And Yet It Moves . . .

In the late 19th-century, Russian composer Rimsky-Korsakov wrote a frantic orchestral interlude, “Flight of the Bumblebee,” for his four-act opera called The Tale of Tsar Saltan.  In the story, a magical swan changes a prince into a bumblebee so that he can fly away to visit his father.  Such flights of fancy seem to happen in operas.  But that’s not my point.  Today’s article takes a look at another kind of flight: the flight of the bumblebee.

Last year, while a friend and I noticed the up-and-down animated flight of a bumblebee (a lumbering flight, it was) over a small plot of wildflowers, he informed me that “it’s impossible, you know, for a bumblebee to fly.”  I was astonished since we were both looking at a flying bee immediately in front of us.

I could not help but recall the much-touted whisper of Galileo, “And yet it moves,” when he was forced by the Inquisition to recant his belief in Copernican theory that the sun was the center of the solar system and then to accept the Church’s false claim that the Earth was unmoving.

And yet there it moved – our bumbling, buzzing bee.

It seems a silly question to ask in retrospect: aerodynamically speaking, is it impossible for the bumblebee to fly?  Somehow the question has embedded itself in folklore, like so many other myths (for example, frog urine causing warts, bats getting entangled in your hair, the daddy-longlegs spider having deadly venom, elephants fearing mice, and numerous others).

According to one account on-line, the story was initially circulated in German technical universities in the early 20th century as a dispute between physical scientists and life scientists.  For example, Ernest Rutherford, the father of nuclear physics, once declared flippantly, “In science there is only physics.  All the rest is stamp collecting.”  That kind of assertion did not endear him to biologists and helped to initiate a long-lasting, agonizing dispute among scientists.  In response, biologists reveled at every opportunity to trounce their opponents.

Supposedly, during a wine-laden dinner, one biologist asked an aerodynamics expert about insect flight.  The aerodynamicist did a few calculations and found that bumblebees cannot generate enough lift to fly.  A bumblebee weighs nearly a gram, has a wing area of one square centimeter, and flies about one meter per second.  Thus, he replied, its wing area is too small in relation to its mass to provide enough lift at its flying speed.  The story then circulated far and wide about an inductive chasm between physical scientists and reality.  Once he sobered up, however, and reworked his equations, the aerodynamicist realized that the problem was a faulty analogy between bees and fixed-wing aircraft.

Bees’ wings are small relative to their body size and are built more like helicopters than like airplanes.  Rather than having an aerofoil shape to generate airflow over the upper wing surface, such as the wings of aircraft or birds, the flat wings of bumblebees are simply hinged like a door to flap up and down, generating just as much downthrust as they generate upthrust.  Bumblebees bring their wings together over the back so they clap, forcing out all the air between the wings to create a partial vacuum.  Atmospheric pressure then pushes air rapidly back into the void, flinging the wings apart and generating enough trailing edge turbulence to provide lift.

Bumblebees buzz because their wings clap!  The louder the noise, the better the vacuum and the more lift on the downstroke.  Further, the wings of bumblebees move faster than the firing rate of their nervous systems.  The thorax muscles that control wing movement do not expand or contract as much as they quiver when a nerve impulse fires.  Like a plucked guitar string, the stimulated muscle vibrates the wing up and down a few times until the next nerve impulse comes along.

Temperature regulation is critical to the flight of the bumblebee.  Apparently, bumblebees cannot fly if their muscle temperature drops below 30 degrees Celsius.  In bumblebee folklore, this little fact may have been extrapolated somehow to its supposedly impossible flight altogether.

Seeing is believing.  That idiom represents a core article of faith for us scientists as we use our senses and our instruments to reveal the secrets of the universe. Only physical or concrete evidence is acceptable.  Of course, “seen evidence” can sometimes be misinterpreted so we scientists also rely on repetition of results and peer review to construct our conclusions about natural phenomena.  The aerodynamicist in our story could see a flying bumblebee but then predicted its impossible flight through his faulty calculations and presumptions.  The biologist challenged, “And yet it moves.”  Healthy skepticism seems appropriate when confronting all societal myths as we piece together our understanding of the natural world and our roles in it.