Toxic and invisible: The defenses of transparent butterflies

Transparency is a powerful tool in nature, helping to hide species from predators. Usually, it's underwater creatures that have the ability to make themselves partially invisible, but some butterflies have window-like wings that help to keep them safe.

In some butterfly species, brightly colored wings are a sign that the butterfly has chemical defenses. This means predators learn to avoid them. And non-toxic species can evolve to mimic the toxic ones so that predators don't bother them either.

"Mimicry rings" can form where both toxic and nontoxic butterfly species all display similar colors and patterns. "The most amazing place to see this is the Amazon," says Nipam Patel, director of the Marine Biological Laboratory in Massachusetts in the US. "You'll find a group of species that are distantly related to each other, yet they've all converged on a similar wing pattern."

In this photo there are three examples of mimicry rings. The middle row is a transparent mimicry ring. The butterflies in the first and second columns are toxic, while the butterflies in the third row are nontoxic. The butterflies might look like they're more closely related by rows, but in terms of evolution they're more related by columns.

Interestingly, mimicry rings have also been seen in clear-wing species in the Amazon. Scientists at the University of Chicago wanted to understand why a butterfly species would be both transparent and toxic at the same time.

The scientists examined the optical and structural properties of transparent butterfly wings within mimicry rings. They found that one of the key toxic butterflies didn't have an anti-glare coating on its wings, so in sunlight it was really easy to see.

"It may be signaling a warning pattern to predators when it's in bright sun, and it's camouflaged when in shadows. So it kind of cheats: it has the best of both worlds," said Patel.