Current information indicates Wisconsin has 154 species of dragonfly. Of these dragonflies, 6 are considered Endangered or Threatened.
Odonata means "toothed ones". Odonata are a very large group of insects, with are over 5,000 species divided into three groups; the Anisoptera (dragonflies), the Zygoptera (includes damselflies) and the third group which only has two living species, Anisozygoptera. These three groups are divided into 25 families.
Odonata are predators in both stages of life. As a nymph they hunt for their prey most often by sitting still and waiting for a suitable food to come to it. As adults they are winged hunters, seeking out their meals with flight moves admired by the best human pilots.
The dragonflies have a unique wing shape among insects, making them easily identified as a group. The wings on these insects are not hidden below a wing cover, are held outward from their body and are in a set of two pair of mostly transparent wings of equal length. Dragonflies also have very large compound eyes.
Generally, the dragonflies rear wing pair is larger than its front wing set. On Damselflies, the front and rear wing pair are similar in size. Damselflies can fold their wings back and often do when resting. Dragonflies cannot fold their wings back over their abdomen.
Dragonflies and Damselflies spend their nymph stage in waters as an underwater predator. Some dragonfly nymphs can remain in this aquatic nymph stage for as little as a few months or for 5-6 years before turning into their adult form, depending on which specie it is.
Typically, when it is time for the dragonfly to hatch out of its nymph shell, it will crawl out of the water at night. As its skin dries, a crack begins to form which allows the mature dragonfly to emerge.
Sources include:
http://dnr.wi.gov/org/land/er/invertebrates/dragonflies
http://www.entomology.wisc.edu/wes/pubs/dragonfly.htm
http://www.ucmp.berkeley.edu/arthropoda/uniramia/odonatoida.html