The wonderful physique of swarming midges

In the early fall afternoon across the temperate world, gnats now gather to swarm: clouds of tiny flies, wings lit by the sun like spark...


In the early fall afternoon across the temperate world, gnats now gather to swarm: clouds of tiny flies, wings lit by the sun like sparks, swirling in patterns too fast and complicated for the eye but leaving a tidy mental afterimage. Not a perfect order, but something more than chaos.

This sense of order is correct, according to scientists who study such swarms: in the movements of midges, one can find mathematical signatures of properties beyond what one would expect from a cloud of insects. As a group, they behave like liquids or gases, and even exhibit the characteristics of “criticality,” that strange stage of matter at which radical transformation from one state to another occurs in the blink of an eye. eye.

“Collective correlation can liberate the system from its microscopic details,” said Dr Andrea Cavagna, a physicist at the Institute for Complex Systems in Rome. A swarm is more than its midges.

Before Dr Cavagna and his partner Dr Irene Giardina, theoretical physicist at La Sapienza University in Rome, looked at midges, they studied flocks of starlings. Using high-speed video cameras to measure the trajectory of each bird in a whisper, as starling flocks are called, researchers found in 2009 that when a starling changes direction or speed, the most birds relatives also change, and in turn the birds closest to those. Every starling in a whisper is so bound, regardless of the distance.

In statistical mechanics jargon, this is called a scaleless correlation. It is a property of criticality – what the liquid undergoes when it becomes a gas, or how the particles of a hot piece of iron, when cooled to a specific temperature, change orientation in the process. unison and create a magnet.

This year, the work of Dr Cavagna and Dr Giardina on starlings earned them the prestigious Max Delbrück Prize for Biological Physics. And during the first years of their research, while taking their young children to the parks of Rome, they marveled at the swarms of midges hovering above the grass and began to wonder about them too.

The swarms of midges didn’t seem as cohesive as the whispers, but the insects didn’t seem to move completely independently of each other either. “We got the idea that the same type of model could also be used to describe midge swarms,” Dr Giardina said.

The researchers focused their cameras on the swarms – which was no small feat, given the evanescence of the swarms and the intrusive curiosity of passers-by – and found that, like starlings in a flock, the midges in a swarm are collectively correlated.

They do not all go in the same direction in near perfect synchrony, and the degree of correlation is not as strong as in starlings. There may also be subgroups within a swarm moving in different directions, with individuals moving from one subgroup to another – hence the appearance of disorder. Nevertheless, the midges are all entangled.

The researchers also found that as the swarms grew larger, they become denser and midges flights become more closely correlated. This is probably a function of how midges react to the sound of their neighbors’ buzzing wings, and it allows them to maintain an optimal degree of correlation.

“It is as if the system is self-organizing in order to have the maximum response possible,” said Dr Giardina. Dr Cavagna described it as a way of “surfing the maximum susceptibility”, allowing sudden and coordinated movements.

“The closest models in physical systems are magnets,” Dr. Cavagna said; that is, the sudden collective change in orientation of particles just before magnetization. But he stressed that the midge swarms aren’t that critical, only nearby.

It can be a physical limitation, he noted. True criticality only occurs in systems with many more units than those found in a swarm. A one gram iron magnet contains around 10,000,000,000,000,000,000 iron atoms, while a decent-sized swarm of midges contains only several hundred midges.

It’s also possible that reaching criticality would be catastrophic for them, making the swarm hypersensitive to every disturbance, puff of air, or whatever the midge equivalent of a sneeze. “The best compromise is to be close to criticism,” said Dr Miguel Muñoz, a physicist at the University of Granada in Spain, who has followed the research closely. “You enjoy the responsiveness but you are not too close because if you are too close you will respond to anything.”

The potential benefits of swarming are evident in whispers, whose synchronized twists can help starlings escape predators.

Swarms of midges, which are made up almost entirely of males, also perform a reproductive function, with females entering and taking their mates in the air. Maybe operating at near-criticality is conducive to gnat romance? It is unknown. It is also possible that the properties of the swarm are not adaptive but simply “a side effect of mathematics,” Dr Cavagna said.

Dr Muñoz considers the findings of Dr Cavagna and Dr Giardina to be “convincing”, but some scientists are opposed. In his own studies of captive midges, Dr. Nicholas Ouellette, a physicist at Stanford University, and his colleagues found that correlations were not quick to arise. When they got up, the correlations did not correspond to the criticality framework.

The swarms were still intriguing, however. In a 2017 paper published in Physical Review Letters, Dr Ouellette and his co-authors described them as containing midges whose flight patterns created a condensed core surrounded by a layer of vapor.

And when the team separated the visual cues that a swarm formed on, the swarm split in two. (In nature, landmarks may be logs or leaves; in the laboratory, they were pieces of paper.) In doing so, swarms did not behave like a fluid but like a solid, “appearing to be under increasing stress before finally breaking down,” said Dr Andrew Reynolds, theoretical biologist at Rothamstead Research in Britain.

“Different stimuli can induce different behaviors,” Dr. Reynolds said. He did not participate in the Stanford experiment but did collaborate on others with Dr. Ouellette, including one in which a laboratory swarm wavered and crushed like Jell-O. Earlier this year, Dr Ouellette and colleagues described how the swarms appear to be governed by the laws of thermodynamics.

Findings like this suggest that a swarm can be understood as a singular entity rather than a collection of individual insects, in the same way that a quartz crystal is seen as a discrete object rather than billions of atoms. “You used to think of it as one thing because you can’t see what it’s made of,” Dr. Ouellette said. “These swarms have well-defined material properties which are not properties of the individuals, but of the group.”

As for the disagreements on correlation and criticality, these will ultimately be resolved with more research. It’s also possible that both groups are right: perhaps swarms of midges can exist, depending on the size and circumstances, in whatever forms the researchers have described.

Wherever this scientific dust settles, one can appreciate how wonderful swarms are and the tantalizing glimpse that they offer underlying principles to seemingly disparate phenomena. Dr. Muñoz’s interest in research was sparked by the findings of criticality in neural networks and cell function; there may be similarities between the dynamics of swarms and the brain transforming cellular excitement into an image, or a genome expressing the instructions in its DNA.

“Criticality could be a unifying principle,” he said, a principle that generates exquisite coordination and complexity from simple components, and which has been repeatedly exploited by evolution. And while the swarms aren’t nearly critical, the connections are still deep.

Dr Reynolds noted that researchers have long compared swarms self-gravitating systems, comparing the forces that help them hold together on a windy day to the forces that hold the planets together. In a recent article, he compared swarms to collecting dust, gas and plasma in interstellar clouds.

“I now see great beauty and subtlety every time I see a swarm of midges,” Dr. Reynolds said. “They stop me in my tracks.”

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