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The dragonfly (suborder Anisoptera) is an insect of the order Odonata, with large multifacted eyes, two pairs of long transparent wings, and a long body. Dragonflies typically eat mosquitos, midges and other small insects. Another name for them is mosquito hawks.
Dragonflies do not bite or sting humans.
The life cycle of the dragonfly, from egg to death of adult, is from six months to as much as six or seven years. Most of this time is spent in the larval (nymph) form, beneath the water surface, catching other invertebrates or even tiny fish. In the adult (flying) stage larger species of dragonfly can live as long as four months. Dragonflies have about 30,000 facets to their eyes, giving them nearly a 360° field of vision.
In the past some much larger dragonfly species existed. The largest found was an extinct Protodonata from the Permian period with a wingspan of 70-75cm (27.5-29.5"). This compares to 19cm (7.5") for the largest modern species, the Central American Megaloprepus coerulatus. The smallest modern species recorded is the Nannophya pygmaea from east Asia with a wing span of only 20mm, or about 3/4 of an inch.
Damselflies are often confused with dragonflies, but the two insects are distinct: damselflies at rest hold their wings vertically above the body, whereas dragonflies at rest hold them horizontally. Also, the eyes on a damselfly are farther apart than the eyes on a dragonfly. Both are members of Odonata, and their life cycles are similar.
Much of the above information derived from the British Dragonfly Society (http://www.dragonflysoc.org.uk/index.htm)