A Moment of Science

Beans That Go Jump in the Hand

On the table in front of you, it looks like a small, dried bean. Pick it up.To your surprise, it starts to wiggle, shifting around nervously in your palm. Suddenly, it squirms back onto the table again. We’ve all heard stories about Mexican jumping beans. What are they, and how do they do that remarkable trick?

Actually, jumping beans aren’t beans at all. They are the seed pods of a certain shrub that grows in the rocky desert areas of northern Mexico and the American south west. These seed pods can’t jump by themselves. What makes a jumping bean so jumpy is the larva of a small grey moth that has burrowed inside the seed pod and eaten the seed. Once the seed is gone, this larva has a peculiar fondness for leaping about inside the empty pod, making its new home jump and roll from place to place. These larvae even rig up a network of silk lining inside the pod, to help transfer more jumping energy into the seed pod wall and make it hop and roll farther.

Why do these larvae want to jump in the first place? Jumping beans tend to jump more when they are warmed up. In fact, the heat of your palm is enough to start that larva wiggling. It’s quite possible that they jump in order to get their seed pod out of the hot desert sun, and into a shadow perhaps. After all, these larvae stay inside the seed pods for months, waiting to change into adult moths. They may jump to avoid being roasted!

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