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, which are similar in structure though usually lighter in build; however, adults can be differentiated by the fact that the wings of most dragonflies are held away from, and perpendicular to, the body when at rest. Dragonflies are agile fliers.
Dragonflies are major predators that eat a variety of other insects. They are usually found around marshes, lakes, ponds, streams, and wetlands because their larvae, known as "nymphs", are aquatic. About 5,900 different species of dragonflies (Odonata) are known in the world today of which about 3000 belong to the Anisoptera.
Though dragonflies are predators, they themselves are subject to being preyed upon by birds, lizards, frogs, spiders, fish, water bugs, and even other large dragonflies.
- 1 Phylogeny
- 2 General description
- 3 Distribution
- 4 Biology
- 5 Predators
- 6 Dragonflies and humans
- 7 Gallery
- 8 Notes
- 9 References
- 10 Sources
- 11 External links
Dragonflies are an ancient group. The oldest fossils are of the Protodonata from the 325 mya Upper Carboniferous of Europe, which had wingspans up to 750 mm; they became extinct in the Triassic. The Protoanisoptera, forerunners of the Anisoptera (true dragonflies), have also been found in Carboniferous rocks from Abilene, Kansas, USA and in Europe. Protoanisopteran fossils from Commentry, France and Bolsover, Derbyshire, England have wingspans up to 700 and 500 mm respectively.
The relationships of Anisopteran families remained not fully resolved in 2013, but all the families are monophyletic except the Corduliidae; the Gomphidae are a sister taxon to all other Anisoptera, the Austropetaliidae are a sister to the Aeshnoidea, and the Chlorogomphidae are a sister to a clade that includes the Synthemistidae and Libellulidae. On the cladogram, dashed lines indicate unresolved relationships; English names are given (in parentheses):
Dragonflies (suborder Anisoptera) are heavy-bodied, strong-flying insects that hold their wings horizontally when at rest. By contrast, damselflies (suborder Zygoptera) are slender-bodied weak-fliers that fold their wings over the abdomen when stationary.
An adult dragonfly has the same typical division into head, thorax and abdomen as do other insects. It has a chitinous exoskeleton of hard plates held together with flexible membranes. The head is large with very short antennae. It is dominated by the two compound eyes, which cover most of its surface, each consisting of nearly 28,000 individual units. There are also three simple eyes. The mouthparts are adapted for biting with a toothed jaw and the flap-like labrum, at the front of the mouth, can be shot rapidly forward to catch prey. The thorax bears two pairs of wings and three pairs of legs. The wings are long, veined and membranous, each pair being of similar size and shape, narrower at the tip and wider at the base. The veins contain hemolymph and both stiffen and strengthen the wings. The leading edge of each wing has a node where other veins join the marginal vein, and the wing is able to flex at this point. In most large species of dragonfly, the wings of females are shorter and broader than those of males. The legs are seldom used for walking, but are used to catch and hold prey and for perching on plants. The abdomen is long and slender and bears the reproductive organs.
Dragonflies, and indeed most families within the Anisoptera, are found on every continent of the world except Antarctica. In contrast to the damselflies (Zygoptera) which tend to have restricted distributions, some genera and individual species are found across whole continents. For example, the blue-eyed darner Rhionaeschna multicolor is found all across North America, and in Central America; emperors Anax are found throughout the Americas from as far north as Newfoundland to as far south as Bahia Blanca in Argentina, across Europe to central Asia, North Africa and the Middle East.
Dragonfly adults hunt on the wing using their exceptionally acute eyesight and strong agile flight.
Many dragonflies, particularly males, are territorial. Some defend a territory against others of their own species, some against other species of dragonfly and a few against insects in unrelated groups. A particular perch may give a dragonfly a good view over an insect-rich feeding ground, and the blue dasher (Pachydiplax longipennis) will jostle other dragonflies to maintain the right to alight here.
Defending a breeding territory is fairly common among male dragonflies, especially among species that congregate around ponds in large numbers. The territory will contain desirable features such as a sunlit stretch of shallow water, a special plant species or a particular substrate that is necessary for egg-laying. It may be small or large, depending on its quality, the time of day and the number of competitors, and may be held for a few minutes or several hours. Some dragonflies signal ownership with striking colours on face, abdomen, legs or wings. The common whitetail (Plathemis lydia) dashes towards an intruder holding its white abdomen aloft like a flag. Other dragonflies engage in aerial dogfights or high speed chases. Any female will need to mate with the territory holder before laying her eggs.
Dragonflies as a group have three distinctive patterns of behaviour:
- Firstly, they are amphibious: while the adults fly, breathe air and live on land, the larvae live in fresh water.
- Secondly, they are predatory: in every species, both the adults and the larvae hunt other animals. Adults generally catch flying insects; the larvae hunt a range of freshwater invertebrates including insects, but the prey of the larger dragonfly larvae ranges up to amphibian tadpoles and small fish.
