On a muggy spring night in 2016, the chemist Bernd Schöllhorn was tromping alone through a forest in northern Vietnam. Into the inky darkness, he raised a black light—and saw an extraordinarily bright shape winking at him in eerie shades of yellowish green.
“I thought it was somebody else,” Schöllhorn, a researcher at the University of Paris, told me. But when he cut his own light, the stranger’s torch instantly extinguished as well. Schöllhorn pushed his way through the vegetation until he reached the source of the glow: a geometric, open-combed nest of a paper-wasp colony.
“It was just incredible,” Schöllhorn recalled. Bathed in ultraviolet rays from his flashlight, the nest looked as though it had been dipped in a vat of highlighter ink, so bright and Day-Glo green that the inches-wide structure was visible from some 60 feet away. The wasps’ home, Schöllhorn realized, was fluorescing, as though prepping for an entomological rave. And he had no idea why.
Over the next several years, Schöllhorn and his colleagues searched for paper wasps in other parts of Vietnam, then in France and French Guiana, until they’d found nests from six different species in the genus Polistes. When fed a steady stream of ultraviolet rays, all of the nests glowed, each with a bit of regional flair: The four from Vietnam all pulsed in green, while the other two, from Europe and South America, were a more muted teal-ish blue. “Finding this in so many species, and across three different continents, is remarkable,” Swanne Gordon, an evolutionary biologist at Washington University in St. Louis, who studies insect signaling and wasn’t involved in the study, told me.
The wasps themselves didn’t light up; neither did the topmost parts of the nests, constructed out of chewed-up wood (hence the “paper” moniker). The glow, the researchers found, came from a layer of silk stitched across the openings of the hexagonal cells at each nest’s base.
Scientists hadn’t pinpointed this silk as fluorescent before. Its primary purpose is to cocoon young paper wasps during their metamorphosis, when larvae “dissolve their bodies” and reform themselves into adults, Sara Miller, a paper-wasp expert at Cornell University, told me. What’s inside the sealed cell is “really like a bag of mush,” Miller said. The larvae excrete silk out of a gland, and it shields the pupa from the ravages of reality—predators, pathogens, harsh weather conditions—much like a chrysalis protects a butterfly-to-be.
In the light of the forest, when viewed with human eyes, paper-wasp silk appears whitish or yellowish and is decidedly matte. But when fed ultraviolet light in the lab, the string-like fibers convert those rays into a fluorescence funky enough for an ’80s aerobics ensemble.
Especially staggering was the silk’s capacity for shine. In the lab, Schöllhorn’s team calculated each nest’s quantum yield, or its capacity to emit light when fed a certain number of photons. “Those kinds of measurements are tricky to get,” especially from flora and fauna, Carlos Taboada, who studies glowy amphibians at Duke University, told me. The few quantum yields that researchers have managed to glean from fluorescent animals tend to fall between 0.3 and 12.5 percent; a few years ago, Taboada uncovered frogs that shone near the top of that range. The brightest nest the researchers collected, a Polistes brunetus creation from Vietnam, registered a whopping 35 percent. “That is incredibly large for a biological tissue,” Taboada, who wasn’t involved in the new study, said.
The silk’s beguiling glow, the researchers confirmed, falls in the range of wavelengths that wasp eyes can see. “They’re very sensitive to green,” Schöllhorn told me. But it’s not yet clear what purpose the fluorescence serves for the insects, if any at all. Plenty of things will glow under black lights, if given the chance. That doesn’t guarantee that these glimmers are more than a coincidence of physics. “It’s still possible this is just an incidental by-product of how the silk is made,” Liz Tibbetts, a paper-wasp expert at the University of Michigan who wasn’t involved in the study, told me.
Schöllhorn and his colleagues haven’t yet figured out whether the silk’s glow-stick effect is important to the wasps, or exactly how the larvae cook it up. The fluorescent molecules could be dietary, for instance, or entirely of the insects’ own making. Still, Schöllhorn thinks the glow probably has some role to play. One possibility is that it serves as a sort of psychedelic beacon for work-weary wasps staggering back home. Several of the insect experts I talked with were a little hesitant to embrace this idea, because paper wasps, which are famously detail-oriented, are already ace at navigation. The silk is also ablaze for only part of the nesting cycle, after eggs had been laid and hatched.
A more intriguing hypothesis, experts told me, might hinge on the silk’s ability to safeguard pupae by waylaying harmful, DNA-damaging UV rays—a sort of DIY sunscreen for baby wasps. “To me, that’s the most appealing idea,” Floria Mora-Kepfer Uy, a wasp expert at the University of Rochester who wasn’t involved in the study, told me. That could come in handy, she said, for nests that hang at the edges of forests, where the vegetation is sparse and the structures are constantly flooded with sunlight. But Uy also told me that she hasn’t yet seen evidence of fluorescence in the nests she’s studied in the subtropics of North and Central America.
Scientists have known for years about bony fish, sharks, worms, jellies, corals, and other marine creatures that light up to attract mates, lure prey, or discombobulate predators. The list of terrestrial examples is sparse, but is growing: Frogs, salamanders, birds, spiders, butterflies, and even flying squirrels all bat for Team Glow. Paper-wasp silk could provide yet more clues about fluorescence’s function on land.
Humans perceive the world “in just one way,” Tibbetts told me. “We’re walking around with blinders on.” Terrestrial fluorescence might not be so rare at all; we just haven’t been searching for it.
Schöllhorn is one of many researchers trying to course-correct—which is why he originally took that fateful black-light-wielding walk in Vietnam. Many of these excursions are solo trips, he told me. “No one wanted to go with me,” he said. “There is no light at all, and snakes, spiders, insects everywhere.” He’s gotten used to it, though, and the rewards are always worth the trek. The next great glower won’t be found unless someone is willing to look.