Summary ⁹

A dragonfly is an insect belonging to the order Odonata, infraorder Anisoptera (from Greek ἄνισος anisos, "uneven" and πτερόν pteron, "wing", because the hindwing is broader than the forewing). Adult dragonflies are 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 usu

General description ¹⁰

Dragonflies (suborder Anisoptera) are heavy-bodied, strong-flying insects that hold their wings horizontally both in flight and at rest. By contrast, damselflies (suborder Zygoptera) have slender bodies and fly more weakly; most species fold their wings over the abdomen when stationary, and the eyes are well separated on the sides of the head.

An adult dragonfly has three distinct segments, the head, thorax, and abdomen, as in all 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. The compound eyes are made up of ommatidia, the numbers being greater in the larger species. Aeshna interrupta has 22650 ommatidia of two varying sizes, 4500 being large. The facets facing downward tend to be smaller. Petalura gigantea has 23890 ommatidia of just one size. These facets provide complete vision in the frontal hemisphere of the dragonfly. The compound eyes meet at the top of the head (except in the Petaluridae and Gomphidae, as also in the genus Epiophlebia). Also, they have three simple eyes or ocelli. The mouthparts are adapted for biting with a toothed jaw; the flap-like labrum, at the front of the mouth, can be shot rapidly forward to catch prey. The head has a system for locking it in place that consists of muscles and small hairs on the back of the head that grip structures on the front of the first thoracic segment. This arrester system is unique to the Odonata, and is activated when feeding and during tandem flight.

The thorax consists of three segments as in all insects. The prothorax is small and is flattened dorsally into a shield-like disc, which has two transverse ridges. The mesothorax and metathorax are fused into a rigid, box-like structure with internal bracing, and provide a robust attachment for the powerful wing muscles inside. The thorax bears two pairs of wings and three pairs of legs. The wings are long, veined, and membranous, narrower at the tip and wider at the base. The hindwings are broader than the forewings and the venation is different at the base. The veins carry haemolymph, which is analogous to blood in vertebrates, and carries out many similar functions, but which also serves a hydraulic function to expand the body between nymphal stages (instars) and to expand and stiffen the wings after the adult emerges from the final nymphal stage. 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 dragonflies, the wings of females are shorter and broader than those of males. The legs are rarely used for walking, but are used to catch and hold prey, for perching, and for climbing on plants. Each has two short basal joints, two long joints, and a three-jointed foot, armed with a pair of claws. The long leg joints bear rows of spines, and in males, one row of spines on each front leg is modified to form an "eyebrush", for cleaning the surface of the compound eye.

The abdomen is long and slender and consists of 10 segments. Three terminal appendages are on segment 10; a pair of superiors (claspers) and an inferior. The second and third segments are enlarged, and in males, on the underside of the second segment has a cleft, forming the secondary genitalia consisting of the lamina, hamule, genital lobe, and penis. There are remarkable variations in the presence and the form of the penis and the related structures, the flagellum, cornua, and genital lobes. Sperm is produced at the 9th segment, and is transferred to the secondary genitalia prior to mating. The male holds the female behind the head using a pair of claspers on the terminal segment. In females, the genital opening is on the underside of the eighth segment, and is covered by a simple flap (vulvar lamina) or an ovipositor, depending on species and the method of egg-laying. Dragonflies having simple flaps shed the eggs in water, mostly in flight. Dragonflies having ovipositors use them to puncture soft tissues of plants and place the eggs singly in each puncture they make.

