Colloquium: Miniature insect flight

Many of the existing winged-insect species are extremely small (wing $\text{length}\approx 0.3–4\mathrm{mm}$); they are referred to as miniature insects. Yet, until recently much of our knowledge about the mechanics of insect flight was derived from studies on relatively large insects, such as flies, honeybees, hawkmoths, and dragonflies. Because of their small size, many miniature insects fly at a Reynolds number ($\mathrm{Re}$) on the order of 10 or less. At such a low $\mathrm{Re}$, the viscous effect of the air is substantial: A miniature insect moves through the air as a bumblebee would move through mineral oil. The great importance of viscosity for miniature insects means that their flight relies on physical mechanisms that are different than those exploited by large insects. These differences range from the nature of the wing stroke to the structure of the wings, with some insects even using porous (bristled) wings to fly. Over the past decade, much work has been done in the study of the mechanics of flight in miniature insects: novel flapping modes have been discovered and new mechanisms of aerodynamic-force generation have been revealed; progress has also been made on fluid-mechanics-related flight problems such as flight power requirements and flight dynamic stability. This Colloquium reviews these developments and discusses potential future directions.