A Sticky Situation for Ants, Bees: Biologists Look at How Insects Adhere to Various Surfaces

AMHERST, Mass. - University of Massachusetts biologist Elizabeth Brainerd is part of a team that recently completed a study on how certain types of ants and bees are able to walk on vertical surfaces or even upside-down. The study, conducted in conjunction with Walter Federle and Bert Höldobler of the University of Warzburg, Germany, and the late Thomas A. McMahon, of Harvard University, was published in a recent issue of the Proceedings of the National Academy of Sciences. The findings have implications not just in the field of biology, but also in the development of miniature robots used in medical procedures.

Thestudy, which included taking videotapes of insects scurrying along glass plates, focused specifically on honeybees and Asian weaver ants. The adhesive organs in these insects are quite different from those of animals such as geckos and most other insects, Brainerd notes. "Geckos have sticky pads on their feet, which peel off at the end of each step. It''s a relatively static system," she said. "The adhesive organs in ants and bees are much more dynamic."

The feet of ants and bees are surprisingly complex structures, says Brainerd. Each foot, viewed through a microscope, has a pair of claws that resemble a bull''s horns, with a sticky footpad called an arolium positioned between the claws. When the insects run along a surface, she explains, the claws try to grasp the surface. If the claws are unable to catch onto the surface, they retract and the footpad comes into action. The footpad quickly unfolds and inflates with blood, protruding between the claws and enabling the adhesive pad to stick to the surface. The footpad then deflates and folds back. The entire process takes just tens to hundredths of a second, and is repeated with each step, rapid-fire, as the insects skitter along. In addition, the footpad secretes a fluid that allows the insects to adhere to smooth surfaces, "the same way a wet piece of paper can stick to a window," says Brainerd. The dynamic nature of the arolium provides varying levels of stickiness, depending on the surface.

Furthermore,researchers found that the claw-flexor tendon not only retracts the claws on a smooth surface, but is responsible for moving the footpad into place. The system, a combination of mechanics and hydraulics, has intrigued robotics engineers who design tiny robotic devices used in the medical field.

Elizabeth Brainerd can be reached at 413/545-0957 or brainerd@bio.umass.edu.