Wednesday, July 1, 2009

What makes a jumping bean jump?

Probably the most interesting thing about Mexican jumping bean shrubs are the remarkable "beans" that jerk and roll about with seemingly perpetual motion. It is doubtful (or very rare) that they actually "jump" above the surface of the ground, but they can certainly roll and tumble along in different directions.
Just as pineapples are not apples and peanuts are not nuts, the jumping bean is not a bean, nor is it a seed.

It is actually a small, thin-shelled section of a seed capsule containing the larva of a small gray moth called the jumping bean moth (Laspeyresia saltitans).After consuming the seed within the capsule section, the robust, yellowish-white larva has the peculiar habit of throwing itself forcibly from one wall to the other, thereby causing the jumping movements of the capsule. Mexican jumping bean capsules typically separate into three parts or sections, some of which contain a moth larva. It is these separate sections (technically called carpels) that are sold as "jumping beans."

Jumping bean shrubs typically bloom during the spring and summer months, and this is when the female moth lays her eggs on the green, immature capsules (ovaries) of female flowers. When the eggs hatch, the tiny, immature larvae bore into the young capsules and begin feeding on the developing seeds inside. [Some references suggest that the eggs are deposited inside the capsules, but C.L. Hogue (Natural History Museum of Los Angeles County Science Series 27, 1974) states that the larvae eat their way into the soft green capsules.] Only seeds without larvae will reach maturity and be viable, and luckily for the plant, not all capsules are infested with moth larvae.

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