For the Bees Glowing paint may highlight the forces that make insects fly 

Sarah Simpson /  Scientific American May00

HOVERING HONEYBEE  proves much more difficult to describe mathematically than does a flying airplane, because insect airfoils flex as they flap.

A honeybee with bright red wings buzzes past a young woman's ear.

The bee's colorful airfoils would seem bizarre if the woman were sniffing an apple blossom, but rather she stands at a counter strewn with pipettes, a video camera, reference books, and a bee filled yogurt container-in a chemistry laboratory at the University of Washing ton. There graduate student Christina M. McGraw paints bee wings in hopes of unraveling the mystery of insect flight.

Insects are often touted as the world's most versatile and maneuverable flying machines. Many of them can hover, loop-even turn in a distance as short as their own bodies. Yet they shouldn't be able to get off the ground, at least not according to the current reaches of solvable mathematics. The laws of quasi-steady-state aerodynamics easily explain the lift capabilities of rigid airplane wings. But insect wings flap and bend, and mapping the flow of air around moving boundaries takes an enormous leap in complexity.

Researchers have attacked this perplexing paradox in several ways, even by building a robotic fly. The problem is that most of these experiments and calculations treat insect wings as if they are stiff and do not describe the forces acting on them. Studying flexible bee wings, painted with a dye that responds to changes in air pressure, may provide the answers.

McGraw's advisers-James B. Callis, Martin Gouterman and their coworkers -- perfected a paint in the early 1990s that can sense air pressure on airplane wings, a technology now exploited at aircraft testing facilities around the world. The paint relies on a chemical dye known as a platinum porphyrin, which phosphoresces a brilliant red under ultraviolet light. Oxygen in the air quickly quenches the glow, a bit the way water thrown on a fire kills the flames. Spots on the wings that experience the highest air pressure

phosphoresce the least, because more oxygen molecules are packed into denser air. By tracking the intensity of the glow, specialists can map out the forces acting on the wings.

Having discussed the mathematical subtleties of insect flight with Stephen Childress and Michael J. Shelley of New York University's Courant Institute, Washington physicist John S. Wettlaufer recently suggested to Callis that they use the same paint to study the flight dynamics of a hovering honeybee. The idea caught on, and bees became part of an ambitious $2.4-million collaborative project, funded by the National Science Foundation, to better understand how air and other fluids flow around moving boundaries-a phenomenon that applies to pumping heart valves as well as to flying insects.

It didn't take long for a problem to surface: the patented airplane paint made bees' wings too heavy and stiff to fly. The Washington group tried mixing new paint, but hordes of bees died from the solvents. Dissolving the fluorescent dye in a fluid that contains honeycomb wax turned out to be the best solution. Using a pipette, McGraw now dabs each wing of an anesthetized bee with a tiny dot of paint, which spreads into a film only about two microns thick. When the bees wake up, almost all of them can fly around the room. "Going from mostly dead bees to mostly flying bees made it all seem a lot more possible," McGraw says.

The team has cleared the first hurdle, but although the bees can fly, Michael H. Dickinson of the University of California at Berkeley points out that even the thin film adds weight and stiffness that may change the way the bee flaps its wings. McGraw hopes to abate Dickinson's concern with the help of Washington zoologist Thomas L. Daniel and his graduate student Stacey Combes. They will glue a painted bee to the tip of a cantilevered syringe needle and reflect a laser beam off the base to measure the lift and thrust created when the bee flaps its wings. If these force measurements match those of unpainted bees, the team will be sure it's on target. "As skeptical as I am, I sure hope it works," Dickinson says.

Recent advances in computational fluid dynamics and computer power will help the team achieve its ultimate goal. Childress, Shelley and their colleagues recently simulated the forces around a two-dimensional insect wing on a computer and have shown that vortices of swirling air produced in an upstroke actually add lift during the down-stroke. If the Washington experiment works, it should be able to show whether the same thing happens in real life.

Still, creating a pressure map of a bee wing in flight will require the detection of changing forces that, Gouterman cautions, may be too subtle. A bee's complete wing-beat cycle takes place in a mere five milliseconds, and even that rapid flapping generates only a hint of lift. But Gouterman says he also reacted with skepticism back when Callis first dreamed of developing pressure-sensitive paint to test airplanes. Now both researchers are enjoying royalties from their patents. "When Jim Callis gets ideas," Gouterman remarks, "he often gets them to work."