That's because you had only seen them in their pre-mating stage. If YOU had to hang around for three years waiting to mate you'd probably have blue parts also.

"The lifespan of a dragonfly is found to consist of two-stages. After breeding the female dragonfly will lay her eggs on or near water or in a place that will fill with water. Once the eggs have hatched the first stage in their lifestyle can begin. At this stage, it is an aquatic larvae, also called 'nymph'. This stage will generally last between one and three years, - See more at: http://www.dragonfly-site.com/how-long-dragonflies-live.html"

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