What’s inside of a Mexican jumping bean?

Nov 09, 2015 • 11:26 PM UTC

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Tom Hata

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Even though I have lived in California for more than a decade, I have never visited Mexico until a week ago. Countless gift shops aimed at tourists lined the streets of Ensenada, but one peculiar item caught my eye: Mexican jumping beans. I had to find out for myself what animal inside caused these beans to jump (although I later found out that these are not beans at all but seed pods)! Yes, I will shamefully admit that this was my only purchase on my first trip to Mexico. And yes, I was so eager to find out what was inside that I spent a good part of one evening in my hotel room Foldscoping the inhabitants of these jumping beans.

It turns out that in each “bean” is a moth larva ( Cydia deshaisiana ), nestled in snugly. These larvae, when exposed to heat (such as the warmth of a hand or direct sunlight), begin to spasm in an attempt to roll their home toward a cooler location. To take a closer look, I opened a bean up and removed it from its home.

Removed from its seed pod, the larva moved much like any other soft-bodied larva: by slowly inching along. Turns out that the jumping motion in the seed pod is achieved by the larva tugging at threads inside of the pods.

The larva itself was rather large (for a Foldscope specimen) and fragile, so I sandwiched it between 2 glass cover slips that were taped to a stack of paper slides. This way, the larva wasn’t stuck directly to tape and could crawl around a bit. The movement of its feet were actually quite fascinating to watch.