Odonates are among the few insects that have secured a major place in folklore and art. In Japan, where a journal (Tombo) has been devoted to reports of their biology since 1958, dragonflies (Odonata) traditionally have been held in high regard. In other Asian cultures they are considered benign and auspicious, but in Europe they have often been perceived as threatening, even though they do not injure humans. Long-standing vernacular names such as horse stinger, snake doctor, and devil's darning needle testify to their formidable appearance.

The young, termed larvae or sometimes nymphs, are functionally wingless and live in a variety of shallow freshwater habitats including tree holes, ponds, marshes, and streams. They are often bottom-dwelling and are well-camouflaged, their mottled or drab colours matching the sediments or water plants around them. Although large numbers of mosquitoes and other insect pests are consumed by larvae and adults, odonates are generally indiscriminate feeders that seldom affect humans economically. Larvae, however, have been used successfully in Myanmar to interrupt transmission of the mosquito-borne disease dengue.

Larval development can range from three weeks to more than 8 years, depending on the species and habitat. Tropical species generally take less time to develop than those of colder climates. Many temperate species spend their last winter in the final instar and emerge during the spring and early summer. Others spend their last winter as eggs, earlier instars, or immature adults. Odonate larvae are preyed upon mainly by fish but also by frogs, birds, crayfish, and each other. Both larvae and adults are sometimes parasitized by flukes, tapeworms, and mites; minute wasps can parasitize eggs. During emergence odonates are particularly vulnerable to predation by birds, spiders, amphibians, and reptiles.

Adult life consists of two phases—the prereproductive, or maturation, period and the reproductive period. Maturation generally lasts about 2 weeks but can take anywhere from 1 to 60 days, depending on species, climate, and weather. When the maturation period serves to bridge dry or cold seasons, however, it can last nine months or more.

The reproductive period begins when a sexually mature adult dragonfly flies to the mating rendezvous, usually the margin of a body of water where the eggs will be laid. Males assemble there slightly earlier than females and space themselves along the shore or over the water. At the rendezvous site defense by males ranges from negligible to intense. Actively defending males establish an area of characteristic extent, much as birds defend territories (territorial behaviour). Where sites are intensely defended, an individual may return to the same perch for many successive days and expel intruders. When a male adult approaches or enters a territory occupied by another individual of the same species, the occupant acts aggressively, and an aerial agility contest often ensues; thus, territories are held by the most vigorous males. Violent confrontations between rival males sometimes result in injury or death. If a female adult approaches or enters a territory, the resident male tries to mate with her. In some species mating is preceded by a courtship display during which the female accepts or rejects the male, who tries to guide her to an egg-laying site in his territory. Usually, however, there is no evident prelude and mating occurs immediately.

Odonates are considered to have excellent vision, and the large compound eyes (eye, human), acutely responsive to movement and form, play an important role in capturing food and interacting with other individuals. In most dragonflies (suborder Anisoptera) the compound eyes meet at the top of the head and can consist of 30,000 individual optical units, or ommatidia. Their large eyes give some anisopterans a nearly 360-degree view of their environment. Among certain dragonflies of the family Aeshnidae that fly only in subdued light (e.g., Gynacantha), the eyes occupy almost the entire surface of the head. Damselflies (suborder Zygoptera), on the other hand, have eyes that are well separated. Odonates also possess three tiny simple eyes called ocelli.

At the opposite end of the body, another structural difference between the two suborders can be seen: male damselflies have four tail appendages, whereas dragonflies have only three.

The large wings (wing) are strengthened by a complex network of veins. Each wing also has a thickened patch (the pterostigma) on the leading edge of the wing tip. The forewings of dragonflies are narrower than the hind wings, which in certain migratory species (genera Libellula, Pantala, and Tramea) are expanded, permitting gliding flight.

Odonates use their wings in a unique manner. Whereas other insects with four wings beat them synchronously, odonates can beat the fore and hind pairs independently. This allows three different modes of flight in which the pairs beat (1) synchronously, as those of other insects, (2) alternately between the two sets, or (3) synchronously but out of phase with each other. Such variations allow odonates a repertoire of aerobatic abilities that include hovering, backward flight, and turns of such tight radius that they are virtually midair pivots. Odonate aerodynamics have even been studied in the hope of applying the flight principles involved to aircraft.

