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Mexican jumping beans (also known as Frijoles saltarines), native to Mexico, are seed pods that have been inhabited by the larva of a small moth (Cydia deshaisiana). The "bean", usually tan to brown in color, "jumps" when heated, because the larva spasms in an attempt to roll the seed to a cooler environment to avoid dehydration and consequent death. They are from the shrub Sebastiania pavoniana, often also referred to as "jumping bean". However, they are not related to actual beans (legume plants), but rather to spurges. The beans are considered non-toxic, but are not intended to be eaten.
After the moth-laid egg on the plant hatches, the larva eats away the inside of the bean (until it becomes hollow) and attaches itself to the inside of the bean with silk-like thread. The larva may live for months inside the bean with varying periods of dormancy. If the larva has adequate conditions of moisture and temperature, it will live long enough to go into a pupal stage. In the spring, the moth forces itself out of the bean through a round "trap door", leaving behind the pupal casing. After its metamorphosis, the small, silver and gray-colored moth lives for no more than a few days.
When the bean is abruptly warmed (by being held in the palm of the hand, for example) the larva twitches and spasms, pulling on the threads and causing the characteristic hop. Leaving the beans in a heated environment (such as direct sunlight) for an extended period of hours can easily kill them.
The beans become energetic when one holds them in the hand (out of the box) for a few minutes. The beans should also appear to be a very slight shade of green on the side. If one shakes a bean near one's ear and hears a rattle inside, the larva inside has either died or entered the pupal stage where its hardened shell makes a softer rattle.
Jumping beans are still available for sale in the United States. In the UK, they were a common novelty item in the 1950s.
A plastic toy under this name was manufactured and sold in packages containing several devices in the 1960s. It resembled a "time release" capsule and had a metal ball inside. When the surface on which the capsule was laid was tilted, the ball would roll to the other end and make the capsule twitch.
Beans do require semi-regular re-hydration, mimicking the monsoon weather of their native Mexico. To rehydrate the beans, they need to be soaked, but not submerged, for a three-hour period in chlorine-free water once or twice a month. The chlorine found in tap water in most cities in the US will kill them.
If one is not sure if their drinking water is chlorinated, the safe thing to do is to use bottled or distilled water. Just spraying the beans with a little water is ineffective in maintaining the larvae's lifespan. Beans should be stored in a cool, dry place, but freezing will kill them.
If beans are shaken too hard, they will sometimes stop movement for a short amount of time as a defense mechanism.
The Mexican jumping bean comes from the mountains in the states of Sonora, Sinaloa, and Chihuahua; indeed, Álamos, Sonora styles itself the "Jumping Bean Capital of the World". They can be found in an area approximately 30 by 100 miles where the Sebastiana pavoniana host tree grows. During the spring, moths emerge from last year's beans and deposit their eggs on the flower of the host tree. Legend says that they fell from the sky one day.
Jumping beans were used as a recurring gag in many cartoons from the 1930s to the 1950s, wherein eating the beans would cause a character's whole body to bounce out of control and land on something painful.
In the fourth episode of An Idiot Abroad, "Mexico", Karl Pilkington spends much of his time in the country unsuccessfully searching for Mexican jumping beans after having seen them as a child on Sesame Street. Frustratingly for Karl, the Mexican locals that he approaches seem to have never heard of the beans. In the last scene of the episode, Karl expresses his relative (compared to other episodes) appreciation of Mexico, but also his one regret: that he couldn't obtain a Mexican jumping bean.