Short answer: A Mexican jumping bean is a name given to a type of Mexcian bean pod that appears to randomly move around. The moving is actually caused by a small moth larva that living inside the pod.
When I was a kid, I remember having Mexican jumping beans. They were usually sold in clear plastic boxes so you could see and hear the Mexican jumping bean moving and rattling around. They don’t actually “jump,” but they do move around quite a bit. I always thought the moving around had something to do with magnets.
What are Mexican Jumping Beans?
Mexican jumping beans (also known as frijoles saltarines in Spanish) are not actually beans. They are brownish seed pods of a Mexican shrub called Sebastiania pavoniana that has been implanted with larva from the Laspeyresia saltitans moth.
What causes the Mexican jumping bean to “jump?”
The larva inside the pod eats the seed leaving a lot of empty space in the pod. Once the seed is gone, this larva has room to leap around, causing the “bean” to move and roll around. They tend to move more the hotter it gets in an attempt to get to a cooler environment to avoid dehydration which is deadly. The larva even creates a complex network of silk lining inside the pod to give it more jumping energy, which results in allowing it to “jump” farther.
Under the right conditions, the larva will live for months inside the pod and eventually go into the pupal stage. If the larva survives until the spring, it will become a moth and exit the pod through a trap door. After all of this work, the moth will only live for a few days.
Mexican jumping beans are usually easy to buy on the internet, but due to the severe weather in Mexico, most vendors are not selling them this year.