The unique wheel position exhibited by Odonata can be regarded as a special way of transferring sperm. Instead of placing sperm on the ground or a leaf for the female to pick up (as certain primitive insects do), the male transfers it from genitalia near the tip of his abdomen to accessory sex organs at the base of his abdomen that are specially constructed to receive and transmit sperm to the female. Odonates that appear very similar may actually be different species; the sex organs are highly intricate and species-specific. Males have evolved various means of displacing the sperm of previous mates. Mating between species is hampered by the fact that odonates of the same species have interlocking structures on the male claspers and female head or prothorax that enable the wheel to be formed and made secure.

Like the adults, odonate larvae have large eyes, and anisopterans are generally stockier than zygopterans. The most immediate method of distinguishing between larvae of the two suborders is by inspecting the abdomen. In most Zygoptera there are three external leaflike plates at the tip of the abdomen that function as gills. Among anisopterans the gills are internal growths of the hindgut wall that are ventilated as water is pumped in and out of the anus. Both respiratory systems are used in emergencies as methods for locomotion; jet propulsion via the anus is a particularly effective escape tactic employed by Anisoptera. Many zygopterans, on the other hand, use their leaflike plates to swim much as a fish uses its tail.

Unlike the adults, the larvae show many structural variations that reflect the demands of different aquatic microhabitats with respect to respiration, locomotion, feeding, and concealment. Species living within fine sediment (Aphylla), for example, are more or less cylindrical in cross-section, with a terminal siphon that maintains respiratory contact with the water above; those residing on the surface of mud or sand (Ictinogomphus) often are flattened, with prominent lateral spines on the abdomen; those living among plants near the surface (Anax, Lestes) are streamlined and very active.

The odonates have an unusually long and rich fossil record. Ancestors date from more than 300 million years ago (the Late Carboniferous Epoch) and predate the dinosaurs by nearly 100 million years. Closely resembling present dragonflies, they had already diverged from other orders of winged insects, including their closest living relatives, the mayflies (order Ephemeroptera). The oldest odonate ancestors are of the extinct order Protodonata, and even they possessed a complete series of alternating upwardly and downwardly curving wing veins, as do today's odonates. Protodonate wings, however, lacked the pterostigma. Also extinct is the suborder Meganisoptera of the Carboniferous and Permian periods. This suborder includes well-preserved fossils of gigantic dragonflies such as Meganeura monyi, which had a wingspan of more than 70 cm (28 inches).

The order Odonata contains four extinct and two living suborders. Extinct suborders are Protanisoptera and Archizygoptera of the Permian Period, Triadophlebiomorpha of the Triassic Period, and Anisozygoptera of the Triassic to Cretaceous periods. The only living suborders are Anisoptera and Zygoptera. Two odd and primitive species, both of the genus Epiophlebia (family Epiophlebiidae), live in the mountains of Nepal and Japan; until recently, they were classified as Anisozygoptera, a suborder intermediate in form between current dragonflies and damselflies. Another relict, or “living fossil,” group is the family Hemiphlebiidae. Like the epiophlebiids, members of this family (found only in a small Australian locale) are primitive and sufficiently different from all other odonates to warrant their own superfamily.

Features used by taxonomists when classifying adult members of the order Odonata are the structure of the male sex organs, shape and vein patterns of the wings, distance between the compound eyes, form and development of rear appendages, and presence of an ovipositor. Larvae are classified according to the type and form of respiratory organ, labial structure, number and arrangement of body spines, and shape of the abdomen.

The members of the order Odonata occupy a uniquely isolated position in the phylogeny of insects, representing a remarkable mixture of primitive and specialized characteristics. The classification given here is essentially that of F.M. Carpenter (1992) and C.A. Bridges (1993); it takes into account the fossil record of ancestral odonates. Other recently proposed classifications exist.

There is general agreement among specialists regarding the status and affinities of living families and genera of the Odonata, and with few exceptions published classifications based on the adult and larva correspond with one another. The phylogeny of the two living suborders, however, remains debatable. The primary issue is whether Anisoptera arose independently from the Protodonata or descended from zygopteroid stock—perhaps the extinct Archizygoptera. The former hypothesis receives wider support.