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For many of us in the animal kingdom, darkness can be scary. At times, though, it can also provide safety. It provides cover. But of course some creatures don't have eyes, which raises the question of how they find the safety of darkness. NPR's Joe Palca tells us about a creature that manages to turn its entire body into a light detector.

JOE PALCA: There's no doubt that for human beings our eyes are where it's at when it comes to detecting light.�

Mr. DAVID BERSON (Neuroscientist, Brown University): We look around the world and we see color and movement and form and we can, you know, think about what we're seeing.

PALCA: David Berson is a neuroscientist at Brown University in Rhode Island. There are special cells inside our eyes called rods and cones that translate light into nerve signals that our brain can turn into pictures. But Berson and others have�found that there are also cells in our eyes that can detect light but don't contribute to making those pictures. These cells simply detect light intensity. But Berson says that's pretty useful too.

Mr. BERSON: As we step into a bright environment, our pupils constrict.

PALCA: A reflex that's triggered by these other light detectors. Keeps us from being blinded by the light.�

PALCA: But now let's step away from the oculocentric world of human light detection and imagine you're a fruit fly larva. Larva is the worm-like stage before the flies pupate and then turn into adults. A larva likes it dark.

Dr. YUH NUNG JAN (Neuroscientist, University of California San Francisco): Larva is known to avoid light.

PALCA: Yuh Nung Jan is a neuroscientist at the University of California, San Francisco.

Dr. JAN: They usually prefer darkness, so if you give them a choice, they would tend to move to the dark side.

PALCA: It's not hard to understand why a fruit fly larva prefers the dark. It's safe and tasty when you've burrowing down deep in a banana. If it starts getting light, you're going the wrong way.�

Jan says scientists know that larvae have primitive eye-like structures that appear to play a role in light detection. But one of his students became interested in another kind of nerve cell that covered the larva's entire body. He tagged these cells with a molecule that glowed green whenever he shone a light on them.

Dr. JAN: And he noticed that every time he shined light on them that the neurons start firing.

PALCA: Firing is what nerve cells do when they're sending signals. At first, they thought it was some mistake, but as they report in the current issue of the journal Nature, Jan and his colleagues now believe these nerves are sufficient to detect light all by themselves.�

Neuroscientist Craig Montell is impressed by the new research. He's at the Johns Hopkins Medical School in Baltimore. What surprised Montell most about the new findings is that the larval nerve cells send their warning signal using something called the TrpA-1 channel.

Mr. CRAIG MONTELL (Neuroscientist, Johns Hopkins School of Medicine): TrpA-1 is already known to be involved in the avoidance of noxious chemical stimuli, noxious olfactory stimuli, and noxious temperatures and uncomfortable temperatures.

PALCA: In other words, if something is annoying and you want to get away from it, TrpA-1 is your channel.

Mr. MONTELL: I find it rather amazing, the very broad role of this one channel in so many sensory avoidance behaviors.

PALCA: A version of the light detectors found in fly larvae have also been found in worms, but so far not in humans.�But it took a hundred years to find them in fruit fly larvae, so they may still turn up in humans.

Joe Palca, NPR News, Washington.

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INSKEEP: And you can find a video of those larval light experiments at our website, NPR.org.

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