Fireflies glow to attract mates. As new research shows, however, a certain species of spider has learned to take advantage of this luminous natural phenomenon.
In a Journal of Animal Ecology paper published August 27, ecologists report that the sheetweb spider (Psekrus key) appears to exploit firefly luminescence to attract more prey. Observational analysis and lab experiments revealed that, by using firefly light as bait, the nocturnal predators improved their hunting success. This is the second time researchers have observed such behavior in spiders, although the new paper describes a completely different species of spider.
“This study sheds new light on the ways that nocturnal sit-and-wait predators can rise to the challenges of attracting prey and provides a unique perspective on the complexity of predator-prey interactions,” said I-Min Tso, study senior author and an ecologist at Tunghai University in Taiwan, in a release.
A penchant for sparkly grub
What alerted researchers to this odd behavior was the spiders’ tendency to immediately consume most grub—such as moths—caught in their webs, but not fireflies. When the glowing creatures flew into the sit-and-wait predator’s web, the spider simply left them hanging for about an hour, even occasionally crawling back to see if the fireflies were alive and glowing.
That seemed odd to the researchers; they knew it wasn’t because the spiders weren’t into fireflies—quite the opposite. Obviously, the researchers couldn’t ask the spiders if they were simply appreciating the pretty fireflies. So instead, they arranged an experiment to test whether the spiders’ behavior could be linked to an evolutionary benefit.
For the experiment, the team prepared LED lights that closely resembled the glows of fireflies, plopping them onto the webs of sheetweb spiders. Webs adorned with these LED lights attracted three times the amount of prey compared to the controls without any lights.
Even more surprising, when the researchers limited the prey to real fireflies, webs outfitted with LED lights attracted ten times more of them—mostly males. That suggests the fireflies were mistaking the artificial glow for potential mates.
“Handling prey in different ways suggests that the spider can use some kind of cue to distinguish between the prey species they capture and determine an appropriate response,” explained Tso. “We speculate that it is probably the bioluminescent signals of the fireflies that are used to identify fireflies, enabling spiders to adjust their prey handling behavior accordingly.”
However, the researchers admitted that their experiment used an artificial replication of the firefly glow. Ideally, a field experiment would use real fireflies, although this would be “extremely difficult in practice,” they noted.
Not to mention unethical; many firefly species are listed as threatened.
It’s tempting to believe that the spiders are simply appreciating the soft glow that’s being added to their webs. But as evolutionary behavior tends to go, the researchers’ hypothesis sounds valid. So until we can figure out spider communication, we’ll just have to assume the latter.
