Insects deploy sticky feet with precision

Some insects can hold an object 100 times heavier than they are–and they can do it while hanging upside down. For ants and bees, the secret to this power resides in adhesive footpads that clamp down tight and hold on to a smooth surface. But for years, scientists have wondered how these insects keep from tripping over their own sticky feet.

The footpad (upper center) of an Asian weaver ant emerges from behind two claws (sides) to make the insect stick in place. Federle

Now, they have the answer. Bert Hölldobler at the University of Würzburg in Germany and his colleagues have found that the pads can repeatedly retract into the foot and then unfold as an insect walks. What’s more, the insects deploy their footpads with a mechanical precision that has piqued the interest of robotics engineers, the researchers report in the May 22 Proceedings of the National Academy of Sciences.

Getting at the biomechanics of insect movement has required some dexterity on the part of the researchers themselves.

First, however, they went to the movies. Videotapes of ants and bees racing through plexiglass runways showed that when an insect walks, two claws at the front of each foot grip the surface and then begin to retract. If the surface is rough, the claws engage and the insect scrabbles along. If the surface is smooth, the hinged claws retract further and adhesive footpads protrude between the claws.

Getting the footpads deployed requires some help from a miniature hydraulic system. To demonstrate this detail, postdoctoral fellow Walter Federle deftly mimicked the hydraulic system by gluing an ant leg into the tip of a fluid-filled syringe. He found that he could inflate and deflate the footpad. When he pricked the taut surface of a footpad, liquid oozed out as if it were under pressure.

Looking at the foot under the microscope, Federle saw a large compartment and a plate-shaped structure. He suggests that the compartment is filled with fluid and compressed by that plate, which hooks up to a tendon. Federle identified that tendon as the same one that controls claw movement.

That mechanical linkage between the claws and the footpad, the researchers say, helps the insects deploy their sticky footpads only when needed–on smooth surfaces. The scientists speculate that the same mechanism enables ants and bees to stay stuck when broadsided by, say, a raindrop. In such collisions, an insect’s feet drag over the surface and the footpads unfold.

Federle showed that this tenacity doesn’t require brain input. When he dragged dead insects, or even severed legs, on a smooth surface, the footpads deployed.

“It’s very fine work,” says Stanislav Gorb, an entomologist at the Max-Planck-Institute in Tübigen, Germany. He notes, however, that in bees, additional footpad-control mechanisms could take precedence over hydraulics. He points to evidence for another mechanism that, in his words, is akin to “elastic springs.”

“The broad implications are really important for millimeter-sized silicon robots,” says University of California, Berkeley researcher Robert Full, for whom Federle now works. Full envisions a future where miniature robots cleanse the surface of tiny machine parts or scurry through the human body on medical missions.