A small invertebrate animal that has a segmented body, six legs, and two or four wings.

A popular anecdote relates that in the 1930’s a student of German aerodynamicist Ludwig Prandtl was asked by a biologist at a dinner party to estimate the lift of a bumblebee. To make this estimate, techniques similar to those commonly used to predict the lift of aircraft wings were used. To the surprise of the dinner guests, the results suggested that bumblebees should not be able to fly, although this is obviously not the case.

Insects are, in fact, the most accomplished of flying animals, with unsurpassed flying abilities. Insects can hover, fly backward, and even perform somersaults. Because insects are small in size and their wings typically move at relatively low speeds, the air through which an insect flies has the same relative feel as syrup to a human.

An important parameter in aerodynamics is the Reynolds number, which relates the motion of the air to its viscosity, where the viscosity is a measure of the stickiness of the air. Thus, for air, a low Reynolds number flow generally indicates that the airspeed is low, and the effects of viscosity are significant. As insects move their wings at comparatively low speed, they fly at very low Reynolds numbers. Few airfoils, or shapes of wings in profile, are capable of creating lift efficiently at the Reynolds numbers of an insect wing in flight or hover.

To circumvent the problem of flying at very low Reynolds numbers, insects have developed sophisticated flight mechanisms for developing lift, mechanisms of which engineers in the 1930’s were unaware. This is one of the reasons for the problems with the analysis by Prandtl’s student. From the 1970’s to the 1990’s, a clearer picture of insect flight techniques developed, based on the data from numerous comprehensive experimental studies and some computer-based numerical analysis.

A wing in steady flight, such that its speed and angle of attack, or the angle of the wing to the oncoming airstream, are unchanging, develops lift due to the air flowing smoothly over its surfaces, and being deflected downward. This type of lift is called attached flow lift. An aircraft wing can also develop lift from the formation of a tornado-like vortex above the wing surface. Such leading edge vortices can clearly be seen above the wings of aircraft, such as the Concorde, at takeoff or landing on humid days. The vortex tends to pull up the wing, thereby increasing its lift. This type of lift is called vortex lift. An aircraft can use both attached flow lift and vortex lift, such that the two lift values can be added together.

Due to viscosity on the wing’s surface, the speed of the air is zero. This represents a condition referred to as the no-slip condition. However, at some distance above the wing’s surface, the airspeed reaches that which would occur if the flow had no viscosity. The region between the surface and this point is referred to as the boundary layer. The nature and behavior of this boundary layer has a significant impact on the ability of a wing to develop lift at very low Reynolds numbers. The boundary layer can either be laminar, turbulent, or transitional from laminar to turbulent. A laminar boundary layer is composed of air moving in orderly lines. A turbulent boundary layer is composed of air moving close to the airfoil surface in swirling motions. A laminar boundary layer is prone to separate from the airfoil surface far more easily than a turbulent boundary layer. At the very low Reynolds numbers at which insects fly, the boundary layer over their wings is always laminar. This causes the air to separate very easily over their wings, with the result that it is difficult for insects to develop enough attached flow lift to support their weight. However, for some insects, even if the boundary layer did not separate, the attached flow lift developed by the insect wing would still not be sufficient to support the insect’s weight. It is thus necessary for insects to use other lift mechanisms, such as vortex lift, to create enough lift to stay aloft and maneuver.

An insect wing is composed of a thin membrane, which uses veins to provide strength. The wings of insects reflect the largest differences between the different insect orders. Some insects have two wings, and others four, but they all use similar flying techniques. Several insects, such as houseflies, originally had four wings. However, evolutionary development modified the two hind wings into small stumps, or halteres, which stabilize the housefly. Some insects with four wings, such as the dragonfly, beat their front and rear wings out of phase, meaning that the two sets of wings do not move forward and backward simultaneously. Other insects that also have four wings, such as butterflies, beat the front and back wings in phase, so both sets of wings flap effectively as a single set of wings.

To create the flapping motion, insects can use either of two muscle systems, direct or indirect. Direct muscles are attached to the wings and the bottom of the insect’s thorax. Separate muscles are used to raise and lower the wings. To use a flapping system with direct muscles, the insect’s brain must continually tell the flight muscles to relax and contract. Generally, insects that use direct muscles for flight cannot beat their wings very quickly because the brain has to coordinate the two sets of flight muscles for the insect to fly successfully. Indirect muscles are not directly attached to the wings. The indirect muscles consist of dorsoventral muscles, which connect to the top and bottom of the thorax, and the longitudinal muscles that run from the front to the back of the insect’s thorax. The insect’s wings are connected to the thorax by hinges. By alternating the contraction of the two muscle sets, the thorax of the insect begins to vibrate. The wings, attached to the thorax, begin to flap, with a contraction of the dorsoventricular muscles moving the wings upward. Contraction of the longitudinal muscles pulls the wings down.

