What Makes Mexican Jumping Beans Jump?

By: Nicole Antonio  | 
Mexican jumping beans
The inside of the seed of the Sebastiania plant, which, when infiltrated by the tiny larvae of the Cydia saltitans moth, or jumping bean moth, will jump around as the larvae develop. Brett Hondow/Shutterstock

If you grew up in southern California, you may remember trips to 7-11, where you'd find Slurpees, gum and a counter display that held little clear plastic boxes of four or five Mexican jumping beans. You could hear the display rattle as the jumping beans clicked against the plastic boxes. Maybe your parents even bought them so you could play games with them on the kitchen table. But what is it that makes a Mexican jumping bean jump?


What Is a Mexican Jumping Bean?

Contrary to what you might think because of its name, Mexican jumping beans are not beans at all; they are actually seed pods from a shrub called Sebastiania pavoniana. These seed pods — about the size of a kernel of corn — contain a miniscule larva known as Cydia saltitans, or jumping bean moth, which is responsible for the bean's peculiar behavior.


The Life Cycle of the Cydia Saltitans Moth

The story starts when a female Cydia saltitans moth lays her eggs on the immature green pods, or ovaries, of the Sebastiana shrub's flower. After a few weeks, the eggs hatch and the insect larvae (worms) emerge from the pod, eating their way to the inside of the flower's capsule. A tiny moth larva is no larger than a grain of rice and immediately begins to feed on the seeds inside the pod.


Why Do Mexican Jumping Beans Jump?

Although there are several theories as to why Mexican jumping beans jump, most indicate that the movement is caused when the tiny larvae inside curl up and have spasms, hitting against the seed pod and causing the unpredictable “jumping” motion. The larvae can survive for months, periodically curling and uncurling and causing the bean to move. Some theories posit that, since high temperatures and direct sunlight can kill the larvae, the movement comes from the larvae attempting to move into a cooler position in order to survive long enough to reach the pupal stage.

After developing and jumping around for months, the moth larvae go dormant for a short time and begin spinning a cocoon inside the capsule. The larvae then undergo metamorphosis, and adult Cydia saltitans moths eventually emerge. The moths disappear into the desert, and the life cycle repeats itself, creating the next generation of Mexican jumping beans.


Where Do Mexican Jumping Beans Come From?

In Mexico, the town of Alamos, Sonora, has the reputation of being the "Jumping Bean Capital of the World" because it is the primary source of Mexican jumping beans. The Sebastiania shrub — which, as we've seen, produces these unique seed pods — is native to the region. Local residents collect the pods containing the tiny moth larvae and sell them as novelty items to people around the world.

This article was created in conjunction with AI technology and edited by a HowStuffWorks editor.