First of all, a jumping bean is really a seed. It’s from a type of shrub that can be found clinging to rocky, dry slopes in the Mexican states of Sonora and Chihuahua.
A tiny moth larva inside makes a jumping bean jump. In the spring, when the shrub is flowering, moths lay their eggs on the shrub’s hanging seedpods. When the eggs hatch, tiny larvae bore into the immature green pods and begin to devour the seeds.
The pods ripen, fall to the ground and separate into three smaller segments, and those segments are what we call Mexican jumping beans. As the tiny larvae inside curl up and uncurl, they hit the capsule’s wall with their heads – and the bean jumps.
No one knows for sure why the larvae curl and uncurl, but it’s been observed that they move more as temperatures rise. It could be that the larvae are trying to get to a cooler spot on the hot ground where they can safely pupate into moths.
By the way, it’s not a good deal for the parent shrub of the “jumping bean” seeds. Only pods without jumping larvae develop a seed and, later, a plant.
Our thanks to:
Dr. Tom Van Devender
Senior Research Scientist
Arizona-Sonora Desert Museum
Steve Prchal, Director
Sonoran Arthropod Studies Institute
Dr. Daniel Rubinoff
Division of Insect Biology
University of California – Berkeley