By Leslie Mertz, Ph.D.

With six legs to coordinate, an insect’s walk involves more than simply putting one foot in front of the other. In fact, the complexity of that movement may just be a good model for the development of multi-legged robots, especially those that can climb walls, according to research published in the Journal of Insect Science.

Leslie Mertz, Ph.D.

The new study focuses on the biomechanics of honey bees’ gait—i.e., their walking pattern—as they maneuver while crawling up smooth vertical surfaces, according to study co-author Shaoze Yan, Ph.D., professor in the Division of Intelligent and Biomechanical Systems at Tsinghua University in Beijing, China.

When moving in a straight line, an insect sets down three feet simultaneously in a stable, tripod formation: either the front right foot, middle left foot, and rear right foot; or the front left foot, middle right foot, and rear left foot. Little was known, however, about how insects turn when walking up walls, Yan says. To investigate this, the researchers built a clever experimental platform that combines a high-speed camera and carefully placed mirrors to obtain three views of the honey bee (Apis mellifera) as it moves. In addition, he says, “We placed different colors on the three pairs of legs of the honeybee to keep clear track of the leg motions.”

In this diagram of honey bee locomotion, the bee’s legs are labelled as right or left (R or L, respectively), and as the front, middle or rear leg pair (1, 2 or 3, respectively). When moving in a straight line, honey bees walk in a tripod gait with three feet touching the ground at all times (two on one side and one on the other). When moving forward four steps (as illustrated at right), alternating feet touch down, and the insect’s body sways from one side to the other. (Image originally published in Zhao et al 2018, Journal of Insect Science)

Computer analysis revealed that whether a bee is moving along the ground or up a wall, it prefers to use a three-legged gait when going straight, but it switches to a four-legged gait when turning. “That especially surprised us. Previous to this, we had always thought that honeybees used the tripod gait to make a turn,” Yan says. To turn, the bee moves both forelegs simultaneously in the direction it wishes to travel, while the other two pair remain on the surface, he describes. Once it sets down its forelegs, the middle pair or hind pair lift and set down, always with four legs in contact with the surface. The synchronized forelegs are therefore responsible for guiding the turning direction of the honey bee, while the central axis of the insect remains consistent with the crawling direction, he says.

In addition, Yan and colleagues saw differences in the swing or sway of bee’s body: The body sways back and forth when moving in a straight line (which helps it walk quickly), but the sway disappears when turning. He says, “The precise connection between the tripod gait and swinging motion of honeybee body remains ambiguous.”

When turning, honey bees switch to a tetrapod gait, with two pairs of legs touching down. In addition, the sway of the body disappears. (Image originally published in Zhao et al 2018, Journal of Insect Science)

These insights into insect locomotion and gait mechanisms may have applications in robot locomotion, Yan says. “Most of the robots that can achieve smooth vertical-wall crawling are not using biologically inspired movement methods. Rather, there are universally used foot-like wheel-type exercises and caterpillar-track robots, but just as wheeled movements in life cannot completely replace walking, this kind of gait is not flexible enough, and its adaptability to macroscopic changes in crawling surfaces is low,” Yan says.

Such robots cannot always navigate well on uneven surfaces, especially when moving vertically. Other designs, such as adhesive-footed robots, also have limitations in that they require multiple motors to drive each joint and control the foot’s special structure, are complicated to operate, and are large in size, Yan says. “Such robots lack the advantages of small robots’ flexibility and cannot be applied in many scenes,” he says. Honey bees, however, are adept at all of these movements, so an examination of their biomechanics may be useful in robot design.

By studying how bees walk, researchers at Tsinghua University in Beijing, China, hope to gain insights that could assist in the design of multilegged robots. Here, Shaoze Yan, Ph.D., (left) and Jieliang Zhao, Ph.D., of the university’s Division of Intelligent and Biomechanical Systems, set up an experimental platform that combines a high-speed camera and carefully placed mirrors to obtain three views of the honeybee as it moves. (Photo courtesy of Shaoze Yan, Ph.D.)

He and his co-authors are currently working on bio-inspired six-legged robots that use the honey bee’s tetrapod gait to make a turn. “As a typical representative of biologically inspired mobile robots, hexapod robots have redundant limb structures and rich gait, and they are flexible in movement. They can achieve non-contact obstacle avoidance, cross barriers, go up and down steps, and accomplish complex terrain movements through posture adjustment and gait switching,” he says. “Six-legged robots will have unique advantages in the fields of service industries, space exploration, anti-terrorism and disaster relief in the future.”

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.