Why do Mexican jumping beans jump?

What makes Mexican jumping means jump? The answer is less cute than you may have suspected.

Often sold as novelty items and exported worldwide, Mexican jumping beans are actually the seed capsules of a shrub (Sebastiania pavoniana) that have been taken over by the tiny larvae of an attractive grey moth (Cydia saltitans). The moth lays its eggs on the green immature capsule of female flowers in the spring and summer, and the immature larvae bore into the young seed capsules.

The developing seed will be home and food for the larva as it grows. By late summer, the capsules separate into three sections, falling to the ground. The seed capsules with larvae inside them hop around on the ground, to avoid overheating in the harsh sun of the Sonoran Desert. To do this, each larva weaves a silk lining along the seed interior, grabs on with its hind legs, and thrashes its head against the walls. The force topples the seed, as the larva uses its finely-tuned sense of temperature to seek shade.