Which of the following insects is not considered a social insect?

Dragonflies are not social insects like, for example, honeybees. They will interact with one another quite directly, though, males in particular, during territorial disputes around ponds and springs. However, they do tend to form swarms that are sometimes so large that they can even be picked up on radar. Feeding swarms arise when there are sudden "outbreaks," or large assemblies of prey insects in a specific location.

Source: The Times Online