2021-10-01 16:00:14 The Marvelous Physics of Swarming Midges

The Marvelous Physics of Swarming Midges

Midges are gathering to swarm on early autumn afternoons across the temperate world: clouds of tiny flies, wings lit by the sun like so many sparks, swirling in patterns too quick and complicated for the eye to follow but leaving a mental afterimage of order. It’s not perfect order, but it’s more than chaos.

According to scientists who study such swarms, that impression of order is correct: The mathematical signatures of properties other than what one would expect from a cloud of bugs can be found in the movements of midges. They behave like liquids or gases as a group, and even exhibit the characteristics of “criticality,” that strange stage of matter in which radical transformation from one state to another occurs in the blink of an eye.

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

Dr. Cavagna and his colleague, Dr. Irene Giardina, a theoretical physicist at Rome’s La Sapienza University, studied starling flocks before turning their attention to midges. The researchers discovered in 2009, using high-speed video cameras to measure the trajectory of every bird in a murmuration, as starling flocks are known, that when one starling changes direction or speed, so do the birds closest to them, and thus the birds closest to those. Each starling in a murmuration is thus linked, regardless of how far apart they are.

This is referred to as a scale-free correlation in statistical mechanics. It is a property of criticality — what happens when a liquid turns into a gas, or how particles in a lump of hot iron change orientation in unison and form a magnet when cooled to a specific temperature.

Dr. Cavagna and Dr. Giardina were awarded the prestigious Max Delbrück Prize in Biological Physics this year for their work on starlings. During the early years of their research, they marveled at the swarms of midges flitting above the grass while taking their young children to Rome’s parks, and they began to wonder about them as well.

Midge swarms did not appear to be as close-knit as murmurations, but they also did not appear to move completely independently of one another. “We thought the same kind of model could be used to describe midge swarms,” Dr. Giardina explained.

The researchers trained their cameras on the swarms — no easy feat given the evanescence of the swarms and the intrusive curiosity of bystanders — and discovered that, like starlings in a flock, midges in a swarm are collectively correlated.

They are not all moving in near-perfect synchrony, nor is the degree of correlation as strong as in starlings. There may also be subgroups within a swarm that move in opposite directions, with individuals switching from one subgroup to the other, giving rise to disorder. Regardless, the midges are all tangled up.

The researchers also discovered that as swarms grow in size, they become denser and the flights of the midges become more closely correlated. This is most likely due to how midges respond to the sound of their neighbors’ buzzing wings, allowing them to maintain an optimal degree of correlation.

“It’s as if the system self-organizes to provide the best possible response,” Dr. Giardina explained. Dr. Cavagna described it as a method of “surfing the maximum of susceptibility,” allowing for quick, coordinated movements.

“The closest physical system models are magnets,” Dr. Cavagna explained, referring to the sudden collective shift in particle orientation just before magnetization. However, he stressed that swarming midges are not at that critical point, but are only near it.

He speculated that this could be due to a physical limitation. True criticality occurs only in systems with many more units than a swarm. A one-gram iron magnet has approximately 10,000,000,000,000,000,000,000 iron atoms, whereas a decently sized midge swarm has only a few hundred midges.

It’s also possible that reaching criticality would be disastrous for them, making the swarm hypersensitive to any disturbance, puff of air, or midge equivalent of a sneeze. “The best trade-off is being close to critical,” said Dr. Miguel Muoz, a physicist at Spain’s University of Granada who has closely followed the research. “You take advantage of the responsiveness, but you don’t get too close, because if you get too close, you respond to everything.”

Murmurations, whose synchronized twists and turns may aid starlings in evading predators, demonstrate the potential benefits of swarming.

Midge swarms, which are almost entirely made up of males, also serve a reproductive purpose, with females entering and taking mates in midair. Perhaps operating at near-criticality promotes midge romance? That is not known. It’s also possible that swarm properties aren’t adaptive at all, but rather “a side effect of mathematics,” according to Dr. Cavagna.

Dr. Muoz believes Dr. Cavagna and Dr. Giardina’s findings are “convincing,” but some scientists disagree. Dr. Nicholas Ouellette, a physicist at Stanford University, and his colleagues discovered that correlations did not emerge quickly in their own studies of captive midges. When they did appear, the correlations did not fit the criticality framework.

The swarms, on the other hand, piqued my interest. Dr. Ouellette and his co-authors described them in a 2017 paper in Physical Review Letters as containing midges whose flight patterns created a condensed core surrounded by a layer of vapor.

When the team separated the visual landmarks over which the swarm formed, the swarm split in half. (Landmarks in nature might be logs or leaves; in the lab, they were pieces of paper.) As a result, the swarms behaved like solids rather than fluids, “appearing to be under increasing tension before eventually snapping,” according to Dr. Andrew Reynolds, a theoretical biologist at Rothamstead Research in the United Kingdom.

“Different stimuli can elicit different behaviors,” explained Dr. Reynolds. He was not involved in the Stanford experiment, but he has worked with Dr. Ouellette on other projects, including one in which a laboratory swarm wobbled and smushed like Jell-O. Earlier this year, Dr. Ouellette and his colleagues described how swarms appear to be governed by thermodynamic laws.

Such findings imply that a swarm can be understood as a single entity rather than a collection of individual insects, similar to how a quartz crystal is perceived as a discrete object rather than a collection of trillions of atoms. “Because you can’t see what it’s made of, you’re used to thinking of it as one thing,” Dr. Ouellette explained. “These swarms have well-defined material properties that are not individual, but group properties.”

Disagreements about correlation and criticality will be resolved through additional research. It is also possible that both groups are correct: midge swarms may exist in all of the forms described by researchers, depending on size and circumstance.

Wherever the scientific dust settles, one can appreciate how amazing swarms are and the tantalizing glimpse they provide of the principles underlying seemingly disparate phenomena. Dr. Muoz became interested in the research after discovering criticality in neural networks and cellular function; there may be parallels between the dynamics of swarms and the brain converting cellular excitation into an image, or a genome expressing the instructions in its DNA.

“Criticality could be a unifying principle,” he said, one that generates exquisite coordination and complexity from simple components and that evolution has exploited numerous times. Even if the swarms aren’t near-critical, the connections are profound.

Dr. Reynolds noted that scientists have long compared swarms to self-gravitating systems, comparing the forces that keep them together on a windy day to the forces that keep planets together. He compared swarms to the accumulation of dust, gas, and plasma in interstellar clouds in a recent paper.

“Whenever I see a midge swarm, I now see great beauty and subtlety,” Dr. Reynolds said. “They come to a halt in front of me.”

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The Marvelous Physics of Swarming Midges