Moths Elude Spiders by Mimicking Them, Study Says

The arrival of a jumping spider sends most moths into a flutter trying to escape the predator's lethal pounce.

Not so for metalmark moths in the genus Brenthia. These moths stand their ground with hind wings flared and forewings held above the body at a slight angle.

In that pose the moth looks like a jumping spider, said Jadranka Rota, a graduate biology student at the University of Connecticut.

"That will actually save [the moth's] life," she said.

"The spider needs to act pretty quickly. Deciding whether the moth is potential prey or another jumping spider could take enough time to offer an advantage, in comparison to other moths."

The trickery usually buys the metalmark moth time for a safe escape.

Sometimes the sight triggers territorial postures—raising and waving of the forelegs—from the spider.

Occasionally the spider even backs off.

Rota and her advisor David Wagner described the metalmark moths' behavior last December in the Public Library of Science's interactive online journal PLoS ONE.

Lab Tests

Mimicry is a well-known trick in the animal kingdom. Many creatures are known to adopt the looks and postures of undesirable prey species to evade their predators.

But rarely have scientists seen prey mimic their predators to successfully avoid becoming dinner.

Rota said she first noticed the metalmark moths "do something weird" when she was walking through the Costa Rican forest and saw them perched on leaves with their wings flared, seeming to jump around.

Wagner suggested that the moths might be mimicking jumping spiders, which are known to employ their unusually keen eyesight to hunt.

To find out, the biologists pitted the presumed mimic moths and normal moths against jumping spiders in the lab. The pair staged 146 of battles; 77 with the mimics, 69 with controls.

When control moths were used, the test spider captured 62 percent of its potential prey.

When paired with the presumed mimic moths, the spider only took 6 percent of its allotted victims.

In addition, the spider made territorial gestures towards 36 percent of the mimics, but no gestures toward the normal moths.

In 11 of the trials, the spiders even backed away from the mimics.

The moths' "ploy of donning the wolf's clothing proves successful," Rota and Wagner conclude in their paper.

Erick Greene is a biologist at the University of Montana. In 1987 he was part of a team that published research in the journal Science on a fly that also mimics jumping spiders.

"These sorts of mimetic interactions may be more common than anyone had suspected," he said in an email from New Zealand, where he is currently on sabbatical.

"And sometimes the patterns are incredibly specific to one very narrow type of interaction, such as [with jumping spiders]."

Evolution Driver

Spider mimicry may also help the moths avoid predation by birds and other animals through what scientists call evasive prey mimicry, the University of Connecticut's Rota said.

This type of mimicry gives insects protection when they look like prey that is hard to catch.

"Birds don't go after hard-to-catch insects, and since jumping spiders are hard to catch, looking like a jumping spider may be advantageous," Rota said.

She has yet to obtain experimental data to back up this theory, but may do so in future research.

The researchers noted that in addition to metalmark moths and flies, mimicking jumping spiders has been suggested for several planthoppers and other moth species.

"The jumping spider predation seems to be an important selective pressure," Rota said. "They are shaping the evolution of all these insects."