What makes the jumping bean jump?

Mexican jumping bean, jumping bean and broncno bean are popular names given to the separate cells of the seed pods of several varieties of swamp shrubs native to northern Mexico and southwestern United States. These shrubs belong to Euphorbiaceae, or spurge family. When the shrub is in bloom or shortly thereafter a bean moth (Carpocarpsa sal titans), closely related to the destructive codling moth or appleworm, deposits its eggs in the blossoms or the green seed pods. After the egg hatches the larva of the bean moth eats the interior of the bean pod cell and lines the sides with silk of its own manufacture. By the time the three-celled pod has separated from the shrub and fallen to the ground the larva is about one-fifth the size and nearly as heavy as the pea-sized arid thin-walled cell that serves it as a shelter. The larva is strong for [455] its size and for a time is very active. It catapults itself with enough force to cause the bean to tilt, roll, tumble, jerk and jump. Many people find this phenomenon so amusing that many tons of “Mexican jumping beans” have been gathered, shipped to market and sold at several cents apiece. Of course, only “wormy” beans are-marketable as oddities. As a rule there is a bean moth larva in only one of the three beans in a pod. Ultimately the larva emerges as a full-fledged bean moth, and the cycle starts over. In the Southwest, broncho beans have been used by gamblers for generations. The wagers are decided by the greatest number of beans that jump over a line or out of a circle within a given time.