Dr. Chun Han/University of California, San Francisco
A fruit fly larva's body is covered with a network of photosensing neurons that help it detect and avoid light. The cells glow under a light with the help of a green fluorescent protein.
Fruit fly larvae like the dark, but they have a problem: They don't have eyes. So finding the dark might seem tricky. Now, scientists in California have shown that the larvae's entire bodies are covered with nerve cells that can detect light.
These light-detecting nerve cells are quite different from what we humans have in our eyes.
"We look around the world and we see color and movement and form, and we can think about what we're seeing," says David Berson, a neuroscientist at Brown University.
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. That, Berson says, is useful too.
"As we step into a bright environment, our pupils constrict," he says. That reflex is triggered by these non-image-forming light detectors.
But let's step away from the oculo-centric world of human light detection and imagine you're a fruit fly larva. Larva is the wormlike stage before the flies pupate and then turn into adults.
It's not hard to understand why a fruit fly larva prefers the dark — it's safe and tasty when you're burrowing down deep in a banana. If it starts getting light, you're going the wrong way.
Yuh Nung Jan, a neuroscientist at the University of California, San Francisco, says scientists know that the larvae have primitive eyelike structures that appear to play a role in light detection. But one of his students became interested in another kind of nerve cell that seemed to spread over the larva's entire body. He tagged these cells with a molecule that glowed green whenever he shone a light on them.
Xiang, et al./Nature
These sequences of images show how two strains of fruit fly larva responded when a white light was shone on their heads. The light spot, indicated by the dotted circle, was turned on for five seconds, from the "0 s" frame to the "4 s" frame.
These sequences of images show how two strains of fruit fly larva responded when a white light was shone on their heads. The light spot, indicated by the dotted circle, was turned on for five seconds, from the "0 s" frame to the "4 s" frame. Xiang, et al./Nature
VIDEO: Look, Ma, No Eyes!
A fruit fly larva's body is covered with light-sensitive cells that allow it to detect and avoid harmful light. When it approaches a beam of light in the video below, it reacts swiftly and changes direction.
"He noticed that every time he shined a bright light on them, the neurons start firing," says Jan. Firing is what nerve cells do when they are 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 as important — if not more important — than the larvae's primitive eyes in detecting light.
Light Triggers An 'Avoidance' Response
Neuroscientist Craig Montell, a researcher at Johns Hopkins University School of Medicine, is impressed by the new research. What surprised him about the new finding is that the larval nerve cells send their warning signal using something called the TrpA1 channel.
"TrpA1 is already known to be involved in the avoidance of noxious chemical stimuli, noxious olfactory stimuli, and noxious temperatures, uncomfortable temperatures," Montell says.
In other words, if something is annoying and you want to get away from it, TrpA1 is your channel.
"I find it rather amazing, the very broad role of this one channel in so many sensory avoidance behaviors," he says.
A version of the kind of distributed light detectors the California scientists have found has also been found in some worms. They haven't been found in humans, at least not definitively. But it took 100 years to find them in fruit fly larvae, so they may still turn up in humans.