Mexican jumping beans are sensitive to the color of light
A video about Mexican jumping beans by KQED/PBS.
Meet the Mexican jumping bean
Mexican jumping beans, sometimes sold as toys in Mexico and the U.S., are not beans. They’re seedpods. And they can jump. But what makes them jump? Each seedpod has a tiny moth larva burrowed inside it. When the larva jerks its body, it bumps into the wall, causing the seedpod to move. Scientists at Binghamton University in New York said on October 8, 2024, that they found that certain colors of light can cause these larvae to produce more forceful jumps. They think it’s a cue for the larvae to move away from potentially harmful temperatures. They also found that if a larva’s seedpod home was damaged, its jumping was impaired.
The researchers published their findings on larvae sensitivity to light in the journal Behavioral Processes on August 30, 2024. They also published a study on how seedpod damage affects jumping in the Journal of Insect Behavior in August 2024.
Lindsey Swierk, a co-author from Binghamton University, said:
When a seed drops to the ground from shrubs, the moth larva inside is at the mercy of whatever environmental temperature the seed experiences. The ground could be scorching hot in direct sunlight. A little moth larva inside of a seed like this can only withstand so much heat, and so they jump away.
Jumping bean larvae response to light
Swierk and her students wanted to better understand how the larvae respond to their environmental conditions. The scientists wondered if the color of light could serve as a signal for larvae to take action to avoid extreme heat. So, they exposed jumping beans to different colors of light: red, purple, green and white.
The larvae responded to the light, despite the fact that less than 1% of light penetrated the seedpod wall. The researchers found that larvae moved more forcefully when exposed to red light, and least under purple light.
Swierk remarked:
Somehow larvae are picking up on these differences. Whether that’s because of very minute temperature changes or because of extremely sensitive photoreceptors, we’re not sure yet. But they’re using light somehow as a cue to change their behavior, which probably has to do with the fact that these different lighting spectra are correlated with different environmental conditions. Red and white light are more characteristic of daytime lighting, while green and purple light are associated with the light under forest canopies or sunset and sunrise.
What happens if the seedpod is damaged?
If a seedpod is damaged, a larva can repair it by weaving silk threads across the opening. But how would it affect jumping? Swierk and her students ran experiments to see how the larvae would respond. They had one set of jumping beans with damaged walls that were repaired by the larvae. The second set of jumping beans were damaged but the larvae were not given enough time to make repairs. And there was a third control group of intact jumping beans.
The team found that both sets of damaged jumping beans were less likely to jump when subjected to high temperatures. But the set with intact jumping beans moved more effectively. Swierk suggested that it was the damaged seedpod, not the energy needed by a larva to repair the hole, that affected movement. Perhaps the damage disconnected silk threads that attached a larva to the inside wall of the seedpod. As a result, the larva was unable to make the seedpod move.
A finely-tuned response to the environment
Swierk commented further on the implications of this study:
These are animals that are extremely sensitive to temperature. A common story here is that we see these larvae using very nuanced cues to change their behavior in response to heat, and we’re also seeing that additional stressors like predation attempts can impair their ability to appropriately respond to temperature.
Responding to temperature change is a big deal. As the climate changes, we need to learn how animals detect imminent thermal stress and what limits their adaptive responses. What we learn about Mexican jumping bean larvae might help us better understand how other insects with limited movement cope with heat stress in their environments.
More about Mexican jumping beans
Mexican jumping beans are seedpods, about the size of a corn kernel, from a shrub called Sebastiania pavoniana. The larvae living inside the pods are the juvenile form of a small moth native to Mexico, the jumping bean moth (Cydia saltitans).
In spring, the moths lay eggs on the shrub’s immature seedpods. Tiny larvae hatch and burrow into the pods to feed on the seed. As the larva grows and feeds, the pod becomes more hollow. Each pod has three segments. When they ripen, the pods fall to the ground and separate into segments, some containing larvae. These pod segments are known as Mexican jumping beans.
But how does a “bean” jump? The larva inside it jerks its body to hit the inner wall, causing the bean to move.
Larvae live inside the seedpods for several months. If it survives, a larva will go through a pupal stage, to eventually emerge as a moth. But before entering that stage, the larva creates a trap door that will later allow the moth to exit the seedpod. After emerging in spring, the small silvery grey moth lives for just a few days, its purpose to repeat the life cycle.
Bottom line: Mexican jumping bean larvae are sensitive to the color of light, jumping more forcefully under red light. Also, damage to the seedpod impairs jumping.
Read more: Find out why moths are attracted to light
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