These are common dragonflies with the males and females looking very different. This is a male with the white (I say actually blue) abdomen. They eat flying insects like mosquitoes and so are welcomed into our yard anytime:>). The males are territorial, meaning they patrol an area they believe is theirs and chase other males away. They perch on low twigs or even on the ground. I find they have favorite spots to sit and wait for prey, and if I startle them but stand still and wait, they generally return.
The nymphs (larval forms) live in water and eat aquatic insects and sometimes even small crayfish. When they are ready to morph into adult dragonflies, they crawl out onto a branch or weed, split their skin, pump up their wings, and fly off to eat, mate, lay eggs in the water.
This picture was taken in Green Lane, Pennsylvania, USA, on May 11, 2012. For more info on these beautiful and interesting insects you can visit: