How do flies know which way they are facing?
A fly keeps an internal compass: a ring of neurons where one patch of activity marks its heading. Turns move the patch, and landmarks hold it steady.
In the middle of a fly’s brain, a set of compass neurons is wired in a ring. At any moment only one patch of them is active, and where that patch sits on the ring stands for the direction the fly is facing. Scientists first watched this patch move in a living fly’s brain in 2015.
When the fly turns, turn neurons push the patch around the ring. That alone would slowly go wrong, so the fly also uses what it sees: ring neurons bring in landmarks such as the sun and hold the patch in place. The compass is not magnetic. It is a pattern of activity that the fly keeps up to date, and in the dark it slowly drifts.
See it in a lesson
Key terms
- Compass neurons
- A ring of nerve cells that keeps track of which way the fly is facing. Scientists call them E-PG neurons.
- Activity bump
- The patch of active compass neurons. Where it sits on the ring shows which way the fly is facing.
- Turn neurons
- Nerve cells that fire when the fly turns and push the bump around the compass. Scientists call them P-EN2 neurons.
- Ring neurons
- Nerve cells that bring what the fly sees, such as landmarks, to the compass. They quiet the compass neurons, and that holds the bump in place.
- Landmark
- Something the fly sees that stays in place, like the sun or a tree. The fly uses it to keep its sense of direction steady.