Firefly caught in a sheetweb spider's web
This news will thrill devotees of a creationist god of divine malevolence; but for those who prefer their deity to resemble the all-loving god of the New Testament, it will be a cause for concern. Even more worrying for the latter, the current campaign by creationist organisations such as the Discovery Institute—trying to promote the pseudoscience of Intelligent Design as a scientific alternative to evolutionary biology—unwittingly strengthens the case for a malevolent designer. After all, Michael J. Behe’s notion of irreducible complexity and William A. Dembski’s concept of complex specified genetic information apply just as well to parasites and stealth predators as they do to supposed “beneficial” features such as human intelligence or the biological systems that keep us alive.
The latest example comes from scientists at Tunghai University, Taiwan, who have shown that a sheet-web spider has evolved a particularly nasty trick for luring fireflies to their deaths. The method is brutally simple: once the spider captures a firefly, it keeps it alive, so its flashing courtship signal continues to glow. Instead of attracting a mate, the unfortunate insect draws more fireflies into the spider’s web—and to certain death.
To an evolutionary biologist, this is a fascinating demonstration of how a mindless natural process can hone behaviour in whatever direction produces more offspring, regardless of whether humans judge the outcome “good” or “evil.”
Taxonomy & Distribution of Psechrus clavis.The scientists report their findings in the Journal of Animal Ecology, published by the British Ecological Society. The study is also explained in lay terms in a news release from the Society.
- Psechrus clavis, described by Steffen Bayer in 2012, is part of the genus Psechrus (family Psechridae)[1].
- It is endemic to Taiwan, according to taxonomic listings [2].
- The broader family, Psechridae, ranges across southeastern Asia — from India through China, Southeast Asia, to northern Australia—inhabiting forests, rocky terrains, and caves up to altitudes of over 2,000 m [3].
Evolution & Phylogeny
- Members of Psechridae, including Psechrus, are cribellate araneomorph spiders — that is, they use a special silk-spinning organ called the cribellum [2].
- Recent research has uncovered that species richness within Psechrus is greater than previously believed, with about 46 species formally recognized at the time of Bayer’s revision — and P. clavis among the newly described [1].
- Phylogenetically, Psechridae are placed near families such as lynx spiders, wolf spiders, and nursery web spiders, within the so‑called RTA clade - a grouping supported by molecular and morphological studies [3].
Ecology – A Dazzling Hunting Strategy
The most compelling insight into Psechrus clavis comes from this study revealing its remarkable predatory behaviour:
- P. clavis is a nocturnal sit-and-wait predator constructing funnel‑shaped sheet webs—typically positioned at the bases of trees or among rocks [4].
- In Taiwan’s forests (specifically in the Xitou Nature Education Area), researchers observed that these spiders capture fireflies but deliberately leave them alive in the web for up to an hour. The trapped fireflies continue glowing, essentially acting as unwitting beacons that lure other insects—including additional fireflies—into the trap [5].
- Experimental setups using LEDs replicating firefly bioluminescence confirmed the strategy’s effectiveness: lit webs attracted three times more prey, and in the case of fireflies specifically, capture rates jumped tenfold compared to unlit control webs [5].
- Observations showed that prey handling varies by species: moths are consumed immediately, while fireflies are left alive—indicating that P. clavis may distinguish prey types, using bioluminescent cues to optimise its hunting strategy [6].
- Ecologists interpret this as an energy-efficient adaptation: instead of investing in its own bioluminescent lure (as some creatures like anglerfish do), the spider outsources prey attraction by repurposing the signalling system of its prey [7].
Summary: Key Facts at a Glance Category Details Species Psechrus clavis Bayer, 2012 Distribution Endemic to Taiwan (subtropical forests) Taxonomic Group Genus Psechrus; Family Psechridae (cribellate araneomorph spiders) Phylogeny Related to wolf, lynx, and nursery web spiders within the RTA clade Hunting Strategy Uses glowing trapped fireflies as live bait to lure further prey; verified via LED mimicry experiments Ecological Significance A novel example of predator-prey co-option of mating signals; energy-efficient evolutionary adaptation
Spider uses trapped fireflies as glowing bait
A species of nocturnal spider attracts prey to its web using the bioluminescent beacons of already trapped fireflies
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.
Sheetweb spiders have outsourced prey attraction to their prey's own signals.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.
Dr I-Min Tso, corresponding author.
Department of Life Science
Tunghai University, Taichung, Taiwan.
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. 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.
Dr I-Min Tso.
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.
Publication:
AbstractThese cunning hunting strategies don’t appear by sheer accident, nor are spiders sitting around working them out and running evening classes for their offspring. Such behaviour arises only through the blind but effective filter of natural selection, honed over deep time in response to fireflies whose own evolved mating system depends on bioluminescent signalling.
- As behavioural and physiological processes can be costly for animals to employ, deception and other dishonest strategies may become necessary for sit-and-wait predators.
- Sheet-web spiders Psechrus clavis have been known to use their body colour and webs as visual cues to deceptively lure and immediately consume lepidopteran insects. However, they do not immediately consume trapped male fireflies Diaphanes lampyroides; instead, the spiders retain them in their webs while the fireflies continue to emit their bioluminescent signal for up to an hour. This observation raises the question: can the spiders exploit prey signals to attract additional prey, thereby enhancing their foraging success?
- We hypothesized that such a unique prey-handling strategy serves to enhance P. clavis' foraging productivity by exploiting the prey's own signal as a deceptive lure. To test this hypothesis, we performed a field manipulative study using light emitting diodes (LEDs) resembling the bioluminescent signal of D. lampyroides and evaluated the responses of fireflies and other prey using infrared video cameras.
- We found that the presence of the simulated firefly signal significantly increased prey attraction and interception rates within P. clavis webs. We also found male D. lampyroides to comprise the bulk of the additional prey obtained. We speculate that this enhanced foraging return allows P. clavis to reduce its energetic and nutritional investment in insect-attracting body coloration.
- 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.
Yip, H. Y., Blamires, S. J., Liao, C.-P., & Tso, I.-M. (2025).
Prey bioluminescence-mediated visual luring in a sit and wait predator. Journal of Animal Ecology, 00, 1–10. https://doi.org/10.1111/1365-2656.70102
© 2025 John Wiley & Sons, Inc.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
Creationists, of course, refuse this explanation, preferring instead to ascribe it all to an intelligent designer. They eagerly point to complexity and Dembski’s so-called “specified genetic information” when it produces something they like—especially when it flatters human vanity—but suddenly fall silent when the same logic leads straight to grisly, predatory tricks like this.
Confront them with the question of why their designer would painstakingly equip fireflies with a bioluminescent courtship system only to turn it into glowing bait for a hungry spider, and the response is depressingly predictable: a retreat into hand-waving cliché about “the Fall” or “Original Sin.” At that moment, the pretence that Intelligent Design is scientific collapses, and it is revealed for what it really is: theology in a lab coat, scrambling to protect Bronze Age mythology from the evidence of nature’s indifference.
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