If you think that fruit flies are tiny, please reconsider. Spectacular radiations of miniaturized beetles, flies and wasps dominate the insect fauna and are characterized by body lengths well below that of the biologist's friend Drosophila melanogaster (∼3 mm). Most remarkably, the smallest flying insect described to date has a length of just 0.16 mm. But just how does flapping flight work at such small scales? The viscous effects of air on a wing increase at smaller sizes and a 1 mm insect moves through the air as would a bumblebee move through mineral oil. Lift forces generated by swirling air around the wing are accordingly impeded given the relative stickiness of the fluid, and alternative mechanisms of force production must be at play. How then do very small insects create the aerodynamic forces necessary to offset body weight and also to generate horizontal thrust to intentionally move about?
Xin Cheng and Mao Sun, from Beihang University, China, took on this question by describing the detailed kinematics of forward flight in the tiny wasp Encarsia formosa, which has a wing length of only 0.5 mm. Writing in 1973 in JEB, Torkel Weis-Fogh first described the novel clap-fling aerodynamic mechanism utilized by this species; Cheng and Sun have now built on their earlier computational work on hovering in E. formosa to analyze its forward flight kinematics and associated forces, albeit at the humble airspeed of ∼0.3 m s−1. They identified a distinctive pattern of wing motions very different from that of larger insects (including the fruit fly). First, the wings accelerate backward at very high incidence angles relative to oncoming flow (‘impulsive paddling’), and then close up on the right and left sides while at the same time moving upward. Then in the downstroke, the wings quickly rotate about their posterior edges (which Weis-Fogh referred to as the ‘fling’) and sweep downward and forward, again moving at high incidence angles to the surrounding air.
But what do these odd features of wing flapping yield in terms of force production? Cheng and Sun used computer simulations to reveal two means by which the effects of small size and fluid stickiness were largely overcome. Firstly, the fast backward acceleration of the wings at high incidence to the air produces a large thrust but also some vertical force supporting the body weight. Secondly, the ‘flinging’ motion produces a low-pressure region of rapidly swirling air which persists through the downstroke, creating the majority of the vertical force required to offset gravity. Thus, the insects employ distinctive wing motions to offset the otherwise formidable effects of increased viscosity. The associated forces last just milliseconds but, at a wingbeat frequency of ∼350 Hz, are clearly sufficient for routine flight. They may be even more effective during maneuvers, a topic destined for future investigation.
Miniaturization is one of the dominant themes in insect evolution and largely underpins the high extant diversity of small insects (and also of their parasitoids). This computational study of aerodynamics in one such lilliputian species suggests that additional fluid dynamic novelties await discovery and analysis by flight biomechanists. Fringed wings with much reduced supporting veins are also well known among the miniaturized fliers, but have largely unknown aerodynamic consequences. Often at the mercy of gentle breezes, these insects live in physical, behavioral and ecological domains supremely distant from our own personal experiences. This aerodynamic study should help us to close that conceptual distance.
