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The "Mexican jumping bean" consists of a hollow "bean" that houses a larva of the tortricid moth Cydia deshaisiana (commonly encountered synonyms for this moth include Carpocapsa saltitans, Cydia saltitans, Carpocapsa deshaisiana, and Laspeyresia saltitans).  Each "bean" is actually a carpel, the basic female reproductive unit in a flower, and the larvae feed on the developing seeds within these capsules. Mexican jumping beans occur in northwestern Mexico (the "Mexican jumping bean capitol of the world" is supposedly near the town of Alamos, Sonora).

In early summer, the female C. deshaisiana deposits her eggs on the developing carpels/capsules of several plant species in the family Euphorbiaceae (Hostplants and Caterpillars Database at the Natural History Museum) and the larva apparently eats its way in. The capsules drop to the ground during the summer rainy season. Summer temperatures in areas where Mexican jumping beans are found may climb extremely high. High temperatures cause the larva to move its capsule more, presumably an adaptive behavior to avoid cooking in the sun. Larvae overwinter within the capsule. In the spring, the larva cuts a circular trapdoor in the capsule, then pupates within it. Following metamorphosis, the newly transformed adult moth pushes open the trapdoor and emerges.

While inside its capsule, the larva attaches itself with silk threads. The motion of the capsule is caused by moving the threads in one of two ways. If the larva walks around the inside surface of the capsule, the capsule rolls. If the larva grabs the shell with its posterior prolegs and rapidly strikes the capsule with its other end, however, the capsule jumps. Activity patterns vary with ambient temperature. For example, at 30° C the capsules may jump 30 times per minute for 15 minutes, followed by an inactive period of  up to 24 h. At 45° C, capsules may move 40 times per minute. West et al. (2012) used computer modeling and experiments with robots and real jumping beans to study the locomotory behavior of these remarkable moth larvae.

The earliest widely disseminated account of Mexican jumping beans was the presentation on "Jumping Seeds and Galls" by the pioneering (British-born) American entomologist C.V. Riley at a meeting of the Academy of Sciences of St. Louis, an account of which was printed in The American Naturalist in 1876. Riley published a version of this account in the Proceedings of the U.S. National Museum in 1883.

(Heckrotte 1983 and references therein; Gilligan and Epstein 2012)

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© Leo Shapiro

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