insect flight

Most insects have four wings. The exceptions are the flies, which have two wings. In addition to performing the movements required for flight, the wings carry touch and strain receptors, so they can be considered as sensory structures too. The wings are driven by proportionally large power muscles, but a set of 13 smaller control muscles act on each wing joint to change the position and angle of the wing during the beat cycle.

In the two-winged flies, the hind wings have become converted into halteres, small club-shaped organs that oscillate up and down as gyroscopic sense organs during flight (Hengstenberg, 1998).

Unlike the relatively steady-state aerodynamics experienced by conventional aircraft wings, flapping wings such as insect wings experience rapid changes in airflow as the wing beats up and down - unsteady airflow.

Insects use a variety of special aerodynamic effects to facilitate flapping flight (Brookes, 1997). The techniques differ according to the size and organisation of the different insects, but include bringing their wings together at the top of the upstroke ('clap') and then peeling them apart, starting at the leading edges, to generate a circulation of air into the enlarging gap ('fling') and generating significant lift (Weis-Fogh, 1975). This process is made possible by having stiff leading edges to the wings and more flexible surfaces behind them. Pleating of the thin membranous wing can influence the patterns of deformation that occur under aerodynamic loading.

As the insect wing continues on its downstroke, a leading edge vortex develops along the top of the wing, significantly contributing to lift. In some species, the leading edge vortex spirals out towards the tip of the wing, while in others there appears to be no spanwise movement of the vortex.

At the bottom of the downstroke in some insects the vortex is 'shed' before the upstroke begins. Another boost in lift can be experienced at the beginning of the upstroke as the wing passes through the wake of the downstroke (Dickinson, 2001).

Transient bursts of lift can also be generated by the wings as they change their angle of attack at the top and bottom of each stroke - rotational lift (Wootton, 1999).

Subtle changes in the degree of wake capture and rotational lift can be used by the fly to induce turns (Dickinson, 2001). Free-flying butterflies use varied combinations of these unconventional aerodynamic mechanisms in successive wing beats to achieve takeoff, steady flight, manoeuvring, and landing - their apparently irregular 'fluttering' flight is in fact highly co-ordinated (Srygley and Thomas, 2002).

clap and fling

leading edge vortices

vortex shedding

wake capture

rotational lift

(For a picture of Dragonfly wings and a summary of dragonfly flight, click here.)

References:

Brookes, M. (1997) On a wing and a vortex. New Scientist, 24-27(Oct 11).

Dickinson, M. (2001) Solving the mystery of insect flight. Scientific American, 34-41 (Jun).

Dudley, R. (2000) The biomechanics of insect flight: form, function, evolution. Princeton University Press.

Grodnitsky, D.L. (1999) Form and function of insect wings: the evolution of biological structures. Johns Hopkins University Press.

Hengstenberg, R. (1998) Controlling the fly's gyroscopes. Nature, 392, 757-758.

Srygley, R.B., and Thomas, A.L.R. (2002) Unconventional lift-generating mechanisms in free-flying butterflies. Nature, 420, 660-664.

Weis-Fogh, T. (1975) Unusual mechanisms for the generation of lift in flying animals. Scientific American, 233(5), 80-87.

Wootton, R. (1999) How flies fly. Nature, 400, 112-113.