A male firefly is caught in the web of an orb-weaving spider, Araneus ventricosus. Researchers believe that these spiders may manipulate the bioluminescent signals that fireflies use to find mates.
PHOTOGRAPH BY XINHUA FU
Spiders have evolved an impressive array of hunting techniques—from trapping their prey with spit to building webs strong enough to capture snakes. Now, researchers have discovered a particularly malicious tactic that some orb-weaver spiders might use to trick fireflies into their webs.
A new study published in Current Biology found that some spiders seem to manipulate the flashing signals of male fireflies they've ensnared in their webs to mimic the signals of a female. This glittering call of love attracts other male fireflies to the spider’s web, just like a siren song attracts sailors to death.
(These spiders lure their prey in very crafty ways.)
The idea that spiders would put off a meal to use their prey as bait is in itself intriguing, says Dinesh Rao, a researcher at the Universidad Veracruzana who reviewed the paper but wasn't involved in the study. "Spiders are always hungry, right? So to say, 'Okay, I'm not going to eat this firefly now and I'll wait for the next one', it's […] very interesting."
But while Rao and other experts agree that something is changing the flashing patterns of the captive males, they say that more research is needed to determine whether the spiders are actually behind it—and how they're pulling it off.
A deadly love call
While observing spider webs in the field in 2004 during his Ph.D. research, Xinhua Fu, a firefly researcher from Huazhong Agricultural University in China and lead author of the study, noticed something peculiar: Only male fireflies were caught in the spider's deadly traps. Even stranger, some trapped males were emitting female-like flashing signals.
Did the spiders have something to do with this phenomenon?
To investigate, Fu and his team set out in a village near Wuhan, China, in an area of farmland dotted with paddy fields and ponds. There, they choose to study Araneus ventricosus, a common orb-weaving spider species which weaves a new web each evening, when fireflies also become active.
The team captured male fireflies with nets and placed them on the spider webs using fine tweezers. Using video cameras, the researchers observed what happened on the webs in different scenarios.
When a male firefly was ensnared in the web, the spider first wrapped up the firefly then bit it in the thorax, injecting a small amount of venom, Fu explained to National Geographic in an email. After that, the spider left the male firefly in the middle of the web and hid in the margin.
(How do fireflies get their glow? We finally have some answers.)
Soon, the trapped firefly started to produce the female-like flashing signals—which consist of single-pulse flashes—attracting other male fireflies looking for a mate to the web. When the firefly stopped flashing, the spider repeated the operation. Fu says the whole process usually lasted for two hours, after which the spider began to feast on its prey.
Fu says the team was surprised by the behavior given that the web spiders are known to have poor eyesight. Despite this, he says, it seems they "still can detect different flash pattern and intensity."
The need for more evidence
But what actually caused the firefly flashing signals to change? Fu and his team of researchers hypothesize that the spiders were manipulating the firefly flashing signals in some way—perhaps, they speculate, through their venom.
But more evidence is needed to support that hypothesis.
"Overall, the paper is very interesting," says Rao. "The only thing that I'm not fully convinced by is whether the spider is actually doing something to change the flashing of the male fireflies." He notes that a neurobiological study would be needed to understand "what exactly is causing the change."
(Everything you think you know about spiders is wrong.)
Kathryn M. Nagel, a Ph.D. candidate who specializes in spider behavior at the University of California, Berkeley, agrees. "It appears something is altering the signals, but the evidence provided in this study isn't enough to identify what specifically is causing the changes," she writes in an email, adding that "further research is needed to determine if the spider's actions […] are directly manipulating the signaling behavior."
As a next step, Fu says he and his team would like to study "how web spider venom affects flash control in fireflies."
Spiders are known to use mimicry and deceptive signals to capture prey. Some species of spider who hunt other spiders, for example, manipulate "web signals to mimic captured prey in order to lure the prey spider towards them," Nagel says.
"Often arthropods are thought of as 'simple' organisms lacking sophisticated behavior, but this is untrue," says Nagel. "This study, and others like it, highlight how organisms previously dismissed are capable of complex behavior, and how we still have so much to learn about spider behavior."
