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September 9, 2019

Caterpillars "see" colors with their skin

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From the New York Times:

Peppered moth caterpillars live across the Northern Hemisphere, from the forests of China to the backyards of North America.

But if you've never seen one, don't feel bad: They're experts at blending in.

Each caterpillar mimics the twig it perches on, straightening its knobbly body into a stick-like shape.

It also changes its hue to match the twig's color, whether birch white, willow green, or dark oak brown.

They're so good at this, in fact, that they can do it blindfolded — literally.

According to a paper published in Communications Biology in early August, the caterpillars sense the color of their surroundings not only with their eyes, but also with their skin.

While other animals, including cuttlefish and lizards, have similar abilities, this is "the most complete demonstration so far that color change can be controlled by cells outside the eyes," said Martin Stevens, a professor of sensory and evolutionary ecology at the University of Exeter.

Dr. Stevens, who was not involved in the study, added that the exact mechanism remains a mystery.

The adult peppered moth is famous for a completely different color journey; After soot from the Industrial Revolution darkened tree bark in Britain, peppered moths there evolved to be darker, too.

Ilik Saccheri, a professor of ecological genetics at the University of Liverpool and an author of the new paper, normally studies the adult moth.

This requires keeping a lot of caterpillars around.

Years of observation sparked his curiosity about their color-changing abilities, which happen individually and in a matter of minutes rather than over generations.

Each caterpillar hatches tiny and black, and in its early days is blown around by the wind.

Once it falls on a plant, it must camouflage itself to avoid being spotted by hungry birds.

This process, which involves producing new pigments, plays out over a period of days or weeks.

"I was a bit disbelieving that they could change that accurately only using their eyes," which are quite simple at the larval stage, Dr. Saccheri said.

Amy Eacock, a graduate student in Dr. Saccheri's lab at the time of the study, and another of the paper's authors, decided to investigate.

First, she tested the caterpillars' eyes for light-sensitive proteins called opsins.

"We did the skin as kind of a negative control," said Dr. Eacock, who is now at the Max Planck Institute for Chemical Ecology in Germany. "But then we found they were expressed in the skin as highly as they were in the head."

That's when she suggested blindfolding the caterpillars.

The researchers could cover their eyes, place them on different-colored dowels, and see if they still changed to match.

"I thought it was kind of a crazy idea," Dr. Saccheri said. The blindfolding itself was a difficult endeavor, involving black paint, a "tiny, tiny paintbrush," a microscope and lots of patience, Dr. Eacock said.

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Caterpillars molt several times over their lives, which required that they be blindfolded again frequently. (This also meant that when the experiment was over, the research subjects got their eyesight back.)

But Dr. Eacock persisted, eventually testing more than 300 caterpillars.

First, they tried green and brown dowels.

For later groups, the researchers used black and white dowels.

Over and over again, the blindfolded caterpillars successfully changed color to blend in.

The team even placed some caterpillars on striped dowels, and "they did go stripey," Dr. Eacock said, although the sample size for that experiment was too small to include in the published work.

In another test, older caterpillars with covered eyes were given a choice of dowel colors, and reliably climbed onto the one that they already matched. (Because it takes a caterpillar many days and a lot of energy to change color, reverse-engineering the camouflage in this way is often more efficient.)

Many questions remain, including exactly how the caterpillars receive and use the color information, and whether the opsins are actually involved in the process.

But the behavior alone impressed the researchers.

Dr. Eacock said: "I remember turning around to Ilik one day and being like, 'You know what? The caterpillars are cooler than the moths.'"

Dr. Saccheri agreed. "They’re pretty amazing," he said.

September 9, 2019 at 04:01 PM | Permalink


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