Why jumping beans jump

It primarily grows in Mexico, in the mountains of the Sonoran Desert. There are three of them that make up this fruit. Some of these sections have a stow-away — a tiny moth larva that burrowed into the seed while it was still on the plant. The larva devours the inside of the seed, hollowing out its new pad to make room for its growing body.

Over the next months our squirmy friend lines the walls with a comfy layer of silk. Just enough air and moisture sneak in through tiny holes in the seed walls. Except for the sweltering desert sun. That heat can dry out and kill our sweet little larva.

So, of course, it starts jumping. A few hops out of the sun can mean the difference between life and death. After a few weeks the eggs hatch and the tiny insect larvae worm eats its way to the inside of the flowers capsule.

As the shrub matures, the capsules that contain the insect larva become hard and changes to a brownish color. The moth larvae, now trapped inside his new home, begins eating the developing seed inside the capsule as food. After the first spring rains occur, the capsules separate into three sections and fall to the ground.

Those capsules that are lucky to contain a moth larvae inside begin to "jump" on the forest floor. Our mysterious Mexican Jumping Bean has come to life. After months of jumping, the moth larvae will go dormant for a short time and begin spinning a cocoon inside the capsule.

This is the stage when the larvae will undergo metamorphosis and an adult Jumping Bean moth will emerge. The larva uses its powerful jaws mandibles to chew through the wall of the capsule. Close-up view of an Arizona jumping bean capsule Sebastiana bilocularis with exit door opened to show pupa inside.

The door was pre-cut by the larva prior to pupation. The door is eventually pushed open by the emerging pupa, and the adult moth crawls out of the pupal case. If a window is cut into the capsule to observe the active larva, it will spin a silken web across the opening. Close-up view of Mexican jumping bean carpel in late spring showing a pupa that has just pushed through the exit door. Soon a small gray moth Laspeyresia saltitans will break out of the pupal case and fly away in search of a mate.

One carpel from a seed capsule of Sebastiana pavoniana sold in San Diego as a Mexican jumping bean.

Note the circular door where the adult moth exited from the carpel. The pupal case pushes forward through the door, and the adult moth crawls out of its pupal case. Empty pupal case formed by the larva inside the carpel. The adult moth emerged from this pupal case during early September in San Diego.

Adult moth Laspeyresia saltitans that emerged from the pupal case. Recent synonyms for this remarkabe species of moth are Cydia saltitans and Cydia deshaisiana. This jumping bean was purchased in San Diego during early summer of in the little plastic boxes shown below. They rock, or, on occasion, scoot a millimeter or two. Imagine a kernel of corn that scoots a millimeter in one direction every 15 seconds or so -- that's about as exciting as jumping beans get.

The thing that makes these beans jump is a tiny moth larvae that lives inside the bean. The moth lays its eggs in the flower of the plant, and the eggs are incorporated into the seeds. The larvae then eat out the interior of the bean and live there. When the larvae move, so does the bean. Eventually, the larvae turn into moths that emerge from the beans to repeat the cycle. Sign up for our Newsletter!