A dragonfly is an insect belonging to the suborder Anisoptera (from Greek ανισος anisos, "uneven" + πτερος pteros, "wings", because the hindwing is broader than the forewing). It is characterized by large multifaceted eyes, two pairs of strong transparent wings, sometimes with coloured patches, and an elongated body. Dragonflies can be mistaken for the related group, damselflies (Zygoptera), which are similar in structure though usually lighter in build; however, the wings of most dragonflies are held flat and away from the body when at rest, while damselflies hold the wings folded, along or above the abdomen. Dragonflies are agile fliers while damselflies have a weaker, fluttery flight. Many dragonflies have brilliant iridescent or metallic colours produced by structural coloration, making them conspicuous in flight.
Fossils of very large dragonfly ancestors in the Protodonata are found from 325 million years ago in Upper Carboniferous rocks; these had wingspans of up to 750 mm. There are about 3000 species of Anisoptera in the world today. Most are tropical, with fewer species in temperate regions.
Dragonflies are predators, both during the aquatic larval stage, when they are known as nymphs, and as adults. Up to several years of the insect's life is spent as a nymph living in freshwater; the adults may be on the wing for just a few days or weeks. They are fast agile fliers, sometimes migrating across oceans, and are often but not always found near water. They have a uniquely complex method of reproduction involving indirect insemination, delayed fertilisation and sperm competition. During mating, the male grasps the female at the back of the head or on the prothorax, and the female curls her abdomen under her body to pick up sperm from the male's secondary genitalia at the front of his abdomen, forming the "heart" or "wheel" posture.
Loss of wetland habitat threatens dragonfly populations around the world. Dragonflies are represented in human culture on artefacts such as pottery, rock paintings and Art Nouveau jewellery. They are used in traditional medicine in Japan and China, and caught for food in Indonesia. They are symbols of courage, strength and happiness in Japan, but seen as sinister in European folklore. Their bright colours and agile flight are admired in the poetry of Alfred, Lord Tennyson and the prose of H. E. Bates.