- Thirdly, their reproduction, unlike that of any other group, involves indirect insemination and delayed fertilisation.
Together, these three features explain many aspects of dragonfly behaviour. Adult males vigorously defend territories beside freshwater; these areas provide suitable habitat for the larvae to develop, and for females to lay their eggs. Swarms of feeding adults are readily explained through predation on swarming prey, such as emerging flying ants. Highly characteristic behaviours are associated with reproduction.
Mating in dragonflies is a complex, precisely choreographed process. First, the male 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 to 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".
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 or continuing to clasp her and flying in tandem, so as to prevent rival males from attempting to remove his sperm and insert their own, something made possible by delayed fertilisation and driven by Sexual selection. To do that, the male uses his penis to compress or to scrape out the sperm inserted by the previous male. This activity takes up much of the time that the copulating pair remain in the heart posture. Egg-laying takes two different forms in different species of dragonfly. The females in some families have a sharp-edged ovipositor with which she slits open a stem or leaf of a plant on or near the water, so she can push her eggs inside. In other families such as clubtails (Gomphidae), cruisers (Macromiidae), emeralds (Corduliidae) and skimmers (Libellulidae), the females lay eggs by tapping the surface of the water repeatedly, by shaking the eggs out of her abdomen as she flies along, or in some cases by placing their eggs on vegetation.
Female dragonflies lay eggs in or near water, often on floating or emergent plants. When laying eggs, some species will submerge themselves completely in order to lay their eggs on a good surface. The eggs then hatch into naiads (nymphs). Most of a dragonfly's life is spent in the naiad form, beneath the water's surface, using extendable jaws to catch other invertebrates (often mosquito larvae) or even vertebrates such as tadpoles and fish. They breathe through gills in their rectum, and can rapidly propel themselves by suddenly expelling water through the anus. Some naiads hunt on land.
The larval stage of large dragonflies lasts up to five years in large species, and between two months and three years in smaller species. When the naiad is ready to metamorphose into an adult, it climbs up a reed or other emergent plant. Exposure to air causes the naiad to begin breathing. The skin splits at a weak spot behind the head and the adult dragonfly crawls out of its larval skin, pumps up its wings, and flies off to feed. In flight, the adult dragonfly can propel itself in six directions: upward, downward, forward, back, and side to side. The adult stage of larger species of dragonfly can last as long as five or six months.
The sex ratio between males and females dragonfly vary both temporally and spatially. Adult dragonflies have a high male-biased ratio at breeding habitats. The male-bias ratio has contributed partially to the females using different habitats to avoid male harassment. As seen in the Hine’s emerald dragonfly (Somatochlora hineana) male populations use wetland habitats, while females use dry meadows and marginal breeding habitats, only migrating to the wetlands to lay their eggs or to find mating partners. Unwanted mating is energetically costly for females because it affects the amount of time that they are able to spend foraging.
- Counter-stroking, with forewings beating 180 degrees out of phase with the hindwings, is used for hovering and slow flight. This style is efficient and generates a large amount of lift.
- Phased-stroking, with the hindwings beating 90 degrees ahead of the forewings, is used for fast flight. This style creates more thrust but less lift than counter-stroking.
- Synchronised-stroking, with forewings and hindwings beating together, is used when changing direction rapidly, as it maximises thrust.
- Gliding, with the wings held out, is used in three situations: free gliding, for a few seconds in between bursts of powered flight; gliding in the updraft at the crest of a hill, effectively hovering by falling at the same speed as the updraft; and in females when "in cop" with a male simply glides while the male pulls the pair along by beating his wings.
The wings are powered directly, with the flight muscles attached to the wing bases. Dragonflies have a high power/weight ratio, and have been documented accelerating at 4g linearly and 9g in sharp turns while pursuing prey.
Dragonflies generate lift in at least four ways at different times, including classical lift like an aircraft wing; supercritical lift with the wing above the critical angle, generating high lift and using very short strokes to avoid stalling; creating vortices; and vortex shedding. Some families appear to use special mechanisms, as for example the Libellulidae which take off rapidly, their wings beginning pointed far forward and twisted almost vertically. Dragonfly wings behave highly dynamically during flight, flexing and twisting during each beat. Among the variables are wing curvature, length and speed of stroke, angle of attack, forward/back position of wing, and phase relative to the other wings.
There are old and unreliable claims that dragonflies such as the southern giant darner can fly at up to 60 miles per hour (97 km/h). However, the greatest reliable flight speed records are for other types of insects. In general, large dragonflies like the hawkers have a maximum speed of 10–15 metres per second (22–34 mph) with average cruising speed of about 4.5 metres per second (10 mph). Dragonflies can fly at 100 body-lengths per second, and 3 lengths per second backwards.
