Scientists can determine which part of the brain is activated by various stimuli, so they now know that the fly processes information about a bitter taste in a different region of the brain than it uses if the taste is sweeter. That's cutting edge stuff, but Scott and her colleagues at Berkeley just needed a little glue and some chemicals to find at least some of the answers to their questions.
Unlike humans, the fruit fly has taste neurons on its wings and its legs, as well as its proboscis, or mouth. Scott wanted to know if the fly has a "body map" in its tiny brain. If it does, when a nerve cell on the fly's leg finds something sweet, the fly should know exactly where that sweet taste is and whip around with its proboscis and take a bite.
"We're meanies," admits Scott, who says her team glued the wings together on a few fruit flies so they could see what happens if just the leg of a fly touches something that is bitter or sweet.
"If you dip a fly's leg in sugar, then the fly's proboscis extends," revealing that the fly immediately knows the location of the sugar, Scott says. "And if you dip the leg in sugar but you have some bitter substance in there, then the proboscis won't extend."
So its taste neurons also serve as locators, but Scott wanted to take it a step further. Flies have two sets of taste neurons, and the researchers wanted to show that one set is used to detect bitterness and the other for sweetness.
So they disabled specific neurons to see which ones did what. When some were disabled, the proboscis did not extend when the fly's leg was poked into sugar water, so the fly no longer knew the sugar was sweet.
In the end they determined that taste receptors among the flies are similar to those found in mammals. Most are used to detect bitterness, not sweetness, because nature provides a more abundant variety of bitter compounds. That's important for a fly to know because bitterness can also signal the presence of poisons.
Taste Has Final Say
All this tells the researchers a bit more about how the fly earns its living. Scott thinks the fly uses odor to detect the presence of food, and that's also similar to how humans do it. It can be hard to pass up the smell of a chocolate-covered donut, for both human and fly.
"Flies can travel long distances for a banana because they can smell it," she says.
But what if they get there and part of the banana is contaminated? How do they find something worth eating in a bucket full of garbage? That's where the taste neurons come in.
"The taste system is just sort of the final checkpoint," Scott says. "The taste system only works when the fly is already on the food. It helps it make the decision to eat it, or don't eat it."
Sort of like us. If it doesn't taste good, we're not likely to eat it, even if it smells like a banana.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.