How Do Bumblebees Fly? The Myth That They Shouldn't Be Able To

Where did the bumblebee flight myth come from?

The claim that “according to science, bumblebees shouldn’t be able to fly” has been repeated for almost a century. Its origin is not certain, but the earliest known printed source is Le Vol des Insectes (Insect Flight), a 1934 book by the French entomologist Antoine Magnan. In the introduction he described how his assistant, the engineer André Sainte-Laguë, had applied the laws of air resistance used for airplanes to an insect and found that its flight should be impossible.

Magnan himself added that it should be no surprise that such calculations did not square with reality. The point was that the equations were wrong for insects, not that insects were doing something impossible. Other versions of the story credit an unnamed aerodynamicist at a dinner party, and the names of famous engineers such as Ludwig Prandtl have been attached to it, but none of these accounts is well documented.

Why the airplane math does not work

An airplane wing is rigid and moves steadily forward through the air. The air flows smoothly over it, and lift depends on speed and wing area. A bumblebee is heavy for its small wings, so if you treat its wing like a fixed airplane wing moving at the bee’s flying speed, it seems far too small to hold the bee up.

But a bee’s wing is not fixed. It flaps rapidly, twists, speeds up and slows down hundreds of times a second, and it works at a tiny scale where air behaves differently. At the size of an insect, the stickiness of air matters much more than it does for an airplane, and the airflow over the wing is constantly changing rather than smooth and steady. Calculations based on steady, smooth flow simply miss most of the lift.

How a bumblebee’s wings move

A bumblebee has four wings. Each forewing is joined to the smaller hindwing behind it by a row of tiny hooks called hamuli, so the two wings on each side beat together as a single surface.

When hovering, the bee sweeps its wings forward and back in a nearly horizontal plane, rather than up and down like a bird. At the end of each stroke it rapidly flips the wings over, so the leading edge always goes first and both the forward and the backward stroke push air downward. The wing tips trace a path a little like a flattened figure eight. In forward flight the stroke tilts so that the wings also push the bee ahead.

Hummingbirds use a similar back-and-forth stroke to hover, although their upstroke produces less lift than their downstroke; see can hummingbirds fly backwards?

The secret: leading-edge vortices

The key to insect flight is a swirl of air called a leading-edge vortex. Each wing moves at a steep angle to the airflow. On an airplane wing, such a steep angle makes the airflow break away and the wing stall, losing lift. On a flapping insect wing, the air rolls up into a small tornado-like vortex that sits on top of the wing just behind its front edge. The low pressure inside the vortex sucks the wing upward, adding a large amount of lift.

Because each stroke is short, the wing reverses before the vortex can break away, so the extra lift is not lost. Researchers first measured this in detail in the 1990s, using large robotic models of a hawkmoth and a fruit fly wing. They also found two other tricks: extra lift as the wing rotates at the end of each stroke, and wings recapturing energy from the swirling wake left by the previous stroke.

Smoke experiments with free-flying bumblebees in 2009 showed a separate leading-edge vortex over each wing. The researchers concluded that bumblebee flight is not especially efficient: these heavy-bodied bees rely on power and fast wingbeats rather than aerodynamic elegance.

Muscles that beat 200 times a second

No nerve can fire 200 times a second for long, so bumblebees use special asynchronous flight muscles. Instead of pulling directly on the wings, two sets of large muscles in the thorax alternately squeeze and stretch the box-like thorax itself, and the wings, attached to its rim, move with it like levers.

These muscles are triggered by being stretched. Once started, each set contracts every time the other set stretches it, so a single nerve signal keeps the cycle going for many wingbeats. The whole thorax vibrates at its natural frequency, like a plucked rubber band, which saves energy. Insects such as dragonflies, which have direct flight muscles attached to each wing, beat their wings far more slowly; compare the common green darner.

Warming up to fly

Flight muscles only work well when they are warm. A bumblebee’s thorax must reach about 30 °C (86 °F) before it can take off, which is why bumblebees are seen shivering on cool mornings. They uncouple the wings and contract the flight muscles against each other, producing heat without moving the wings. At an air temperature of 13 °C (55 °F), warming up takes about five minutes.

Their dense coat of hair keeps that heat in, and on hot days they can shunt surplus heat from the thorax into the abdomen to avoid overheating. This control over body temperature, studied in detail by the biologist Bernd Heinrich, lets queens of species such as the buff-tailed bumblebee forage on cold spring days when most insects stay grounded.

Bumblebee flight performance

Bumblebees are strong, capable fliers:

• Speed: foraging bumblebees have been recorded at ground speeds of up to about 15 m/s (54 km/h, 34 mph), with the help of the wind.
• Loads: workers regularly fly home carrying heavy loads of nectar in their honey stomachs and pollen on their hind legs.
• Altitude: in a 2014 study, alpine bumblebees from western China kept flying in a chamber with air as thin as at more than 9,000 m (29,500 ft), by making bigger wing strokes rather than faster ones.
• Wing wear: the wings are slightly flexible, which experiments suggest helps bees lift heavy loads, but repeated knocks against flowers and leaves gradually fray their edges, and foragers with badly worn wings tend to die sooner.

Far from being an aerodynamic puzzle, the bumblebee is a well-studied example of how insects solved flight in a different way from birds and airplanes. For how birds generate lift, see how do birds fly?