Have you ever watched or participated in a mosh pit dance? Essentially, every person is moving together while bumping into each other and getting hit. There are plenty of animals that move in groups, such as shoals of fishes, flocks of birds, or swarms of locusts, but most manage to proceed without continually bumping into one another. One of the unique behaviours that has been observed when animals are in groups is when individuals stop before coordinating their movement with that of the animals that surround them. This behaviour is known as intermittent motion. Some scientific work has suggested that the use of intermittent motion is linked to an energy saving response; it has also been associated with a way for animals to gather and analyse information about the environment to guide their movement when in a crowd. Knowing this, Yossef Aidan, Itay Bleichman and Amir Ayali from Tel Aviv University, Israel, decided to find out more about how locusts process visual information from their surroundings and then act on it when moving in a swarm.

The team collected wingless desert locust (Schistocerca gregaria) nymphs – before the final moult when they transform into adults – from a colony at Tel Aviv University. Each locust was tethered so that it could walk on a large rotating ball, essentially a treadmill that allowed the locust to walk in any direction. The ball and insect's movements were tracked using a mouse sensor and a camera located above the insect while the locusts watched a movie of four dots played on monitors in front of them, either moving forward or backward, to simulate the view of nearby insects in a swarm. Aidan and colleagues then created three artificial visual situations for the locusts and monitored how they moved in response to the dots’ movements. In the first experiment, the dots moved continuously, regardless of whether the locust was walking or not. In the second experiment, the dots moved when the locust moved, but stopped when the locust was static. In the third experiment, the dots on the screen moved while the locust was stationary but were motionless when the locust walked on the treadmill.

What the researchers found was very interesting: the locusts moved the most when the moving dots appeared to be going backwards and they moved the least when the dots on the display moved forward continuously. In addition, the team discovered that locusts moved their body to the sides more when the display was moving backwards, and the greatest side movement was observed when the display was moving and the locust was static.

In summary, when a locust is not moving and its surroundings appear to move backwards, it triggers a change in the direction of their movement to stay within the group. This suggests that taking a small break can provide animals with time to process visual information about the movements of neighbouring locusts so they can adjust their own movements accordingly. At the same time, it may also help explain how other animals use the ‘pause-and-move’ response. For example, shoals of fish might use a break to assess the environment and move in time with each other if a predator is approaching. Lastly, the growing field of insect-inspired robotics has produced robots that can fly in formation, but they are not typically programmed to use intermittent motion to help them to remain in sync. Implementation of this behaviour in robots may be beneficial for their performance, so long as they don't get too smart and try to take over the world.

Pausing to swarm: locust intermittent motion is instrumental for swarming-related visual processing
Biol. Lett.