Spider uses trapped fireflies as glowing bait to attract more prey

(Press-News.org) Ecologists have observed a species of nocturnal spider attracting prey to its web using the bioluminescent beacons of already trapped fireflies. This rare example of a predator exploiting its prey’s mating signal for its own gain is documented in the British Ecological Society’s Journal of Animal Ecology.

Researchers at Tunghai University, Taiwan have observed sheet web spiders Psechrus clavis capturing fireflies in their webs and leaving them there while they emitted bioluminescent light for up to an hour. The researchers even observed the spiders going to check on the captured fireflies from time to time.

Intrigued by this unusual behaviour the researchers set up an experiment to test whether this was a strategy used by the spiders to increase their hunting success. In the experiment, they placed LEDs that resembled fireflies, in real sheet spider webs and left other webs clear as controls.

They found three times the amount of prey was attracted to webs with the LEDs compared to the control webs. This increased to ten times more prey when they only looked at fireflies being captured.

The findings confirm that captured fireflies left as bait increase the hunting success rate of the spiders. The researchers also noticed that the majority of captured fireflies were male, who were likely mistaking the glow for potential mates.

Dr I-Min Tso, the lead author of the study said: “Our findings highlight a previously undocumented interaction where firefly signals, intended for sexual communication, are also beneficial to spiders.

“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.”

The researchers suggest that this behaviour could have developed in sheet web spiders to avoid costly investment in their own bioluminescence like other sit-and-wait predators, such as anglerfish. Instead, the spiders are able to outsource prey attraction to their prey’s own signals.

The sheet web spider Psechrus clavis is a nocturnal sit and wait predator found in subtropical forests of East Asia. It’s main source of prey, the winter firefly Diaphanes lampyroides, uses continuous, non-flashing bioluminescence to attract mates.

Video footage captured by the researchers in their experiment shows sheet web spiders employing different strategies when interacting with different prey species. Spiders would immediately consume any moths captured in their webs but would not immediately consume fireflies they captured.

“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 Dr I-Min 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.”

The researchers conducted their field experiment in the conifer plantation forest at National Taiwan University’s Xitou Nature Educational Area.

Because they used LEDs to mimic the light signal emitted by fireflies, the researchers warn that although the wavelength and intensity of the LED set up was a close match to fireflies, it would be best if real fireflies were used in the field experiment. But they admit that this would be extremely difficult in practice.

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