Engineers may recognize the internal muscle structure of a honey bee (left) abdomen for its resemblance to a Stewart platform, a mechanical device that enables six degrees of freedom in movement. Researchers who have found its natural equivalent in bees say the discovery is already informing their work in designing articulating nose cones for rockets. The diagram at right shows a schematic view of a Stewart platform designed based on the honey bee abdominal segment structure. (Honey bee photo credit: Russ Ottens, University of Georgia, Bugwood.org. Diagram originally published in Liang et al 2019, Journal of Insect Science)

By Leslie Mertz, Ph.D.

The next time you see a bee land on a flower, watch how busy its abdomen is. It scrunches up, it lengthens, and it curls this way and that with amazing flexibility. A group of engineers at Tsinghua University in Beijing has now found that a quite surprising mechanism is involved in that movement, and this mechanism could possibly help in the design of rocket nose cones that need to morph into different shapes to accommodate the aerodynamics, mobility, and flight control required to punch through and re-enter the atmosphere. The findings are reported in a new study published in May in the open-access Journal of Insect Science.

Leslie Mertz, Ph.D.

The engineers focused on honey bees (Apis mellifera), which have very lively abdominal movements that are showcased during the waggle dance they perform to communicate with other members of their hive about the location of a prime flower patch. To figure out how the insects move, the engineers carefully dissected honey bees and viewed microstructures of their abdomens using a scanning electron microscope, and they also used a high-speed camera to record the wiggles of additional living honeybees, according to study co-author Shaoze Yan, Ph.D., of Tsinghua University’s Department of Mechanical Engineering.

With those observations, the engineers identified something unexpected: six “lateral connection structures” that look rather like the hydraulic shocks on a car and that extend and contract. One end of each of the six lateral connection structures, which are made of muscle, attaches to the flexible upper exoskeletal plate (the tergum) of an abdominal segment, and the other end attaches to the flexible lower exoskeletal plate (the sternum). The six lateral connection structures move independently of one another, Yan says, and because of the location of their connection points on the two plates, they allow for extensive and quite pliable movement of the tergum and sternum, therefore giving the abdomen its impressive scope of movement.

Researchers at Tsinghua University in Beijing have found that the internal muscle structure of a honey bee (Apis mellifera) abdomen resembles a mechanical device known as a Stewart platform. In this schematic diagram of the honey bee abdomen segment structure, each tergum is connected to a sternum by six “lateral connection structures” (L1-L6) that look rather like the shocks on a car. The lateral connection structures, which are made of muscle, have connection points—b1-b6 on the tergum (trg4) and B1-B6 on the sternum (stm4)—such that their contraction and extension offer considerable flexibility, especially to the tergum. (MP denotes moving platform, and BP denotes base platform.) (Image originally published in Liang et al 2019, Journal of Insect Science)

This stereoscopic image of a honey bee’s abdomen shows the upper and lower exoskeletal plates, known as the terga (trg) and the sterna or (stm), respectively. The plates are flexible. (Image originally published in Liang et al 2019, Journal of Insect Science)

Engineers at Tsinghua University, including Ph.D. student Youjian Liang (left) and Professor Shaoze Yan, Ph.D. (right), identified a surprising mechanism behind the highly maneuverable honey bee abdomen, and the hope to use this finding to help design nose cones for aerospace vehicles. (Photo courtesy of Shaoze Yan)

Engineers may recognize the arrangement and functioning of lateral connection structures as “the Stewart platform,” which is commonly used in flight simulators to imitate the change of attitude and speed of an aircraft, Yan says. “Most flight simulators adopt the Stewart platform as the kinematic mechanism, which is composed of a moving platform linked to a fixed base through six extensible legs. The simulation cockpit is installed on the platform, [and] the coordinated motions of the six extensible legs can drive the platform and make the cockpit simulate the movement of the aircraft,” he says.

Once the engineers saw the lateral connection structures, they realized they had discovered an equivalent Stewart platform structure in insects. “We were very excited!” Yan says. According to their paper, this type of structure and mechanism is rarely seen in animals and has never previously been reported as regulating and controlling physiological activities. Yan adds, “We believe that it is the result of natural evolution adapting to the needs of the honey bee abdomen.”

This finding combines with the group’s earlier study revealing the role of folded intersegmental membranes in abdominal deformation in bees and its yet-to-be-published research indicating that abdominal muscles also function as linear actuators to control coordinated movement of adjacent abdominal segments. (Engineers would describe this as a parallel mechanism.)

Although the research group studied only honey bees, Yan believes other insects’ abdomens may move in the same way. “This model may be [able] to elucidate the abdominal deformation mechanism of butterflies, dragonflies, and drosophilae, since the abdomens of these three insects have similar physiological structures,” he says.

In yet another example of bioinspiration, the group is now applying their understanding of honey bee abdomens to the design of aerospace nose cones. “As mechanical engineers, we generally consider that the motion ability of machine—or animal—is determined by its body structure, [and] we have designed a morphing nose cone for the aerospace vehicle after optimizing the honey bee abdominal deformation mechanism,” Yan says. “Next, we will develop a physical prototype and carry out a series of experiments to verify its morphing ability.”

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.