Dragonflies, from the order Odonata, have been around for over 250 million years. I've seen fossil dragonflies from the Eocene some 50 million years ago that would have been exquisite monsters to behold. Huge beasts taking to the skies and enjoying the warmth that the Earth was experiencing at the time.
The most conspicuous difference in their evolution over time is the steady shrinking of their wingspan from well over two and a half feet down to a few inches.
Ancient Perfection: Dragonflies