气动力被动控制。气动输出是机翼运动的结果，因此了解机翼在拍打过程中的形状变化是至关重要的。在我的研究的第二个轨迹中，一个博士生将记录来自相同代表性昆虫的个体的运动学。学生将通过人工选择不同形态的果蝇品系(例如，更细长的翅膀)并表征新变种的飞行性能，来测试关于机翼形状在生态学中的作用的预测。同时，学生将通过根据飞行性能选择菌株，并测量由此产生的翅膀形态修改来验证他们的结果。这两个轨道的输出将是：1)从生物力学适应的角度解释昆虫翅膀形状的多样性；2)计算流体力学研究的详细运动学数据；3)工程师建造昆虫大小的飞行器的明确设计指南。

Insects are the most diverse order of animals on earth and flight may be the key to this success. However, despite hundreds of millions of years of evolution, insect wings have not converged on a single optimal shape. Instead, there is an extraordinary range of wing morphologies visible in the world today (and even more fossilized), yet fundamentally, they all perform the same task - to enable flight. This led me to ask 'why is there no single wing shape that is best-suited to flapping flight?'The answer may well lie in assorted locally optimal solutions, specifically adapted to the tasks each insect undertakes during its life. The mission-profile of flight is unique for each insect species and so the selection pressures on wing morphology and kinematics is also species specific. A dragonfly that catches its prey on the wing and engages in aerial combat against rivals must be fast and manoeuvrable. Contrast this with the death's-head hawkmoth, migrating across Europe raiding bees' nests. They must be highly efficient since energy is at a premium during migration, but also robust enough to withstand attacks from bees when in their honey-stores. Understanding the morphologies of over a million described flying insect species is unfeasible, yet trends run through them which are exciting for aerodynamic engineering because they show solutions to specific requirements that have been tried, tested, and proven to succeed.My research seeks to understand how and why insect wing shapes have such variation despite intense selective pressure for aerodynamic performance, and why morphologies change when transitioning between ecological niches. The best way to examine this is to look at examples of convergent evolution, species which have similar ecology and morphology, yet originate from disparate taxonomic branches. Selecting species which are quite unrelated from one another allows discrimination of the aspects of wing shape which are part of design optimisation as opposed to those which are simply due to their historical starting point. My experiment therefore utilizes a comparative approach to evaluate representative species from across the class.In Track 1 of my research programme, a Postdoc will measure the aerodynamic output of flying insects directly, because it is essential to know how fast and in which direction the air is moving around the wings and in the wake. Flow velocities will be calculated around insects tethered in a wind tunnel by seeding the air with a light fog, and illuminating the particles with pulsing laser light. This technique is called Digital Particle Image Velocimetry and is the technique of choice for engineers studying complex flows. Recently, I successfully applied the technique to flying insects despite their small size and high wingbeat frequencies.Insects have no musculature in their wings. All the deforming complexities of the flapping cycle are controlled either actively by muscles at the wing hinge, or passively by inertial and aerodynamic forces on the wing architecture. The aerodynamic output is a result of wing motion so it is vital to know how the wing shape changes during flapping. In Track 2 of my research, a PhD student will record the kinematics of individuals from the same representative insects. The student will test predictions about the role of wing shape in ecology, by artificially selecting strains of fruit fly for alternate morphologies (e.g. more slender wings) and characterising the new morphs' flight performance. Simultaneously, the student will validate their results, by selecting strains based upon flight performance, and measuring the resulting modification in wing morphology.The output from these two tracks will be: 1) an explanation for the diversity of insect wing shapes from the perspective of biomechanical adaptation; 2) detailed kinematic data for Computational Fluid Dynamics studies; 3) clear design guidelines for engineers constructing insect-sized vehicles.