Despite not being terribly smart as individuals, wasp (黄蜂) colonies build and maintain a complex nest that lasts many generations. Just how these social wasps coordinate this task has always been a mystery. But now a mathematical model suggests that one key factor drives their behavior. the amount of water in the nest. Social wasps cannot learn from one another—unlike bees, which use a complex dance to tell nest mates where sources of nectar (花蜜) are. Nor do they use pheromones (信息素) the way ants do to lead other ants to food. Robert Jeanne of the University of Wisconsin-Madison proposed that wasps set up a demand-driven chain of information. At the end of the chain, builder wasps monitor the nest and when necessary, request pulp from pulp forager wasps. They in turn demand water from water foragers in order to make the pulp. But biologist Istvan Karsai of East Tennessee State University in Johnson City, Tennessee, and his team found that social wasps in Panama don't actually work that way. They removed either builders or pulp foragers from a colony of a species called Metapolybia aztecoides. Although that should break up the so-called information chain, it did not significantly alter the amount of water being brought into the nest. They also found that the wasps could change roles, something that Jeanne didn't expert. For instance, when the researchers sprayed a surplus of water onto the nest, water foragers quickly became builders, and nest building increased. Based on their observations, Karsai's team developed a mathematical model that shows that wasps achieve their complex behaviour simply by monitoring the level of water in the nest—what he calls the'common stomach' of the colony. He believes wasps infer what the level is when they exchange fluids on meeting each other, a behaviour called trophallaxis that is common in many social insects. To test the model, Karsai simulated changes in the model colony, for example by removing pulp foragers or builders. 'What's interesting is that in every case the model responds like the actual colony in Panama, 'says Karsai. According to the passage wasps ______.