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Mexican jumping beans are seeds which have been colonized by moth larvae. The larvae move around inside the seeds, causing them to roll or tumble, although they rarely literally jump. The seeds have long been sold as novelty items in Mexico, where they are called brincadores, and their popularity has spread to other parts of the world as travelers bring them home. Although Mexican jumping beans are certainly fun, they also illustrate a very interesting relationship between plants and insects, and it is only one of many such relationships in the natural world.
The name “bean” is a bit of a misnomer, as Mexican jumping beans are actually individual carpels or sections of the seeds of shrubs from the genus Sebastiana. These plants have dark green leathery leaves which turn a rich red in the winter, and they are native to Mexico and parts of the American Southwest. In the spring, when Sebastiana plants flower, small gray moths lay their eggs in some of the flowers, and when the flowers develop into seeds, the eggs are trapped inside.
Once the eggs mature into larvae, the larvae eat out the inside of the seed to make a cozy home for themselves. They also spin a series of threads, creating a complex internal web inside the seed. For unknown reasons, the larvae tumble around inside the seeds, causing them to move. These movements may encourage dispersal of larvae across a wide region, and they appear to be stimulated by exposure to warmth.
As the larva grows inside the seed, it ultimately turns into a moth, and drills a small trapdoor into the seed so that it can escape. Once it emerges, the moth has only a few days to live; it generally seeks out a mate so that the life cycle of the Laspeyresia saltitans moth can begin all over again. Fortunately for the host plants, the moths do not colonize all of the seeds on a shrub, ensuring that the plants will be able to perpetuate themselves, thus providing future homes for the moths.
The seeds do rather resemble small beans, which explains the name. A collection of Mexican jumping beans can be quite interesting to watch, as the seeds will rattle against each other and shake out of shallow containers if given the opportunity. The Mexican jumping beans will continue to move for weeks or months if cared for properly, although they may experience periods of dormancy. Since the moths only colonize plants in the Sebastiana genus, the cycle will stop unless the young moths have access to the right hosts after they emerge.