jumping bean 
noun
A plant seed that jumps as a result of the movement of a moth larva which is developing inside it.
- Affected seeds are found in several plants of the family Euphorbiaceae, in particular the Mexican plant Sebastiana pavoniana, the seeds of which can contain larvae of the moth Cydia saltitans.
- The movements of a jumping bean are actually caused by a caterpillar that lives inside the seed.
- The movement of a Mexican jumping bean will cause the tumbler to turn and toss about.