Eyeless Larvae 'See' With Full Body Nerve Cells
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.
Light-Sensing Bodies
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.