At night in the forests of East Asia, fireflies glow to attract mates. But sometimes their signals are hijacked. Sheet web spiders have been caught turning these insects into unwilling accomplices, leaving them glowing in their webs to draw in more prey.

Researchers in Taiwan filmed Psechrus clavis spiders snaring winter fireflies (Diaphanes lampyroides) and then refusing to eat them right away. Instead, the spiders let the beetles shine for up to an hour. That’s about the same length of time a female firefly normally glows from a fixed spot.

“Our findings highlight a previously undocumented interaction where firefly signals, intended for sexual communication, are also beneficial to spiders,” said Dr. I-Min Tso of Tunghai University.

Fireflies as Bait

The strategy works. Spiders that left fireflies glowing in their webs attracted far more visitors than those with empty webs. To test this, Tso’s team placed LED lights that mimicked fireflies into some webs, while leaving others dark. The glowing webs caught three times more total prey. When only fireflies were counted, glowing webs caught ten times more.

And the spiders were well aware of their prisoners. The researchers saw them check back on the fireflies, as though monitoring their light levels. This is unusual among predators. Anglerfish and glow-worms evolved their own bioluminescence to lure victims. But these spiders have taken a shortcut. They “outsourced” the work of glowing to the fireflies themselves.

Most of the tricked and trapped fireflies were male. They may have mistaken the steady glow of a trapped firefly for a potential mate. Instead, they ended up entangled in the spider’s silk.

Outsmarting the Night

The video footage shows how selective the spiders are. When moths crash into their webs, they’re eaten right away. But fireflies are kept for their utility. “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,” Tso explained. He suspects the glow itself is the signal.

This discovery expands our understanding of predator-prey relationships. It also adds another layer to the evolutionary arms race between insects and their hunters. Fireflies use light to find love. Spiders use that same light to turn romance into a death trap.

For the fireflies, their sexy glow has become a liability. For the spiders, it’s a clever hack and a way to rise to the challenges of hunting without expending extra energy. As Tso put it: “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.”

In the end, it’s a story about what happens when a mating signal doubles as a dinner bell.

The findings appeared in the Journal of Animal Ecology.