What’s inside of a Mexican jumping bean?

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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.

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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.
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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.

6 Comments Add yours

  1. Manu Prakash says:

    @Tom; that’s fantastic. I was quiet surprised with the miniature legs it uses to generate traction.

    Now to something not so serious; are they sold as a toy? Food? Are they yummy?

    Cheers
    Manu

  2. Saad Bhamla says:

    @tom – nice post! same reaction on the tiny feet ! Did not expect the feet be so ‘wedge’-like.

    Also i’m curious about the jumping – how high can they jump? In your video, they barely just ‘shudder’. If they can actually jump (disconnect from the ground), then there might be interesting dynamics, including elasticity of the pod etc. to think about..

    I wonder what is the highest jump recorded..

    Saad

  3. Saad Bhamla says:

    @tom – also since you mentioned the new html/video format you’re using – one of the things I usually like with videos is the audio – which seems to be missing from this format. For example, I was curious when the ‘jumped’, is there a popping noise, like popcorn?

    saad

  4. Tom Hata says:

    Manu – they’re pretty much a novelty. I couldn’t resist buying a small container of them for 1 dollar.

    Saad – Good point about the sound. They make a pleasant rattling noise. I think “jumping” is a misnomer, they mostly rattle around.

  5. laksiyer says:

    Fascinating story.

  6. Matt.Rossi says:

    @tom I love looking at the texture on that larva. It’s a smooth looking creature at a distance, but up close, you can see how porous and goose-pimpled its skin is. And bristled. This technique of wedging it between two slip covers to gain width is also great. How much room can we give our larger specimens to breath, I wonder, and still keep a good focus.

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