Insect Larvae Inspire Rolling Robots
Autonomous machines built from soft materials offer advantages in medicine and disaster relief, but designing soft robots that can move around by themselves has been challenging. Now a research team has constructed a soft robot that can roll continuously without the benefit of an external force or any rigid structures [1]. The team studied the mechanisms that enable fruit-fly larvae to generate a rolling motion and replicated the larvae’s technique by using a pneumatic system to drive a sequence of shape changes in the robot. The demonstration shows that a simple shape-changing mechanism can generate a self-propelled, rolling motion in a soft robot.
Soft-robotics researchers aim to create autonomous machines from flexible materials and electronics, often inspired by soft-bodied animals. Their pliable nature makes these robots ideally suited for interacting with the human body and for squeezing into tight spaces that cannot be reached by rigid structures. But self-propelled locomotion remains a key challenge, since an entirely soft robot has no rigid limbs and cannot contain motors or gears. Xudong Liang of the Harbin Institute of Technology in Shenzhen, China, and his colleagues wondered how soft-bodied animals have solved this problem, so they studied the larvae of fruit flies. One of these worm-like larvae can roll away from danger by rotating around its long axis while its body is curled into a C shape.
However, the mechanism for this rolling behavior remains unclear. A previous study theorized that the larva generates rotation by continuously redistributing its weight so that it’s always “tipping over” [2]. But Liang and colleagues found that this process is too weak: Preliminary measurements of the larva motion showed that this gravitational effect cannot supply the force required to roll a larva. To investigate the biomechanical mechanisms at play, they tracked the shape and activity of two different types of muscles as the larvae rolled: circular muscles, which wrap around the larval body, and axial muscles, which extend along the body’s long axis.
By imaging the individual muscles and using laser ablation to deactivate each muscle group in turn, they found that the axial muscles play a more important role in sustaining the rolling motion. The experiments revealed a sequential process that begins when one of the axial muscles contracts to bend the body into a C shape. That muscle then relaxes, allowing the body to recover slightly from the bend, while the neighboring muscle starts to contract. As each axial muscle contracts and relaxes in sequence, the larva is able to roll.
Liang and colleagues developed a mechanical model that describes how the sequential activation process can generate a rolling motion. This detailed analysis enabled the team to design a soft robot that can generate and sustain rotation. The cylindrical “body” is made of silicone rubber, while actuation is provided by four internal chambers under pneumatic control. Pressurizing one of the chambers causes it to lengthen, making the robot bend toward the opposite side. When the adjacent chamber is pressurized and the first one deflated—mimicking the sequential muscular contractions of the larvae—the robot starts to roll. According to Liang, this proof of concept shows that even simple actuation sequences can yield complex motion.
“This simple yet effective strategy provides a foundational framework for developing bioinspired robots that offer energy-efficient and adaptive rolling locomotion,” says Qiguang He, an expert in soft robotics at the Chinese University of Hong Kong. The team now hopes to integrate sensory feedback into the design, which would make it possible to dynamically adjust the actuation sequences and enable the robot to roll across uneven terrain.
–Susan Curtis
Susan Curtis is a freelance science writer based in Bristol, UK.
References
- X. Liang et al., “Mechanics of soft-body rolling motion without external torque,” Phys. Rev. Lett. 134, 198401 (2025).
- P. C. Cooney et al., “Neuromuscular basis of Drosophila larval rolling escape behavior,” Proc. Natl. Acad. Sci. U.S.A. 120 (2023).