July 11, 2019 report
Midge swarms show mechanical properties, behave as a viscoelastic material
A team of researchers from Stanford University and Rothamsted Research, has found that midge swarms have some types of mechanical properties and also respond to a stimulus at times as a viscoelastic. In their paper published in the journal Science Advances, the group describes their study of swarm behavior in a species of midges and what they found.
Midges are an unofficial classification of two-winged fly. There are many species, most of which are associated with swarms that live near water or marshy areas. In this new effort, the researchers wanted to learn more about the physical properties of swarming behavior from the perspective of an entire swarm, rather than the individuals in it. To that end, they obtained a mass of male Chironomus riparius, which are known to swarm over a given object such as a tree stump as part of their mating ritual—the swarm allows the females to find them from a distance. In their lab, the researchers placed a square of black felt on the bottom of a tank for the midges to use as their orienting object. When the piece of felt was moved, the swarm responded to it. To test swarm responses to it, the felt square was attached to a small oscillating device.
The researchers report that they focused specifically on how the swarm responded as an entire unit to the moving object below them. They report that it behaved in a layered manner, with those closest to the moving object responding faster than those in more distant layers. The researchers suggest this was likely due to propagation delays in messaging between individuals. They also observed mechanical properties—and sometimes the swarm behaved elastically, while at other times it behaved more like a viscous liquid—which, in physics terms, meant it was behaving like a viscoelastic material. They also found that sometimes, the viscosity properties were overcome by the elastic properties, which led to an observed dampening effect of the swarm's overall movement. The researchers suggest this made the swarm more stable, making it easier for the females to find it. They also suggest their findings might have some relevance to swarming robotics efforts.
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