One advantage of an indirect muscle system is that the insect’s wings can beat at very high frequencies, up to 1,000 times per second for a gnat. Flies typically beat their wings approximately two hundred times per second, whereas beetles may beat them about eighty times per second, and butterflies about thirty times per second. The major advantage of indirect muscles, though, is that they require fewer instructions from the insect’s brain than do direct muscles. Once the muscles are rhythmically contracting, they no longer need instructions from the brain. Houseflies use an indirect muscle system, whereas locusts use a direct muscle system. Insects using an indirect muscle system are far better fliers than those using a direct muscle system; the latter often appear clumsy in flight.

Meticulous experiments performed in the 1990’s clarified the methods that insects use to develop lift in the potentially unfavorable environment in which they fly. Experiments by Cambridge University zoologist Charles Ellington and colleagues showed that insects use attached flow lift as well as vortex lift, similar to the lift developed over highly swept wings such as those of the Concorde or numerous fighter aircraft. Computer simulation of the flow over a large moth, the hawkmoth, showed similar results.

Insects generally move their wings in a pattern that may resemble a figure eight or some variant of it. The plane in which the wings are moved backward and forward is the so-called stroke plane. A wing stroke begins with the wings almost touching above the insect’s body. The front of the wing is rotated down rapidly, and the wing is accelerated downward; this is the downstroke. For most insects, the wings are rotated through approximately 120 degrees in the stroke plane. The boundary layer, the thin layer of air adjacent to the wing surface that is affected by viscosity, separates from the wing surface at the front of the wing, and no longer conforms smoothly to the wing’s surface. Due to the motion of the insect wing, this boundary layer forms a tornado-like vortex above the wing. The vortex has the effect of increasing the lift of the insect’s wing by causing vortex lift to develop.

At the beginning and end of the insect’s wing stroke, the wing is rotated or flipped rapidly. Through careful timing of the point at which the wing is flipped, the insect is able to develop extra lift by causing the air over its upper surface to effectively speed up and that on the lower surface to slow down. This lift is similar to that developed by a spinning tennis ball. Some insects may also move their wings in such a fashion that they move through the wake, or energized air left behind by the previous wing cycle. As the insect wing moves through the air, air that comes into contact with the wing is set in motion and may begin to rotate. This air can then increase the lift of the wing if the motion of the wing is correct. For many insects, the angle of the stroke plane to the insect’s body is essentially fixed. Thus, for the insect to change from hovering to forward flight, it rotates its body, so that the force from the wings will generate both some lift to support its weight as well as some thrust to propel it forward. Generally, insects generate most of the lift to support their weight on the downstroke, where the wings are moved from above to below the insect’s body. The duration of the downstroke is also typically twice as long as the upstroke.

Another flight technique, referred to as the clap-and-fling technique by scientist Torkel Weis-Fogh, is used by some smaller insects such as Chalcid wasps. Initially, the insect’s wings are positioned above the body with the wings touching, as may be seen when a butterfly is at rest. The insect then rapidly draws the front, or anterior, edges of its wings apart, and then rotates the two wings down. The advantage of such a flight method is that both wings instantly develop maximum lift. Normally, when a stationary wing is initially accelerated, it takes some time for the lift developed by the wing to reach its final steady value. However, the clap and fling represents a brilliant biological adaptation for circumventing the reasons for delay. Presumably, due to wear on the wings from continual colliding, few insects actually use the clap and fling.

• Dickinson, M. H., F. O. Lehmann, and S. P. Sane. “Wing Rotation and the Aerodynamic Basis of Insect Flight.” Science 284 (1999): 1954-1960. A thorough paper giving detailed, if somewhat technical, explanations of the flight mechanisms used by flies.
• Dudley, R. The Biomechanics of Insect Flight: Form, Function, Evolution. Princeton, N.J.: Princeton University Press, 1999. An exceptionally thorough compendium of information relating to insect flight, with an exhaustive reference list.
• Ellington, C. P., C. van den Berg, A. P. Willmott, and A. L. R. Thomas. “Leading Edge Vortices in Insect Flight.” Nature 384 (1996): 626-630. A clear description of the leading-edge vortices that develop over insect wings and their importance to the overall flight of insects.
• Weis-Fogh, T. “Quick Estimates of Flight Fitness in Hovering Animals, Including Novel Mechanisms for Lift Production.” Journal of Experimental Biology 59 (1973): 169-230. A groundbreaking paper on insect flight that remains an excellent source of information on insect flight mechanisms, with considerable experimental data pertaining to various insects.

Aerodynamics

Animal flight

Bats

Birds

Forces of flight

Ludwig Prandtl