Dragonfly nymphs vary in form with species, and are loosely classed into claspers, sprawlers, hiders, and burrowers. The first instar is known as a prolarva, a relatively inactive stage from which it quickly moults into the more active nymphal form. The general body plan is similar to that of an adult, but the nymph lacks wings and reproductive organs. The lower jaw has a huge, extensible labium, armed with hooks and spines, which is used for catching prey. This labium is folded under the body at rest and struck out at great speed by hydraulic pressure created by the abdominal muscles. Whereas damselfly nymphs have three feathery external gills, dragonfly nymphs have internal gills, located around the fourth and fifth abdominal segments. Water is pumped in and out of the abdomen through an opening at the tip. The naiads of some clubtails (Gomphidae) that burrow into the sediment, have a snorkel-like tube at the end of the abdomen enabling them to draw in clean water while they are buried in mud. Naiads can forcefully expel a jet of water to propel themselves with great rapidity.

Many adult dragonflies have brilliant iridescent or metallic colours produced by structural colouration, making them conspicuous in flight. Their overall colouration is often a combination of yellow, red, brown, and black pigments, with structural colours. Blues are typically created by microstructures in the cuticle that reflect blue light. Greens often combine a structural blue with a yellow pigment. Freshly emerged adults, known as tenerals, are often pale-coloured and obtain their typical colours after a few days, some have their bodies covered with a pale blue, waxy powderiness called pruinosity; it wears off when scraped during mating, leaving darker areas.

Some dragonflies, such as the green darner, Anax junius, have a noniridescent blue that is produced structurally by scatter from arrays of tiny spheres in the endoplasmic reticulum of epidermal cells underneath the cuticle.

The wings of dragonflies are generally clear, apart from the dark veins and pterostigmata. In the chasers (Libellulidae), however, many genera have areas of colour on the wings: for example, groundlings (Brachythemis) have brown bands on all four wings, while some scarlets (Crocothemis) and dropwings (Trithemis) have bright orange patches at the wing bases. Some aeshnids such as the brown hawker (Aeshna grandis) have translucent, pale yellow wings.

Dragonfly nymphs are usually a well-camouflaged blend of dull brown, green, and grey.

Life cycle ¹⁰

Dragonflies are hemimetabolous insects; they do not have a pupal stage and undergo an incomplete metamorphosis with a series of nymphal stages from which the adult emerges. Eggs laid inside plant tissues are usually shaped like grains of rice, while other eggs are the size of a pinhead, ellipsoidal, or nearly spherical. A clutch may have as many as 1500 eggs, and they take about a week to hatch into aquatic nymphs or naiads which moult between six and 15 times (depending on species) as they grow. Most of a dragonfly's life is spent as a nymph, beneath the water's surface. The nymph extends its hinged labium (a toothed mouthpart similar to a lower mandible, which is sometimes termed as a "mask" as it is normally folded and held before the face) that can extend forward and retract rapidly to capture prey such as mosquito larvae, tadpoles, and small fish. They breathe through gills in their rectum, and can rapidly propel themselves by suddenly expelling water through the anus. Some naiads, such as the later stages of Antipodophlebia asthenes, hunt on land.

The larval stage of 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 stops feeding and makes its way to the surface, generally at night. It remains stationary with its head out of the water, while its respiration system adapts to breathing air, then climbs up a reed or other emergent plant, and moults (ecdysis). Anchoring itself firmly in a vertical position with its claws, its skin begins to split at a weak spot behind the head. The adult dragonfly crawls out of its larval skin, the exuvia, arching backwards when all but the tip of its abdomen is free, to allow its exoskeleton to harden. Curling back upwards, it completes its emergence, swallowing air, which plumps out its body, and pumping haemolymph into its wings, which causes them to expand to their full extent.

Dragonflies in temperate areas can be categorized into two groups, an early group and a later one. In any one area, individuals of a particular "spring species" emerge within a few days of each other. The springtime darner (Basiaeschna janata), for example, is suddenly very common in the spring, but disappears a few weeks later and is not seen again until the following year. By contrast, a "summer species" emerges over a period of weeks or months, later in the year. They may be seen on the wing for several months, but this may represent a whole series of individuals, with new adults hatching out as earlier ones complete their lifespans.

