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Suborder of insects

Damselflies are flying insects of the suborder Zygoptera in the order Odonata. They are similar to dragonflies (which constitute the other odonatan suborder, Anisoptera) but are usually smaller and have slimmer bodies. Most species fold the wings along the body when at rest, unlike dragonflies which hold the wings flat and away from the body. Damselflies have existed since the Jurassic, and are found on every continent except Antarctica. All damselflies are predatory insects; both nymphs and adults actively hunt and eat other insects. The nymphs are aquatic, with different species living in a variety of freshwater habitats including acidic bogs, ponds, lakes and rivers. The nymphs moult repeatedly, at the last moult climbing out of the water to undergo metamorphosis. The skin splits down the back, they emerge and inflate their wings and abdomen to gain their adult form. Their presence on a body of water indicates that it is relatively unpolluted, but their dependence on freshwater makes them vulnerable to damage to their wetland habitats. Some species of damselfly have elaborate courtship behaviours. Many species are sexually dimorphic, the males often being more brightly coloured than the females. Like dragonflies, they reproduce using indirect insemination and delayed fertilisation. A mating pair form a shape known as a "heart" or "wheel", the male clasping the female at the back of the head, the female curling her abdomen down to pick up sperm from secondary genitalia at the base of the male's abdomen. The pair often remain together with the male still clasping the female while she lays eggs within the tissue of plants in or near water using a robust ovipositor. Artificial fishing flies that mimic damselfly nymphs are used in wet fly fishing. Damselflies are sometimes represented in personal jewellery such as brooches.

The Zygoptera are an ancient group, with the earliest fossils dating to the Late Jurassic, around 153 million years ago. Well preserved Eocene damselfly larvae and exuviae are known from fossils preserved in amber in the Baltic region. 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. The Protoneuridae in particular is shown to be composed of six clades from five families. The result so far is 27 damselfly families, with 7 more likely to be created. The discovered clades did not agree well with traditional characteristics used to classify living and fossil Zygoptera such as wing venation, so fossil taxa will need to be revisited. The 18 extant traditional families are provisionally rearranged as follows (the 3 paraphyletic families disappearing, and many details not resolved):

The general body plan of a damselfly is similar to that of a dragonfly. The compound eyes are large but are more widely separated and relatively smaller than those of a dragonfly. Above the eyes is the frons or forehead, below this the clypeus, and on the lower lip or labium, an extensible organ used in the capture of prey. The top of the head bears three simple eyes (ocelli), which may measure light intensity, and a tiny pair of antennae that serve no olfactory function but may measure air speed. Many species are sexually dimorphic; the males are often brightly coloured and distinctive, while the females are plainer, cryptically coloured, and harder to identify to species. For example, in Coenagrion, the Eurasian bluets, the males are bright blue with black markings, while the females are usually predominantly green or brown with black. A few dimorphic species show female limited polymorphism, the females being in two forms, one form distinct and the other with the patterning as in males. The ones that look like males, andromorphs, are usually under a third of the female population but the proportion can rise significantly and a theory that explains this response suggests that it helps overcome harassment by males. Some Coenagrionid damselflies show male limited polymorphism, an even less understood phenomenon. In general, damselflies are smaller than dragonflies, the smallest being members of the genus Agriocnemis (wisps). However, members of the Pseudostigmatidae (helicopter damselflies or forest giants) are exceptionally large for the group, with wingspans as much as in Megaloprepus and body length up to in Pseudostigma aberrans. The first thoracic segment is the prothorax, bearing the front pair of legs. The joint between head and prothorax is slender and flexible, which enables the damselfly to swivel its head and to manoeuvre more freely when flying. The remaining thoracic segments are the fused mesothorax and metathorax (together termed the synthorax), each with a pair of wings and a pair of legs. A dark stripe known as the humeral stripe runs from the base of the front wings to the second pair of legs, and just in front of this is the pale coloured, antehumeral stripe. Species markers include quadrangular markings on the wings known as the pterostigma or stigma, and in almost all species, there is a nodus near the leading edge. The thorax houses the flight muscles. Damselfly nymphs swim by fish like undulations, the gills functioning like a tail. Dragonfly nymphs can forcibly expel water in their rectum for rapid escape.

Odonates are found on all the continents except Antarctica. Although some species of dragonfly have wide distributions, damselflies tend to have smaller ranges. Most odonates breed in fresh water; a few damselflies in the family Caenagrionidae breed in brackish water (and a single dragonfly species breeds in seawater). Dragonflies are more affected by pollution than are damselflies. The presence of odonates indicates that an ecosystem is of good quality. The most species rich environments have a range of suitable microhabitats, providing suitable water bodies for breeding.

Adult damselflies catch and eat flies, mosquitoes, and other small insects. Often they hover among grasses and low vegetation, picking prey off stems and leaves with their spiny legs (unlike dragonflies which prefer catching flying prey). Although predominantly using vision to locate their prey, adults may also make use of olfactory cues. 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. 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. The nymphs of damselflies have been less researched than their dragonfly counterparts, and many have not even been identified. They choose their prey according to size and seem less able to overpower larger prey than can dragonfly nymphs. The major part of the diet of most species appears to be crustaceans such as water fleas. 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. The scarce blue tailed damselfly (Ischnura pumilio) in contrast requires base rich habitats and water with a slow flow rate. It is found in ditches, quarries, seeps, flushes, marshes and pools. It tolerates high levels of zinc and copper in the sediment but requires suitable emergent plants for egg laying without the water being choked by plants. 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. Another tropical species, the cascade damselfly Thaumatoneura inopinata (Megapodagrionidae), inhabits waterfalls in Costa Rica and Panama. Damselflies, both nymphs and adults, are eaten by a range of predators including birds, fish, frogs, dragonflies, other damselflies, water spiders, water beetles, backswimmers and giant water bugs. Damselflies have a variety of internal and external parasites. Particularly prevalent are the gregarine protozoans found in the gut. In a study of the European common blue damselfly, every adult insect was infected at the height of the flying season. When present in large numbers, these parasites can cause death by blocking the gut.

