In the summer of 2019, when joking about omens of the apocalypse still seemed fun, a swarm of grasshoppers descended on the Las Vegas Strip.

For weeks, every evening after sunset, their flapping wings filled the Sky Beam shining up from the pyramid of the Luxor casino, and their dead exoskeletons littered the sidewalks. The news media speculated that the city's artificial lights lured in grasshoppers like moths to flame.

A new analysis substantiates the link to the city's lights — with worrying implications for the grasshoppers. Elske Tielens, an insect ecologist at the University of Oklahoma, found that on July 26, 2019, the peak night of the invasion, 46 million grasshoppers took wing and then clustered over the brightest parts of the city.

"It's really hard to wrap your mind around that volume," she said. "We're getting more grasshoppers in the air on a single day than you get humans coming to Vegas to gamble across an entire year."

Visitors, of course, already knew Las Vegas cranked up its wattage at night. But some of that glow escapes straight up into space, where satellites measure it as the brightest city on the planet by a wide margin. The rest of that light, overflowing up into the atmosphere, forms a glowing dome that the U.S. National Park Service recently measured from 200 miles away, at Great Basin National Park in Nevada.

Insect ecologists have spent years studying how individual lamps and nighttime traps can be a silent siren call for insects, tempting them to their deaths. But Tielens and her colleagues saw an opportunity to hunt for a wider pattern. They found that the roving clouds of grasshoppers had also been visible in weather radar data. Then they overlaid those radar movement patterns with separate maps of the city's vegetation and its nighttime lighting.

Their study, published in Biology Letters, suggested a daily commute. Before dusk, the grasshoppers began to spread out and gathered near vegetation. But as daylight faded, they took to the skies. Then they clustered up to dozens of miles away, traveling not just toward individual bright points but toward the glowiest regions of the Vegas sky.