Why are flies so hard to swat?
A fly sees the swatter coming early, and a fast escape circuit launches it into the air in a few hundredths of a second, before the swatter lands.
Three things work in the fly’s favor. Its eyes cover almost the whole space around it, so it is hard to sneak up from behind. Its vision is faster than ours, so a hand looks slow to it. And it has neurons with one job: they fire when something grows quickly in view, the way a swatter does as it comes closer.
Those looming neurons connect to the giant fiber, a very thick nerve cell that runs from the brain down to the neurons that drive the jumping legs. A thick fiber carries its signal fast, so the order to jump arrives almost at once. When it has a moment, a fly first shifts its legs so the jump carries it away from the swatter. In an emergency, the giant fiber can launch it at once.
See it in a lesson
Key terms
- Looming neurons
- Nerve cells behind the fly’s eyes that fire when something grows quickly in view, like a swatter coming closer. Scientists call them LC4 and LPLC2.
- Giant fiber
- A very thick nerve cell that runs from the brain down into the nerve cord. It carries the fly’s fastest escape signal. A fly has two, one on each side.
- Jump neurons
- Motor neurons that make the jump muscles of the middle legs contract, so the fly springs into the air.
- Reflex arc
- The route a reflex takes: from sensory neurons, through one or more relay cells, to the motor neurons that move a muscle.