Fly Bearings

A fruit fly, tethered and exposed to a simulated sun, demonstrates an ability to navigate by the sun’s position.

Ysabel Giraldo

Nearly forty years ago, scientists found that fruit flies, Drosophila—not known for their navigational abilities—can fly in a straight path across open desert, for as much as nine miles in a single evening, without losing their bearings. Researchers have recently discovered that the insects have specialized neurons in their tiny brains, called compass neurons, which make this feat possible.

In the new study, researchers from the California Institute of Technology in Pasadena, California, used metal wires to tether flies inside a “flight simulator,” a space surrounded by light-emitting diodes (LEDs). A camera filmed the movements of their tethered, immobilized wings. Data were fed into a computer program that displayed a small bright spot on the LED panels to simulate the Sun. When the flies moved their wings, the “sun stimulus” location would change, based on the flies’ intended direction. This allowed researchers to gauge the flies’ responses to the appearance and disappearance of the light source.

The researchers found that the fruit flies, like many insects and other animals, use the position of the Sun to navigate. “Flies adopt arbitrary headings with respect to the Sun and remember their heading for hours,” says lead author Ysabel M. Giraldo. When the flies were exposed to the sun stimulus, which was turned off and then on again up to six hours later, the flies were capable of continuing on their original headings, suggesting that fruit flies can remember their direction vectors after sunset.

In order to further characterize the brain mechanisms behind fruit fly navigation, the team accessed each fly’s head to examine the activity of fruit fly neurons in real-time, using a galvanometric scan mirror-based two-photon microscope. Activity in “compass neurons” tracked the position of the sun stimulus. When the compass cells were rendered inoperable, however, the flies reverted to a less-effective means of orientation by simply flying toward the brightness of the sun stimulus.

The findings could offer new insights into the navigational neural circuitry in an array of creatures. The Caltech team is examining neurons that connect to the compass neurons in an effort to illustrate how the overall navigational neural system works. Other migrating insects, such as locusts or monarch butterflies, likely rely on the same, conserved neural circuitry. (Current Biology)