Araneus ventricosus Spiders Manipulate Male Firefly Signals

Araneus ventricosus spiders manipulate firefly signals

A seemingly ordinary orb-weaving spider found throughout China, Japan and Korea appears capable of remarkable manipulation.

Researchers have seen Araneus ventricosus capture male fireflies, then prompt them to produce flash patterns normally associated with females.

The outcome is a macabre honeypot: further male fireflies are drawn directly into the spider’s web.

Exactly how the spiders prompt male Abscondita terminalis fireflies to produce these female-like flashes remains uncertain, but fresh research points to an apparent causal link.

"Drawing from extensive field observations, we propose that Araneus ventricosus practices deceptive interspecific communication by first ensnaring firefly males in its web and then predisposing the entrapped male fireflies to broadcast bioluminescent signals that deviate from female-attracting signals typically made by A. terminalis males and instead mimic the male-attracting signals typically made by females," writes a team led by entomologist Xinhua Fu of Huazhong Agricultural University in China.

"The outcome is that the entrapped male fireflies broadcast false signals that lure more male fireflies into the web."

Fu had been studying bioluminescent courtship signals among wild fireflies when he first noticed something unusual involving these spiders during a field expedition.

Nearly every firefly trapped in the webs was male.

On its own, that observation might have meant little. Yet Fu encountered the same trend on later expeditions: female fireflies were only rarely caught in A. ventricosus webs, making the pattern far too pronounced to dismiss as chance.

Firefly courtship flashes in spider webs

After spending many nights observing the spiders, the team discovered that the fireflies’ behaviour was stranger still.

Male and female fireflies communicate in distinct ways when signalling for mates. Males use two abdominal lanterns to create chains containing multiple pulse signals. Females, by contrast, rely on one lantern to make single-pulse signals. These differences allow the insects to distinguish male flashes from female ones and locate one another in darkness.

Males caught in webs were frequently observed producing the single-lantern, single-pulse flashes females use to attract partners. Since this might simply have been coincidental, the researchers carried out a field experiment to investigate further.

They watched free-flying fireflies approach webs classified according to whether spiders or trapped fireflies were present. Across each of the 161 webs monitored, they also recorded the capture rate: the number of fireflies each web had trapped.

Although every ensnared male produced flashes more closely resembling those of female fireflies, the spider’s presence was significantly linked to a greater capture rate. Males caught in webs where no spider was present were less likely to draw other fireflies towards their fatal end.

That finding does not support the idea that trapped fireflies are producing distress signals.

The spider’s wrap-and-bite attack

The team also identified another detail. Once a flashing firefly became caught in a web, the spider approached and carried out a distinctive, repeated wrap-and-bite assault. It performed this sequence only when the firefly’s flashes could be seen; spiders did not use it when fireflies with blackened lanterns were put into the web.

Whether the shift in behaviour is caused by venom or by the bite itself is not known. However, the researchers think that some aspect of the attack triggers a neurotransmitter that changes the insect’s signalling style.

This would not be the first observation of spiders using puppets. Orb-weaving spiders in the genus Cyclosa construct large spider-shaped decoys from dead insects and debris at the centre of their webs. Their purpose may differ, however: to discourage predators rather than attract prey.

Manipulating a living, struggling and doomed insect into summoning its companions to the same fate is an altogether more sinister display of ingenuity. We are probably very fortunate that A. ventricosus venom is harmless to vertebrates.

The research was published in Current Biology.