Dragonfly


A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of dragonflies are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threatens dragonfly populations around the world. Adult dragonflies are characterised by a pair of large, multifaceted, compound eyes, two pairs of strong, transparent wings, sometimes with coloured patches, and an elongated body. Many dragonflies have brilliant iridescent or metallic colours produced by structural coloration, making them conspicuous in flight. An adult dragonfly's compound eyes have nearly 24,000 ommatidia each.
Dragonflies can be mistaken for the closely related damselflies, which make up the other odonatan infraorder and are similar in body plan, though usually lighter in build; however, the wings of most dragonflies are held flat and away from the body, while damselflies hold their wings folded at rest, along or above the abdomen. Dragonflies are agile fliers, while damselflies have a weaker, fluttery flight. Dragonflies make use of motion camouflage when attacking prey or rivals.
Dragonflies are predatory insects, both in their aquatic nymphal stage and as adults. In some species, the nymphal stage lasts up to five years, and the adult stage may be as long as 10 weeks, but most species have an adult lifespan in the order of five weeks or less, and some survive for only a few days. They are fast, agile fliers capable of highly accurate aerial ambush, sometimes migrating across oceans, and often live near water. They have a uniquely complex mode of reproduction involving indirect insemination, delayed fertilisation, and sperm competition. During mating, the male grasps the female at the back of the head, and the female curls her abdomen under her body to pick up sperm from the male's secondary genitalia at the front of his abdomen, forming the "heart" or "wheel" posture.
Fossils of very large dragonfly-like insects, sometimes called griffinflies, are found from 325 million years ago in Upper Carboniferous rocks; these had wingspans up to about, though they were only distant relatives. True dragonflies first appeared during the Early Jurassic.
Dragonflies are represented in human culture on artefacts such as pottery, rock paintings, statues, and Art Nouveau jewellery. They are used in traditional medicine in Japan and China, and caught for food in Indonesia. They are symbols of courage, strength, and happiness in Japan, but seen as sinister in European folklore. Their bright colours and agile flight are admired in the poetry of Lord Tennyson and the prose of H. E. Bates.

Etymology

The infraorder Anisoptera comes from Greek ἄνισος anisos "unequal" and πτερόν pteron "wing" because dragonflies' hindwings are broader than their forewings.

Evolution

Dragonflies and their relatives are similar in structure to an ancient group, the Meganisoptera or griffinflies, from the 325 Mya Upper Carboniferous of Europe. This group includes one of the largest insects that ever lived, Meganeuropsis permiana from the Early Permian, which had a wingspan of around. The Protanisoptera, another ancestral group that lacks certain wing-vein characters found in modern Odonata, lived in the Permian.
Anisoptera first appeared during the Toarcian age of the Early Jurassic, and the crown group developed in the Middle Jurassic. They retain some traits of their distant predecessors, and are in a group known as the Palaeoptera, meaning 'ancient-winged'. Like the gigantic griffinflies, dragonflies lack the ability to fold their wings up against their bodies in the way that many modern insects can, although some evolved their own different way to do so. The forerunners of modern Odonata are included in a clade called the Panodonata, which include the basal Zygoptera and the Anisoptera. Today, some 3,000 species are extant around the world.
The relationships of anisopteran families are not fully resolved as of 2021, but all the families are monophyletic except the Corduliidae, and the Aeshnoidea are sister to the remaining families:

Distribution and diversity

About 3,012 species of dragonflies were known in 2010; these are classified into in. The distribution of diversity within the biogeographical regions is summarized below.
FamilyIndomalayaNeotropicalAustralasianAfrotropicalPalaearcticNearcticPacificWorld
Aeshnidae1491297844584013456
Austropetaliidae7411
Petaluridae161210
Gomphidae36427742152127101980
Chlorogomphidae46547
Cordulegastridae2311846
Neopetaliidae11
Corduliidae2320336185112154
Libellulidae192354184251120105311037
Macromiidae5021737710125
Synthemistidae37946
Incertae sedis37242115299

File:PantalaFlavescensTalakaveri.jpg|thumb|upright|An aggregation of globe skimmers, Pantala flavescens, during migration
Dragonflies live on every continent except Antarctica. In contrast to the damselflies, which tend to have restricted distributions, some genera and species are spread across continents. For example, the blue-eyed darner Rhionaeschna multicolor lives across North America, and in Central America; emperors Anax live 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. The globe skimmer Pantala flavescens is probably the most cosmopolitan, occurring on all continents in the warmer regions. Most Anisoptera species are tropical, with far fewer species in temperate regions. Rhionaeschna variegata, with populations on Tierra del Fuego, is the most southern known species of dragonfly or damselfly in the world.
Some dragonflies, including libellulids and aeshnids, live in desert pools. In the Mojave Desert they are active in shade temperatures between ; these dragonflies survive body temperatures fatal to individuals not adapted to that environment, even those of the same species.
Dragonflies live from sea level up to the mountains, decreasing in species diversity with altitude. Their altitudinal limit is about 3700 m, represented by a species of Aeshna in the Pamirs.
Dragonflies become scarce at higher latitudes. They are not native to Iceland, but individuals are occasionally swept in by strong winds, including a Hemianax ephippiger native to North Africa, and an unidentified darter species. In Kamchatka, only a few species of dragonfly including the northern emerald Somatochlora arctica and some aeshnids such as Aeshna subarctica are found, possibly because of the low temperature of the lakes there. The treeline emerald also lives in northern Alaska, within the Arctic Circle, making it the most northerly of all dragonflies.

Description

Anatomy

Dragonflies are heavy-bodied, strong-flying insects that hold their wings horizontally both in flight and at rest. By contrast, damselflies 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. 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 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 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.
File:A migrant hawker dragonfly.jpg|thumb|upright=0.7|left|Migrant hawker, Aeshna mixta, has the long, slender abdomen of aeshnid dragonflies.
The abdomen is long and slender and consists of 10 segments. Three terminal appendages are on segment 10; a pair of superiors and an inferior. The second and third segments are enlarged, and in males, 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 ninth 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 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. Both damselfly and dragonfly nymphs ventilate the rectum, but just some damselfly nymphs have a rectal epithelium that is rich in trachea, relying mostly on three feathery external gills as their major source of respiration. Only dragonfly nymphs have internal gills, called a branchial chamber, located around the fourth and fifth abdominal segments. These internal gills consist originally of six longitudinal folds, each side supported by cross-folds. But this system has been modified in several families. Water is pumped in and out of the abdomen through an opening at the tip. The naiads of some clubtails 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.