Volume 2, Issue 15 October 2005
Insect Flight, Sans Wings
Next month, UC Berkeley integrative biologist Robert Dudley will travel deep into the rainforests of Panama. What will he and his research team do during this field study? "Mostly we'll be tossing ants," he says. Dudley isn't joking. He's a leading expert in animal flight — bees, birds, hummingbirds, and even ants that manage incredibly sophisticated airborne maneuvers without wings.
Robert Dudley also conducts research on Southeast Asian flying lizards and the flight performance of hummingbirds and bumblebees at various elevations.
"I'm interested in the biodynamics of flight, including aerodynamics, physiology, form, structure, and evolution," Dudley says. "There are a lot of things up in the air, so the motto of our lab is essentially 'all taxa, all the time.'"
Dudley's trip this fall is the next step in a groundbreaking exploration of how wingless flight has evolved throughout myriad species in the animal world. The project began last year when insect ecologist Stephen P. Yanoiviak of the University of Texas Medical Branch was high in the forest canopy outside Iquitos, Peru, studying mosquitoes. Brushing some annoying ants off his arm, he noticed that they managed to land back on the tree trunk below. It turned out that the ants were capable of directed descent — they literally glided to their target.
Shortly after his initial observation, Yanoiviak called in Dudley and University of Oklahoma ant ecologist Michael Kaspari to study the surprisingly excellent pilots.
The Darth Vader of the ant world, Cephalotes atratus, lives in the tropical forest canopy of South and Central America. Almost a centimeter long, its long hind legs and flanged head shield may be the key to its newly discovered ability to glide back to its home tree after falling or dropping. (photo by Steve Yanoviak)
"The ants glide backwards but we don't understand the mechanisms," Dudley says. "They start tumbling and then swoosh... they land right where they want to be."
Meeting in Panama where the same species of ants are found, the three researchers captured high-speed video of the bugs falling. The special camera enabled them to slow down the footage to dissect the ants' subtle and fast motions. Apparently, the insects use their legs as flaps.
"As the ant falls away from the tree trunk, the left hind leg goes toward to the left to increase the drag and provide rotation as it comes in for landing," Dudley says. "The amazing thing is that you can cut off the hind legs and it'll still land on the target trunk. The arthropods have incredible structural redundancy."
To identify the minimal structures required for gliding, the researchers are surgically removing body parts and observing how flight is affected. In one such ablation experiment, they left the legs intact but removed the abdomen. Even with 30 percent of their bodyweight gone, the ants still managed to hit their targets, Dudley says. Just don't blind them.
Recently, Dudley and Yanoviak determined that the ants rely on the reflectance and color of the tree trunk to orient their trajectories as they fall. By hanging strips of colored cloth and "tossing the ants," as Dudley phrases it, they determined that the ants direct their descent toward lighter colors.
The researchers hung fabric strips in the forest as surrogate tree trunks to examine how color affects directed aerial descent behavior. (photo courtesy the researchers)
"In the forest, tree trunks are covered with lichen crusts that look white against the vegetation," Dudley says. "So the high contrast certainly provides a good visual target for the falling ants."
The researchers recently received a National Geographic Society grant to continue the gliding ant research in Amazonian Peru and Panama. By collecting more high-speed video, Dudley hopes to better understand the gliding mechanism. For example, even though the ants fall at nearly 4 meters per second at a steep angle and sometimes bounce off the trunk, they're capable of quickly turning around mid-air and trying again. These kinds of aerodynamic tricks are most likely achieved through precisely orchestrated movements of the legs, abdomen, and head.
While ant gliding has much to tell us about morphology and flight, the airborne ants will probably not provide much insight into the evolution of flying insects. That's because ants once had wings and lost them to history, Dudley explains. A more primitive taxon that never had wings but still exhibit directed descent, such as silverfish, would be more likely to reveal the evolutionary connection between gliding and winged flight, Dudley says.
The ant C. atratus is only one of many ant species found to glide when dropped from a branch high in the forest canopy. They may have developed this ability to avoid almost certain death if they were to land on the forest floor or in water that floods the Amazonian forest much of the year. These ants were photographed near Iquitos, Peru. (photo by Steve Yanoviak)
"Flight may have started out as a way of controlling landing which otherwise could be pretty bad if an insect falls," Dudley says. "And so gliding might be a pre-existing behavioral substrate for subsequent elaboration into morphological structures that could perhaps turn into wings."
The beauty is that silverfish can be easily reared in the laboratory. All of the necessary experiments, from ablation to high-speed video, could be conducted right on the Berkeley campus instead of far away in the field, he says. Time will tell if silverfish, a pest to most of us, might just hold the secret history of insect aerodynamics.
"The origins of flight are murky," Dudley says. "The fossil record is either not good or totally absent in this regard. But studying directed descent could give us a stunning experimental avenue into the evolution of insect flight."