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Forward flapping flight in insects

The study of insect flight has a long history, but the focus has mainly been on the aerodynamics of hovering flight. However, some insects can reach impressive flight speeds, there are for example large beetles that can fly at up to 8 m/s. Insect flight is generally characterized by high wing beat frequency, low Reynolds number (the ratio of inertial and viscous forces) compared to most birds and bats, and wings that stay fully extended during the wing entire wing beat.

The low Reynolds number of insect flight suggests rather low lift to drag ratios and relatively high body drag and as a consequence insects need to produce a lot of thrust. The inability to adjust wing length (as otherwise done by birds and bats) and the high wing beat frequency result in an upstroke mainly generating thrust during forward flight. However, the high diversity in insect morphology and behaviour suggests a potential for large variation in the aerodynamics of insect flight.

Flying insects typically possess two pairs of wings, which allow for aerodynamic interactions between the wings. One of our interests concerns how these interactions may affect the performance of the animal. In beetles, the front wing pair has evolved into short, hardened structures, the elytra, which protect the second pair of wings and the abdomen. This allows beetles to exploit habitats that would otherwise cause damage to the wings and body. Many beetles fly with the elytra extended, suggesting that they influence aerodynamic performance, and in the first wake study of a beetle during fast forward flight we have shown that the presence of the elytra increases vertical force production by approximately 40 %, indicating that they contribute to weight support.

The wing-elytra combination creates a complex wake compared with other studied animal wakes. At mid-downstroke, multiple vortices are visible behind each wing. These include a wingtip and an elytron tip vortex with the same sense of rotation, a body vortex and an additional vortex in between the two tip vortices of the opposite sense of rotation to the tip vortices. This latter vortex reflects a negative interaction between the wing and the elytron, suggesting that the extra weight support of the elytra comes at the price of reduced efficiency. As expected, the upstroke generates mainly thrust. The result thus pinpoints a potential evolutionary conflict between aerodynamic performance and selection for other uses of the wings. Understanding the factors influencing the performance of insect flight is one of the main concerns in our lab.

A characteristic of many insects is the use of unsteady aerodynamic mechanisms to boost the force production. The most commonly used mechanism is the leading edge vortex (LEV), generally illustrated as a single vortex attached to the wing throughout the downstroke. Although a commonly studied phenomenon, LEVs have until recently not been studied quantitatively in freely flying insects. Our measurements, the first on freely flying insects, of the flow above the wing of hummingbird hawkmoths (at 1-2 m/s) showed multiple simultaneous LEVs of varying strength and structure along the wingspan. Due to the high force demands, LEVs have mainly been studied during hovering flight and less commonly during forward flight, but our study shows that also during forward flight insects may use LEVs to boost their force production. At the inner wing there is a single, attached LEV, while at mid wing there are multiple LEVs, and towards the wingtip flow separates. The complexity of the LEV suggests a bursting behaviour, which in flapper studies have been associated with increased force production. The strong and complex LEV suggests relatively high flight power in hawmoths. In addition, the variable LEV structure may result in variable force production, which should influence flight control.

Insect flight has traditionally mainly been studied using tethered insects, and although sometimes necessary, the main focus of AFL is to explore insect aerodynamics using freely flying insects. The high diversity of insects promises novel insights adaptations in morphology and behaviour relating to aerodynamic performance.

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Movie of a dung beetle in tethered flight in the wind tunnel.