Mexican jumping beans, actually seeds of plants in the Croton family, are invaded by larvae of a moth, Cydia saltitans, and it is the larvae that make the beans move.
The larvae are aroused by the warmth of someone’s hand, making the “beans” jump.
The insect, commonly called the Mexican jumping bean borer, is native to Mexico and is now found in Arizona and Florida.
Plants in the Croton family are chiefly weeds.
The moth is related to some serious pests, like the codling moth and the hickory shuck worm.
The feeding habits of its larvae could be of economic concern in Mexico, but the larvae are not yet known to threaten any crops in the United States.