I have actually heard other people comment that there seems to be a lot more dragonflies this year, and I would have to agree. I've had a pond in my yard for years, and I have never seen so many! The most common one is about 2" long. The males are blue and the females are brown. When they mate, the male sticks the back end of his body into the back of the female's head, and they will fly around together like that, with the female dropping eggs onto plant stems and directly into the water. Most of the males are sort of a medium to light blue, but I saw one yesterday that was nearly purple.
I have also seen some very large ones that fly around the neighborhood in groups. Based on their size, I believe these larger ones are responsible for all of the nymphs I found in my pond last weekend while draining it down to work on the waterfall. There were a LOT of them, and I'm really surprised that the fish had not eaten them. Then again, I'm also surprised that this is the first time I've noticed them. I'm really hoping for a good success rate with these larger ones, because we need more of them!
Something I didn't see while skimming through this thread... despite their alien appearance, both the nymphs and the adult dragonflies are extremely good to have around. They eat other insects, and will clean out anything your fish may have missed. The adults are expert aerial hunters, and when you see them darting and weaving through the air, be sure to thank them for reducing the mosquito population in your yard. I really love having them around, and with all the types of plants I have growing in my pond now, I hope to see more varieties of dragonflies pass through over the years.