Why Do Mexican Jumping Beans Jump?
A jumping bean’s movement is caused by a larva, or caterpillar, wriggling around inside it. The “beans” are actually the seeds of a shrub that grows in Mexico. A gray moth lays its eggs on the plant. Each egg hatches into a larva that chews its way inside one of the plant’s young seeds and eats the soft center.
The seed later falls to the ground. When the sun warms the seed, the larva gets hot, and jerks its body. Each time it moves, the jumping bean jumps and rolls as it is trying to either move to a cooler spot or is responding to sounds and light.
It holds onto the very fine web it has woven on the walls of the bean, and then pushes away with its lower body hitting the opposite wall with its head. This transfers the energy to the bean (seed) and moves the seed a short distance. After the larva becomes a moth, it chews a hole in the seed and crawls out.