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Species: Metalmark moth (Brenthia coronigera)

Habitat: Leaping about vegetation in Indo-China

A moth that looks and acts just like a spider is so convincing that it receives elaborate courtship displays from its predator.

Many prey species mimic other poisonous prey or blend into the background to escape predators. The metalmark is one of the few that mimics its predator.

The impersonator’s black, beady “eyes” are actually patterns on its wings, and its “furry legs” are contorted wings with a striped pattern. This gives the impression that it is a big spider. And instead of fluttering like other moths, the metalmark makes jerky leaps like the jumping spiders it mimics.

“It confuses the spider. If the spider is smaller, it even intimidates the spider”, says , an entomologist at the University of Connecticut in Storrs, who was not involved in the study. Cannibalism is common in spiders, so smaller ones prefer to run away rather than risk being eaten.

The moths also display a peacock-like behaviour. They raise their forewings and twist their hindwings to show off eyespots and stripes to maximum effect. These appear on the upper and lower surface of the wings, so the moth looks like a spider from the back as well as the front.

Winning combination

To find out which among these strategies – the wing pattern, jerky flight or peacock posture – is most critical for the mimicry to work, at the National Sun Yat-sen University in Taiwan first presented a Brenthia coronigera to jumping spiders in the lab.

The spiders responded by performing courtship displays, raising and waving their first pair of legs at the moth. Other moths used as a control did not elicit any such response from the spiders – they just got eaten.

“The reason the spider exhibited ‘leg-waving’ behaviour to the moth is that the moth was mistaken by the spider as conspecific,” says Yen.

When the researchers painted over the moth’s eyespots, the mimicry seemed to fall apart – the spiders did not perform courtship displays, and instead spent some time examining the moths, then ate them.

To check the importance of the jumpy flight pattern and the peacock posture, the researchers froze some metalmarks and presented them to the spider. These frozen moths came under attack.

The experiments showed that the loss of any one component made the charade much less effective.

“If I had to guess, I might put the posture as the last thing to evolve… and the other two would have come first,” says Wagner. “To me, it is the icing on the cake that really sells the disguise.”

Journal reference: Animal Behaviour, DOI: 

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