This story originally appeared on Atlas of Obscura and is part of Air conditioning desk cooperation.
As he walked through the Peruvian rainforest, on an eight-hour boat trip from the nearest jungle village, biologist Aaron Pomerantz saw what looked like small invisible streams crossing the trail. “I was there with a net, trying to catch things,” he says, “and they just changed direction and disappeared.”
This was his first close encounter with clear-winged butterflies, insects that inhabit the forests of Central and South America and have remarkable means of camouflage: transparent or “glass” wings that make them particularly difficult to spot in dense undergrowth.
“It’s like the power of invisibility,” said Pomerantz, lead author of a recent study in Journal of Experimental Biology which examines how pure wings develop. “If you can put on an invisible cloak, it’s much harder for predators to find you. There are many transparent species in the ocean, but on land this is much rarer. And that really goes into the question, “What does it take to be transparent on land?”
By studying the wings of the species Greta oto, also known as the glass-winged butterfly, at various stages in the development of pupae, Pomerantz and colleagues at the University of California, Berkeley, the Marine Biology Laboratory at Woods Hall and Caltech discovered several factors. There are changes in the shape and density of microscopic scales, which usually create colorful patterns of butterfly. A layer of tiny wax pillars also acts as an additional anti-glare coating.
If it looks like a unique adaptation, it is not. “It has evolved many times,” says Pomerantz. There are several hundred species of glass-winged butterflies and moths, he notes. Although they represent only a small part of the order Lepidoptera, they constitute most of the rare cases of such transparency on land. Glass frogs, which show varying degrees of skin transparency, are another example.
The ocean, on the other hand, overflows with transparent species, from jellyfish and mushrooms to crustaceans, cephalopods and even fish. Earlier in the summer of 2021, two rare observations of a glass octopus were made during an expedition aboard the research ship of the Schmid Ocean Institute Falcor, to watery depths off the remote Phoenix Islands in the Pacific Ocean. It turns out that being invisible is much easier to achieve in the ocean than on land, in part because of the visual and physical properties of water.
“You can think of it as a piece of glass in water,” says marine biologist Laura Bug. “This environment is much more impersonal than on land, and you don’t have to deal with gravity. So most of these animals are some kind of aquatic, floating thing, without the backbone or dense structures that are needed to survive on land.
Imagine this classic Jaws a scene – the one from the shark’s point of view – where the silhouette of a swimmer stands against the light flowing from above. Where the sun shines, underwater predators can easily see opaque shapes, so transparency helps you sneak in. Deeper in the ocean, it remains useful because even in the aphotic zone – the depths where little or no sunlight penetrates – many bioluminescent animals emit their own light, says Bug.
Now a senior biologist at Torch Technologies in Florida, Bagge is fascinated by the transparency of animals during a research cruise for his dissertation at Duke University. She had dipped her hand into a bucket of sea creatures and pulled out a mysterious specimen. “It was hard, like a lobster, but it was a perfectly clear animal,” she says. It was a crustacean, like a shrimp, Cystismwhich can grow as much as a human hand. “They’re so cool because they have a hard shell and they’re full of muscle. How do you clarify this? “