It is pretty easy to tell if a dragonfly is a member of the darner family. Darner dragonflies have large eyes that come together in a long line. They are often described as looking as though they are wearing a helmet. Unfortunately when it comes to distinguishing one darner species from another it is not usually so easy. Several species are very distinct, however those that are what are often referred to as blue or mosaic darners can be very difficult to identify.
One of the most difficult is the Canada darner. It is very similar to the green striped and lake darner. The way to identify the species is to look at the two stripes on the side of the thorax, called side stripes. In the photos above you can clearly see a notch in the front side stripe of each dragonfly. If this notch is greater then 90 degrees then it is probably a green striped darner, If it is less then 90 degrees then it is probably a lake darner, if it is 90 degrees then it is probably a Canada darner. There are other things such as size, and the existence and thickness of a rearward flag at the top of the front side stripe. In the bottom photo the pair of Canada darners are mating.