Tom Johnson-Medland usually walks over the Veterans Memorial Bridge in Columbia, Pennsylvania, every morning with no problem.
But on Tuesday, he stumbled upon a revolting — yet not totally surprising — scene.
A massive mound of dead mayflies standing about 3.5 feet high blanketed the bridge’s sidewalk and spilled onto the road, video shows.
“I can’t even walk through this pile… unbelievable,” Johnson-Medland can be heard saying in the video provided to Storyful .
But what caused the apocalyptic scene, and is it normal for these flies to die in masses?
Turns out the aquatic insects, also known as the summertime swat-worthy “fish flies,” only live for about two days, which they spend aggressively swarming above bodies of water to mate. Once the deed is done, females release their eggs into the water, where they hatch and turn into mayfly larvae, eventually emerging as adult flies.
The Pennsylvania bridge where the die-off occurred spans the Susquehanna River. Swarms sometimes get so bad that the Pennsylvania Department of Transportation tries to prevent an overload of mayfly hatches by shutting off the bridge’s lights, which attract the flies, from May through late June, according to Storyful.
So, while they can be annoying and coat your car or sidewalks with their lifeless bodies, mayflies are actually telltale signs of healthy water ecosystems.
“Mayflies are a good thing when they emerge. They indicate better quality water, higher oxygen levels and less pollution … It’s a good sign the waterways are still supporting these kinds of insects,” David Lowenstein, a garden and pest expert at Michigan State University, told The Detroit News. ”It’s a small price to pay for a sign there is good water around. Keep your porch lights off at night and get your brooms ready to sweep them away.”