When Ian Stevenson returned home from a walk on Sunday morning, he thought he saw an earthworm with debris stuck to it. He probably didn’t expect to see what he actually saw: A rope of wriggling worms snaking along his walkway. Looking at the nightmarish scene, the famous line from Dawn of the Dead might come to mind: “When there’s no more room in Hell, the dead will walk the Earth.”

Stevenson, an assistant professor of neuroscience at the University of Connecticut, tweeted a video of what he saw, which appears to be a slithering mass of worms moving as one.

“I have a serious fear of snakes so the first video was to get a closeup and confirm that they were insects,” Stevenson tells Inverse. “After that I was just curious — emergent behavior like flocking and swarming is super fascinating. I completely understand some of the squeamish reactions the video got, though.”

He asked his followers for their help figuring out what was going on: “Anyone on science twitter know what this thing on my walkway is? Diptera larvae migration/earthworm biomimicry? Deleted scene from Akira?”

It turns out this flowing mass of semi-transparent creatures is not demonic in origin (probably), and as Stevenson suspected, the larva mass’s hypnotic motion illustrates a highly efficient collective strategy for locomotion. Take a look:

A fellow neuroscientist named Heather Read, Ph.D. replied to the video, identifying the phenomenon as dark-winged fungus gnat larvae, which are known to move in this kind of snake-like mass. So Stevenson’s initial guess, that the larvae were some kind of Diptera, might be correct. Diptera is the taxonomic order to which flies belong to, and the fungus gnat is a type of fly.

But why do they move like a snake from hell? It turns out this mass movement, in which the larvae crawl over each other like a conveyor belt, is a process the species uses to maximize the entire swarm’s speed.

In 2013, Aatish Bhatia reported in Wired that this strategy, which is also employed by other caterpillar species, helps the entire mass move 1.5 times faster than an individual can move, if it’s a two-layer mass. And if it’s a three-layer mass, the group can move nearly twice as fast as an individual can. Bhatia likens it to walking on a moving sidewalk at the airport, but with the major advantage that it can go anywhere it pleases.

“Unlike a typical conveyor belt, this one never runs out, because the caterpillars keep disassembling and re-assembling it,” he wrote.

In short, since the larvae on top of the pile crawl over their neighbors — who are also moving — the second layer is moving twice as fast. But since they also have to spend time underneath once they work their way all the way up front, they will spend part of the time going normal speed. For a two-layer mass, this averages out to about 1.5 times the speed of an individual.

In Stevenson’s video, though, the fungus gnat larvae appear to be moving in a mass greater than just two layers deep, so their speed advantage is probably greater than 1.5 times. Regardless, we can rest assured that this writhing rope is not a sign of an impending zombie apocalypse. And in fact, it’s not even that big.

“The video makes it look huge, but in-person it’s just a cute little wriggly swarm,” says Stevenson. “Not that I’m volunteering to touch it.”

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