Many damselflies have elaborate courtship behaviours. These are designed to show off the male's distinctive characteristics, bright colouring or flying abilities, thus demonstrating his fitness. 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. Platycypha males will hover in front of a female, thrusting their bright white legs forward in front of their heads. Flattened tibia and bright leg colouring are seen in Platycnemis phasmovolans and a few other Platycnemididae, including the extinct Yijenplatycnemis huangi. Rhinocypha bobs up and down, often low over fast flowing forested and shaded streams, displaying its bright coloured body and wings. Vibrating the tibia is seen in Libellago semiopaca despite it lacking bright colouration on the tibia suggesting that foot waggling is a generalized excitary signal in Chlorocyphidae damselflies. and Platycypha caligata. Male members of the family Protoneuridae with vividly coloured wings display these to visiting females. Swift forktail (Ischnura erratica) males display to each other with their blue tipped abdomens raised. Other behaviours observed in damselflies include wing warning, wing clapping, flights of attrition and abdominal bobbing. Wing warning is a rapid opening and closing of the wings and is aggressive, while wing clapping involves a slower opening of the wings followed by a rapid closure, up to eight times in quick succession, and often follows flight; it may serve a thermo regulatory function. 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. Characteristics of displays and coloration of males are suggested to be the common cues used by females to choose mates. In at least one species, Mnais costalis, males with more sunlight in their territories had higher wing beat frequency and were more likely to mate. Females preferred "hotter" males because they would be on warmer territories for egg laying. At night, damselflies usually roost in dense vegetation, perching with the abdomen alongside a stem. If disturbed they will move around to the other side of the stem but will not fly off. Spreadwings fully fold their wings when roosting.

Mating in damselflies, as in dragonflies, is a complex, precisely choreographed process involving both indirect insemination and delayed fertilisation. The male first has to attract a female to his territory, continually driving off rival males. When he is ready to mate, he transfers a packet of sperm from his primary genital opening on segment 9, near the end of his abdomen, to his secondary genitalia on segments 2–3, near the base of his abdomen. The male then grasps the female by the head with the claspers at the end of his abdomen; the structure of the claspers varies between species, and may help to prevent interspecific mating. The pair fly in tandem with the male in front, typically perching on a twig or plant stem. The female then curls her abdomen downwards and forwards under her body to pick up the sperm from the male's secondary genitalia, while the male uses his "tail" claspers to grip the female behind the head: this distinctive posture is called the "heart" or "wheel"; the pair may also be described as being "in cop". Males may transfer the sperm to their secondary genitalia either before a female is held, in the early stage when the female is held by the legs or after the female is held between the terminal claspers. This can lead to variations in the tandem postures. The spermatophore may also have nutrition in addition to sperms as a "nuptial gift". Some cases of sexual cannibalism exist where females (of Ischnura graellsii) eat males while in copula. Parthenogenesis (reproduction from unfertilised eggs) is exceptional, and has only been recorded in nature in female Ischnura hastata on the Azores Islands. 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. The male attempts to prevent rivals from removing his sperm and inserting their own, a form of sperm competition (the sperms of the last mated male have the greatest chance of fertilizing the eggs, also known as sperm precedence) made possible by delayed fertilisation and driven by sexual selection. For example, the red eyed damselfly Erythromma najas lays eggs, in tandem, into leaves or stems of floating or sometimes emergent plants; in contrast, the scarce bluetail Ischnura pumilio oviposits alone, the female choosing mostly emergent grasses and rushes, and laying her eggs in their stems either above or just below the waterline. 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. 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. 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.

Damselflies are hemimetabolous insects that have no pupal stage in their development. 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. which are terrestrial in their early stages. The nymphs proceed through about a dozen moults as they grow. In the later stages, the wing pads become visible. 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. This has a soft body at first and hangs or stands on its empty larval case. It pumps haemolymph into its small limp wings, which expand to their full extent. The haemolymph is then pumped back into the abdomen, which also expands fully. The exoskeleton hardens and the colours become more vivid over the course of the next few days. Most damselflies emerge in daytime, and in cool conditions the process takes several hours. On a hot day, the cuticle hardens rapidly and the adult can be flying away within half an hour. 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. The presence of alien species can also have unintended consequences. The ancient greenling Hemiphlebia mirabilis has been an important flagship species for conservation action in preserving its habitat in Australia.

Fishing flies that mimic damselfly nymphs are sometimes used in wet fly fishing, where the hook and line are allowed to sink below the surface. Damselflies have formed subjects for personal jewellery such as brooches since at least 1880. Damselfly is a 2005 short film directed by Ben O'Connor. Damselfly is the title of a 2012 novel in the Faeble series by S. L. Naeole and of a 2018 novel by Chandra Prasad. Modern poems with the damselfly as a subject include a 1994 poem by August Kleinzahler, which contains the lines "And that blue there, cobalt / a moment, then iridescent, / fragile as a lady's pin / hovering above the nasturtium?" The poet John Engels published “Damselfly, Trout, Heron” in his 1983 collection Weather Fear: New and Selected Poems.