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Why Do Mexican Jumping Beans Jump?

You may have held Mexican jumping beans as a child and wondered how they work. The answer may surprise you...

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.

Mexican Jumping Beans

We’ve all heard stories about Mexican jumping beans. What are they, and how do they do that remarkable trick?

They Aren’t Beans…

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 southwest. These seed pods can’t jump by themselves.

Why Do They Jump?

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 larva 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.

But, Why Do These Larvae Want to Jump?

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!  A search on YouTube for “Mexican Jumping Beans” turns up many results, such as this one:

http://www.youtube.com/watch?v=o4t5lZCRb-0

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