The nymphs are aquatic, with different species living in a variety of freshwater habitats including acidic bogs, ponds, lakes and rivers.

The Zygoptera are an ancient group, with the earliest fossils dating to the Kimmeridgian age of the Late Jurassic, around 152 million years ago.

[3] Molecular analysis in 2021 confirms that most of the traditional families are monophyletic, but shows that the Amphipterygidae, Megapodagrionidae and Protoneuridae are paraphyletic and will need to be reorganised.

[5] The forewings and hindwings are similar in appearance and are membranous, being strengthened and supported by longitudinal veins that are linked by many cross-veins and that are filled with haemolymph.

The secondary genitalia in males are on the undersides of segments two and three and are conspicuous, making it easy to tell the sex of the damselfly when viewed from the side.

[22] These are known to leave their waterside habitats, flying upwards till lost from view, and presumably being dispersed to far off places by the stronger winds found at high altitudes.

[24] No species are known to hunt at night, but some are crepuscular, perhaps taking advantage of newly hatched flies and other aquatic insects at a time when larger dragonflies are roosting.

[26] There are few pools and lakes in these habitats, and these damselflies breed in temporary water bodies in holes in trees, the rosettes of bromeliads and even the hollow stems of bamboos.

[28] The European common blue damselfly (Enallagma cyathigerum) for example can occur at high densities in acid waters where fish are absent, such as in bog pools.

[29] The scarce blue-tailed damselfly (Ischnura pumilio) in contrast requires base-rich habitats and water with a slow flow-rate.

[30] In the tropics, the helicopter damselfly Mecistogaster modesta (Pseudostigmatidae) breeds in phytotelmata, the small bodies of water trapped by bromeliads, epiphytic plants of the rainforest of northwest Costa Rica, at the high density of some 6000 larvae per hectare in patches of secondary forest.

[31] Another tropical species, the cascade damselfly Thaumatoneura inopinata (Megapodagrionidae), inhabits waterfalls in Costa Rica and Panama.

Calopteryx males will hover in front of a female with alternating fast and slow wingbeats; if she is receptive she will remain perched, otherwise she will fly off.

The male river jewelwing (Calopteryx aequabilis) performs display flights in front of the female, fluttering his forewings while keeping his hindwings still, and raising his abdomen to reveal the white spots on his wings.

Flattened tibia and bright leg colouring are seen in Platycnemis phasmovolans and a few other Platycnemididae, including the extinct Yijenplatycnemis huangi.

[37] Some species (R. biceriata, R. humeralis) have a foot waggling behaviour: they thrust a leg forward and vibrate it towards ovipositing females while in flight.

[41] Flights of attrition are engaged in by the ebony jewelwing (Calopteryx maculata) and involve males bouncing around each other while flying laterally and continuing to do so, sometimes over a considerable distance, until one insect is presumably exhausted and gives up.

[45] Mating in damselflies, as in dragonflies, is a complex, precisely choreographed process involving both indirect insemination and delayed fertilisation.

[21][54] Egg-laying (ovipositing) involves not only the female darting over floating or waterside vegetation to deposit eggs on a suitable substrate, but the male hovering above her, mate-guarding, or in some species continuing to clasp her and flying in tandem.

[47][48] If successful, a rival male uses his penis to compress or scrape out the sperm inserted previously; this activity takes up much of the time that a copulating pair remain in the heart posture.

[58] The willow emerald Chalcolestes viridis (a spreadwing) is unusual in laying eggs only in woody plant tissue, choosing thin twigs of trees that hang over water, and scarring the bark in the process.

[59] A possible exception is an apparent instance of ovo-viviparity, in which Heliocypha perforata was filmed in western China depositing young larvae (presumably hatched from eggs inside the female's body) onto a partly submerged branch of a tree.

[60] Many damselflies are able to produce more than one brood per year (voltinism); this is negatively correlated with latitude, becoming more common towards the equator, except in the Lestidae.

[62] The female inserts the eggs by means of her ovipositor into slits made in water plants or other underwater substrates and the larvae, known as naiads or nymphs, are almost all completely aquatic.

They breathe by means of three large external, fin-like gills on the tip of the abdomen, and these may also serve for locomotion in the same manner as a fish's tail.

When fully developed, the nymphs climb out of the water and take up a firm stance, the skin on the thorax splits and the adult form wriggles out.

The main threats experienced by odonates are the clearance of forests, the pollution of waterways, the lowering of groundwater levels, the damming of rivers for hydroelectric schemes and the general degradation of wetlands and marshes.

[64] The clearance of tropical rainforests is of importance because the rate of erosion increases, streams and pools dry up and waterways become clogged with silt.

[64] In Hawaii, the introduction of the mosquitofish (Gambusia affinis) was effective in controlling mosquitoes but nearly exterminated the island's endemic damselflies.

[65] The ancient greenling Hemiphlebia mirabilis has been an important flagship species for conservation action in preserving its habitat in Australia.