One of my favorite scientists is Deborah Gordon. She teaches at Stanford and studies harvester ants in the desert of southeast Arizona. She has published two excellent books, Ants At Work: How An Insect Society Is Organized and Ant Encounters: Interaction Networks and Colony Behavior. Yesterday, she published a paper in Nature about how ants forage when conditions are ideal and stay put when they are not. The decision to stay or go is made collectively. The Nature paper is summarized in this post on Science Daily:

"Natural selection is not favoring the behavior that sends out the most ants to get the most food, but instead regulating foraging to hold back when conditions are bad," Gordon said. "This is natural selection shaping a collective behavior exhibited by the entire colony."
Gordon's group is still investigating how the ants gauge humidity, but they have determined that the collective response of the colony to conditions is heritable from parent colony to offspring colony. Even though a daughter queen will establish her new colony so far from the parent colony that the two colonies will never interact, the offspring colonies resemble parent colonies in their sensitivity to conditions.
Gordon has also discovered that colonies change their behavior as they age—meaning, young colonies make different collective decisions than older ones. This discovery allows one to see the ant colony as a kind of primitive (or even phantom) form of, say, an individual ant, which itself is an advanced multicellular organism.