A new study has documented something that appears to be intentional cruelty in spiders. Not only are they catching fireflies, they’re keeping them alive just long enough to turn them into bait.
Researchers studying Psechrus clavis, a sheet web spider native to East Asia, noticed their webs often held live, glowing fireflies. The spiders didn’t eat them right away. They waited. Other insects, drawn to the light, were also trapped. Those insects were eaten immediately. The fireflies were left to glow until they weren’t helpful anymore.
Published in the Journal of Animal Ecology, the study describes how the spiders’ hunting success increased dramatically when a firefly was left hanging in the web. “Our findings highlight a previously undocumented interaction where firefly signals, intended for sexual communication, are also beneficial to spiders,” said lead author I-Min Tso of Tunghai University in a statement.
Spiders Spotted Trapping Fireflies in Their Webs for Bait
To confirm the glow was doing the work, the researchers set up fake webs with LED lights that mimicked fireflies. Those webs attracted three times more prey than unlit controls. They even drew in ten times more fireflies than the unlit ones, which suggested the spiders weren’t just getting lucky.
Most of the trapped fireflies were male, likely mistaking the steady glow for a female waiting nearby. But rather than attacking immediately, the spiders waited. They circled back to check on the still-living insects, only feeding once the glow had faded and the lure had done its job. Brutal.
Tso and his team think the spiders may be responding to bioluminescent cues, using them to identify the fireflies and delay feeding. “Handling prey in different ways suggests that the spider can use some kind of cue to distinguish between the prey species,” he said.
Predators like anglerfish evolved their own glow to lure food. These spiders don’t need to. They just let their prey do the work for them.
There’s no delicate way to frame it. A glowing male, broadcasting his availability, gets trapped and left alive while moths and midges die instantly. Then more insects arrive.
The spider remains still beneath the web while the firefly keeps glowing above it. The light does what it was meant to do, just not for the reason it was made.
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