Phylogeny ¹⁰

Dragonflies and their relatives are an ancient group. The oldest fossils are of the Protodonata from the 325 Mya Upper Carboniferous of Europe, a group that included the largest insect that ever lived, Meganeuropsis permiana from the Early Permian, with a wingspan around 750 mm (30 in); their fossil record ends with the Permian–Triassic extinction event (about 247 Mya). The Protanisoptera, another ancestral group that lacks certain wing vein characters found in modern Odonata, lived from the Early to Late Permian age until the end Permian event, and are known from fossil wings from current-day United States, Russia, and Australia, suggesting they might have been cosmopolitan in distribution. The forerunners of modern Odonata are included in a clade called the Panodonata, which include the basal Zygoptera (damselflies) and the Anisoptera (true dragonflies). Today, some 3000 species are extant around the world.

The relationships of anisopteran families are not fully resolved as of 2013, but all the families are monophyletic except the Corduliidae; the Gomphidae are a sister taxon to all other Anisoptera, the Austropetaliidae are sister to the Aeshnoidea, and the Chlorogomphidae are sister to a clade that includes the Synthemistidae and Libellulidae. On the cladogram, dashed lines indicate unresolved relationships; English names are given (in parentheses):

Gomphidae (clubtails)

AustropetaliidaeAeshnoidea (hawkers)

Petaluridae (petaltails)

Macromiidae (cruisers)

NeopetaliidaeCordulegastridae (goldenrings)

Libellulidae (skimmers)

"Corduliidae" [not a clade] (emeralds)

Synthemistidae (tigertails)

Chlorogomphidae

Predators and parasites ¹⁰

Although dragonflies are swift and agile fliers, some predators are 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 wasps, too, prey on dragonflies, using them to provision their nests, laying an egg on each captured insect. In the water, various species of ducks and herons eat dragonfly larvae and they are also preyed on by newts, frogs, fish, and water spiders. Amur falcons, which migrate over the Indian Ocean at a period that coincides with the migration of the globe skimmer dragonfly, Pantala flavescens, may actually be feeding on them while on the wing.

Dragonflies are affected by three major groups of parasites: water mites, gregarine protozoa, and trematode flatworms (flukes). Water mites, Hydracarina, can kill smaller dragonfly larvae, and may also be seen on adults. Gregarines infect the gut and may cause blockage and secondary infection. Trematodes are parasites of vertebrates such as frogs, with complex life cycles often involving a period as a stage called a cercaria in a secondary host, a snail. Dragonfly nymphs may swallow cercariae, or these may tunnel through a nymph's body wall; they then enter the gut and form a cyst or metacercaria, which remains in the nymph for the whole of its development. If the nymph is eaten by a frog, the amphibian becomes infected by the adult or fluke stage of the trematode.

Sources and Credits

1. (c) lundbergj, some rights reserved (CC BY-NC)
2. (c) Thomas Bresson, some rights reserved (CC BY), https://www.flickr.com/photos/computerhotline/3920451490/
3. (c) Pablo Leautaud, some rights reserved (CC BY-NC), https://www.flickr.com/photos/pleautaud/3020725607/
4. (c) satish nikam, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/wwwssncomphotos/50765824741/
5. (c) Paul Ritchie, some rights reserved (CC BY-NC-ND), https://www.flickr.com/photos/thelizardwizard/50040734711/
6. (c) Camilo Hdo, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/camilohdo/6205613721/
7. (c) Paul Ritchie, some rights reserved (CC BY-NC-ND), https://www.flickr.com/photos/thelizardwizard/28045968851/
8. (c) Juan Carrasco, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/36113253@N06/5996101298/
9. Adapted by Greenpigeonsnaturelk from a work by (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Dragonfly
10. (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Dragonfly

More Info

• iNat taxon page
• Atlas of Living Australia
• Biodiversity Heritage Library
• BOLD Systems BIN search
• Global Biodiversity Information Facility (GBIF)
• Rowe (1987) Dragonflies of New Zealand

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