As the Sun sets over the paddy fields and ponds of Wuhan, China, male fireflies seek out the distinctive flickering of females. But what appears to be an opportunity to mate could be a deadly ambush. Some orb-weaving spiders force trapped male fireflies to mimic the flashes of females, researchers report today in Current Biology, drawing ever more males into their webs.

"It's a really fascinating study," says Sara Lewis, a biologist emeritus at Tufts University not involved with the work. Male fireflies sometimes change the patterns of their flashes when they try to stand out among other males, she notes, but the new study is the first to document a pattern change potentially triggered by a predator.

Among tropical Asian fireflies of the species Abscondita terminalis, both males and females flash to find mates, but their flashes are not the same. Females make slow, single pulses with a sole "lantern" in their abdomen, whereas males flash in quick succession from dual abdominal lanterns.

Both sexes should be easy prey for Araneus ventricosus, a nocturnal orb-weaving spider found in China. However, during his field trips to Wuhan, Xinhua Fu, a biologist at Huazhong Agricultural University, noted several webs with only male fireflies trapped in them. He wondered whether the arachnids were manipulating these captured males to attract more male prey.

"The first time he told me, I couldn't believe it," says Daiqin Li, a biologist at the National University of Singapore and a co-author of the new study. Driven by curiosity, the two researchers embarked on a field experiment in Hubei province. Fu, Li, and their team analyzed 161 spiderwebs in the field and divided them into four groups.

For the first group, they placed a male firefly on a web where a spider was present. For the second, they placed a firefly on a web with no spider. In the third, they placed a firefly in a web with a spider, but covered its lantern with black ink to block its light. In the fourth, neither fireflies nor spiders were present.

Webs with a flashing firefly guarded by a spider caught up to seven male fireflies, the team found. Webs without spiders, in contrast, caught no more than two fireflies.

The researchers also found that the spiders behaved differently depending on whether the fireflies' lights were visible: If the fireflies' lanterns were not covered, the spiders would promptly wrap and bite them, keeping the insects alive and allowing their lights to continue to flicker. The spiders would also do that with males that fell into the trap after the first male was captured. Conversely, if the firefly lanterns were obscured with ink and did not flicker, the spiders ate the insects right away. "Perhaps spiders simply misidentify the firefly with the blackened-out lanterns as some other insect," the authors say.

After analyzing the light patterns of the fireflies trapped in the webs, Fu and Li confirmed their suspicions: The males were only using one of their lanterns, mimicking the pattern of the females.

Two male fireflies were attracted to a web by the flickering of the firefly in the center.XINHUA FU

In the animal kingdom, some predators can manipulate their environment or their own behavior to lure in additional prey. However, a predator changing the behavior of prey to attract more prey is not as commonly observed, Lewis says. The closest case is some parasites that alter host behavior, such as the "zombie ant fungus" (Ophiocordyceps unilateralis), which infects ants and compels them to climb vegetation, helping the fungus grow.

The work is a "stepping stone" for the field, says Orit Peleg, a biological physicist at the University of Colorado Boulder. However, she says, more research is necessary to establish a cause-and-effect relationship. "It's clear that there's a change in the behavior of the firefly," Peleg says. "But is the spider really actively manipulating the firefly, or is it just a firefly's natural predation response from which the spider indirectly benefits?"

Li and his team propose that the spider venom might block oxygen from reaching the firefly organs that produce the light. Or it could affect neurotransmitter action in the insects, leading to changes in their flashing.

For Lewis, an obvious follow-up experiment would be to inject male fireflies kept in the lab with spider venom and see how it affects their flash behavior. "I'm surprised that they didn't do that," she says. "But hey, you can only do so much."