Tony Mathis, age 11, of High Point, N.C., for his question:

What causes a Mexican bean to jump?

It is customary, of course, for a grown plant to remain rooted to the spot. However, various seeds do tend to travel to new locations. Dandelion seeds are wafted far and wide by the breezes and certain lily seeds are shipped afar by flowing streams. Grass seeds often hitch rides on passing animals. Many ripe bean pods pop open like pistols and shoot their seeds to new locations. But the famous Mexican bean seems to leap from here to there under its own power.

This is the story of a one sided partnership between a plant and an animal. The so called arrow plant is a five foot shrub with long, pointed leaves. It grows in parts of Mexico and got its name because, in bygone days, its milky sap was used to poison the tips of arrows. This hardy desert plant is a member of the spurge family and a cousin of our lovely Christmas poinsettiasAfter the spring rains, the arrow plant sprouts spikes of small flowers. At about this time, a drab little moth arrives on the sandy scene. Her fancy name is Laspeyresia saltitans, and she is related to the pesky coddling moth which hides maggots in our apples.

As the arrow plant loses its petals, the moth places an egg in this and that seed pod. When an egg hatches, a tiny grub bores into a growing bean. There he feeds, eating out a hollow to serve as a home as he grows bigger. When the bean is a mere shell, he lines it with silken thread.

At last the ripe pods on the plant pop open and scatter their seeds. Some will sprout roots and become new plants. But not the one with a hollow inside. If this bean happens to fall where the sand is very warm, the grubby caterpillar inside is uncomfortable. So he grips the silken webbing, coils like a spring and lets go with a sudden jerk

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This is enough to move the hollow, lightweight bean. It may roll or jump in short hops, hopefully to a shady, cooler spot. This is how and why the jumping bean jumps. The frisky bean may be captured and sold as a novelty item. when placed in a warm place, perhaps near a radiator, the caterpillar inside the little brown bean will try to move to a cooler spot.   He will coil like a spring, let go with a jerk and the jumping bean is forced to jump.  

 

Left in the wilds, the grub eats a round hole partway through his pantry wall. Then he becomes a pupa and his jumping days are done. Come spring, when the arrow plants bloom again, he emerges through his prepared escape hatch  as a winged adult moth.