In territorial battles between male Australian emperors Hemianax papuensis (Aeshnidae), the fighting dragonflies adjust their flight paths to appear stationary to their rivals in "breakneck territorial dogfight[s]", minimizing the chance of being detected as they approach.[a] To achieve the effect, the attacking dragonfly flies towards its rival, choosing its path to remain on a line between the rival and the start of its attack path. The attacker thus looms larger as it closes on its rival, but does not otherwise appear to move. Researchers found that 6 of 15 encounters involved motion camouflage.
Although dragonflies are swift and agile fliers, there are predators fast enough to catch them. These include falcons such as the American kestrel, the merlin, and the hobby; nighthawks, swifts, flycatchers and swallows also take some adults; some species of wasp, too, prey on dragonflies, using them to provision their nests, laying an egg on each captured insect. In the water, various species of duck and heron eat dragonfly larvae.
Dragonflies and humans
Habitat degradation has reduced dragonfly populations across the world, for example in Japan. Over 60% of Japan's wetlands were lost in the twentieth century, so its dragonflies now depend largely on rice fields, ponds and creeks. Dragonflies feed on pest insects in rice, acting as a natural pest control. Dragonflies are steadily declining in Africa, and represent a conservation priority.
The dragonfly's long lifespan and low population density makes them vulnerable to disturbance, such as from collisions with vehicles on roads built near wetlands. Species that fly low and slow may be most at risk.
In Europe, dragonflies have often been seen as sinister. Some English vernacular names, such as "devil's darning needle" and "ear cutter", link them with evil or injury. A Romanian folk tale says that the dragonfly was once a horse possessed by the devil. Swedish folklore holds that the devil uses dragonflies to weigh people's souls.:25–27 The Norwegian name for dragonflies is Øyenstikker ("eye-poker"), and in Portugal they are sometimes called tira-olhos ("eye-snatcher"). They are often associated with snakes, as in the Welsh name gwas-y-neidr, "adder's servant". The Southern United States term "snake doctor" refers to a folk belief that dragonflies follow snakes around and stitch them back together if they are injured.
For some Native American tribes, dragonflies represent swiftness and activity, and for the Navajo they symbolize pure water. They are a common motif in Zuni pottery; stylized as a double-barred cross, they appear in Hopi rock art and on Pueblo necklaces.:20–26 They have been used in traditional medicine in Japan and China. In Indonesia. adults are caught on poles made sticky with birdlime, then fried in oil as a delicacy.
In the United States dragonflies and damselflies are sought out as a hobby similar to birding and butterflying, known as oding, from the Latin name of the dragonfly order, Odonata. Oding is especially popular in Texas, where 225 different species of odonates have been observed. With care, and with dry fingers, dragonflies can be handled and released by oders, as can be done with butterflies, though it is not encouraged.
Images of dragonflies are common in Art Nouveau, especially in jewelry designs. They have also been used as a decorative motif on fabrics and home furnishings. Douglas, a British motorcycle manufacturer based in Bristol, named its innovatively designed postwar 350cc flat twin model the Dragonfly.
As a seasonal symbol in Japan, the dragonfly is associated with summer and early autumn. More generally, dragonflies are symbols of courage, strength, and happiness, and they often appear in art and literature, especially haiku. Japanese children catch large dragonflies as a game, using a hair with a small pebble tied to each end, which they throw into the air. The dragonfly mistakes the pebbles for prey, gets tangled in the hair, and is dragged to the ground by the weight.:38 Beyond this, one of Japan's historical names – Akitsushima (Kanji: 秋津島 Hiragana: あきつしま) – is an archaic form meaning "Dragonfly Islands". This is attributed to a legend in which Japan's mythical founder, Emperor Jinmu, was bitten by a mosquito, which was then eaten by a dragonfly. Human-sized, prehistoric dragonfly nymphs called "Meganulon" appear in the 1956 film Rodan, based on the extinct genus Meganeura. The main kaiju antagonist in the 2000 film Godzilla vs. Megaguirus is a variation of this fictional creature.
Broad-bodied chaser (Libellula depressa)
Tau emerald (Hemicordulia tau)
Australian emperor (Hemianax papuensis)
Common glider (Tramea transmarina), Darwin, Australia
Scarlet percher (Diplacodes haematodes), Queensland, Australia
Blue dasher, (Pachydiplax longipennis), Wichita, Kansas, USA
Jewel flutterer, (Rhyothemis resplendens) with colorful wing panels.
- This is not to say that other species may not use the same technique, only that this species has been studied.
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- Dragon fly naiad labium extended to capture prey
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- Dragonfly at DMOZ
- Identification key to dragonflies found in Ireland
- British Dragonfly Society
- Dragonflies and Damselflies (Odonata) of the United States
- Phaon (Pinhey's Heritage African Odonata Network)
- Dragonflies and damselflies on the UF / IFAS Featured Creatures Web site
- Photos of dragonflies from Asia-dragonfly.net, Africa-dragonfly.net, America-Dragonfly.net, Libellulasman.com and Odonata.su
- List of field guides to dragonflies, from the International Field Guides Database