An insect might zigzag because it is trying to avoid an enemy or because it doesn't have an accurate sighting of a potential food source. A predatory insect might be flying in a straight line because it is unafraid of other predators or because it is trying to "make time" when migrating; the same insect in search of food might zigzag if its target wasn't yet selected. Flight paths are usually determined by visual, auditory, or olfactory stimulation. For example, bees and butterflies orient to the color and size of flowers; dragonflies orient to prey items; moths orient to a wind carrying a specific smell, usually a "pheromone."
http://www.wikipedia.org/wiki/Insect_flight
