hopping peas leave it to chance

Have you ever seen peas from Mexico, these funny seeds that were spread in France by Pif Gadget magazine in 1971? All you have to do is heat one up, holding it in the palm of your hand, for example, until the peas jump out. This unpredictable behavior is caused by a small inhabitant that has made the seed its home: the caterpillar of the butterfly, the euphorbia moth, which is moving in search of a cooler environment. And it’s not bad at that, according to the work of Devon Mackey and Pasha Tabatabaei of Seattle University in the US. The researchers highlighted the random nature of the jumping pea movement while showing that this strategy is more effective than one might imagine.

The pea in question is not actually a pea, but a sporangia seed belonging to two species of the genus Sebastiana, from Mexico. Codling moth larvae settle there and feed on its contents. When the seed falls from the tree, only a thin shell remains, inside which its inhabitant clings, anchored with threads of silk.

The caterpillar will then have to survive on the ground until next spring and metamorphose into a butterfly. But the climatic conditions are harsh in Mexico. In order not to become dehydrated, the caterpillar should not be exposed to the sun for a long time. If you notice a very high temperature, usually above 20°C, they move erratically within their shelter, making the famous jerks of jumping peas. By movement, the seed moves to reach a shaded corner with more moderate temperatures.

In an experiment conducted in 2012, Daniel West of the Georgia Institute of Technology in the US and his colleagues deposited a seed with its tenant on a flat surface subject to a temperature gradient. They found that the caterpillar moves away from the heat source very effectively by adopting a movement called run and stumblewith a sequence of straight displacement followed by a change in direction, which responds to a certain probability law, a behavior found in bacteria.

Devon McKee and Pasha Tabatabaei have recently expanded this research and sought to understand the movement strategy of young moths in Euphorbiaceae in the absence of a temperature gradient. After a statistical study of the trajectories followed by jumping peas placed on a uniformly heated plate, the researchers showed that the movement of the weights follows a diffusion pattern similar to that determined thermodynamically. In other words, without information about which direction to take cover, the pea’s movement is random, and the direction of movement changes with each shake.

The interest of such a strategy is not clear. One might indeed be inclined to think that the caterpillars are thus doomed to spin in circles, or at least to depart very slowly from their starting point. However, researchers have shown that being harassed in this way increases the chances of accidentally finding a gray area. By choosing one direction to follow, the paths will certainly move faster, but they will have a higher risk of missing their target. Chi Fa Piano Fa Sano!