Dragonfly

• 1. Introduction

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  Each compound eye of an adult dragonfly holds nearly 24,000 tiny lenses called ommatidia, and the two eyes together cover most of the insect's head. Few animals are built so completely around sight. The dragonfly belongs to the infraorder Anisoptera, part of the order Odonata, and it is a hunter from the day it hatches.

  An adult has four strong, transparent wings and a long, narrow body. Many species flash with iridescent or metallic colours that come from the physical structure of the body surface rather than from pigment alone. That makes them hard to miss as they patrol the edge of a pond.

  How does a creature this complicated trace back across hundreds of millions of years? What does it do with the long stretch of its life that is spent out of the air? How does it fly, hunt and court a mate? And why do people in Japan and Europe read the same insect in such different ways?

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• 2. Ancient Lineage, Wide Range

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  Meganeuropsis permiana, from the Early Permian, spread its wings across about 750 millimetres, which makes it one of the largest insects that ever lived. It belonged to the Meganisoptera, also called griffenflies, a group known from Upper Carboniferous rocks in Europe about 325 million years ago. They were only distant relatives of today's dragonflies, though the body plan is similar. Another ancestral group, the Protanisoptera, lived in the Permian and lacked certain wing-vein features found in modern odonates.

  The true dragonflies show up much later, during the Toarcian age of the Early Jurassic. Their crown group developed in the Middle Jurassic. Scientists place them among the Palaeoptera, a name meaning ancient-winged, because like the griffenflies they cannot tuck their wings back against the body as many modern insects do.

  The name of the infraorder records one detail of the wings. Anisoptera comes from the Greek for unequal wing, because the hindwings are broader than the forewings.

  By 2010, researchers had described 3,012 species, sorted into 348 genera and 11 families. Dragonflies live on every continent except Antarctica. Most species are tropical, and far fewer live in temperate regions.

  The globe skimmer, Pantala flavescens, is probably the most widespread, turning up on every continent in the warmer regions. Emperors of the genus Anax range from Newfoundland in the north to Bahia Blanca in Argentina in the south, and across Europe to central Asia. Rhionaeschna variegata lives on Tierra del Fuego, the southernmost known dragonfly or damselfly anywhere.

  Altitude and cold set other limits. A species of Aeshna in the Pamirs reaches about 3700 metres, the highest known. Iceland has no native dragonflies, but strong winds occasionally carry in a visitor such as Hemianax ephippiger from North Africa. In the Mojave Desert, some dragonflies stay active in shade temperatures between 18 and 45 degrees Celsius.

  The northern record belongs to the treeline emerald, which lives in northern Alaska, inside the Arctic Circle.

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• 3. Years Beneath the Surface

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  In large species, the nymph stage can last up to five years, while in smaller species it runs between two months and three years. Most of a dragonfly's life passes underwater, in a form that looks like a wingless version of the adult. The young are called nymphs or naiads.

  A clutch can hold as many as 1500 eggs, and they take about a week to hatch. Eggs pushed inside plant tissue are often shaped like grains of rice. Others are the size of a pinhead and nearly round. The hatchling nymph moults between six and 15 times as it grows, depending on the species.

  The nymph's signature weapon is its labium, a hinged, toothed lower mouthpart sometimes called a mask. It stays folded under the head until prey comes close. Then hydraulic pressure from the abdominal muscles shoots it forward at great speed. Targets include mosquito larvae, tadpoles and small fish.

  Only dragonfly nymphs have internal gills, housed in a branchial chamber around the fourth and fifth abdominal segments. Water is pumped in and out through an opening at the tip of the abdomen. A sudden jet of water also drives the nymph forward. Burrowing clubtails in the family Gomphidae add a snorkel-like tube at the end of the abdomen, so they can draw clean water while buried in mud.

  Some nymphs leave the water altogether. Cordulegaster bidentata sometimes hunts small arthropods on the ground at night. Certain Anax nymphs have been seen leaping out of the water to kill full-grown tree frogs. Phanogomphus militaris may even act as a parasite, feeding on the gills of gravid freshwater mussels.

  When a naiad is ready to change, it stops feeding and rises to the surface, usually at night. Its breathing system adapts to air while its head stays out of the water. It then climbs a reed, anchors its claws, and splits its skin behind the head. The adult crawls free, leaving the empty shell called the exuvia. It swallows air to plump out its body and pumps haemolymph into its wings until they open fully.

  After all those years below, the winged stage is brief. Most species live about five weeks or less, some only a few days, and the longest adult lives reach around 10 weeks.

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• 4. Wings and Heat

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  An adult dragonfly can move in six directions: up, down, forward, backward, left and right. It can also change course suddenly, and some species migrate across the sea. Documented accelerations reach 4 G in a straight line and 9 G in sharp turns while chasing prey.

  Unlike most insect families, dragonflies power their wings directly, with the flight muscles attached at the wing bases. They use four styles of flight. In counter-stroking, the forewings beat 180 degrees out of phase with the hindwings, for hovering and slow flight. Phased-stroking puts the hindwings 90 degrees ahead of the forewings, which gives more thrust and less lift for fast flight. Synchronised-stroking beats all four wings together to maximise thrust during quick turns. Gliding with the wings held out covers short breaks between bursts of power.

  Gliding has one unusual use during mating in darters. The female sometimes simply glides while the male beats his wings and pulls the pair along.

  Lift comes from more than one mechanism. At different moments a dragonfly uses classical lift like an aircraft wing, supercritical lift with very short strokes to avoid stalling, and the creation and shedding of vortices. The wings flex and twist on every beat.

  An old claim says the southern giant darner can reach 97 kilometres per hour. The source calls such claims old and unreliable, and the greatest reliable speed records belong to other insects. Large hawkers reach a maximum of 36 to 54 kilometres per hour and cruise at about 16. A dragonfly can cover 100 body lengths per second going forward and three going backward.

  Being cold-blooded, a dragonfly needs the sun to warm its flight muscles. Early in the morning it may perch vertically with its wings outstretched, and at midday it may choose a horizontal stance. Some larger species, including the long-distance migrant green darner, Anax junius, whirr their wings before dawn so the vibration heats the muscles for an early start.

  Overheating is the opposite problem. Some species use the obelisk posture, a kind of handstand with the abdomen pointing at the sun to cut exposure. On a hot day, a dragonfly may skim over water and touch the surface, often three times in quick succession.

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• 5. Eyes and Prey

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  Dragonfly vision is thought to resemble slow motion for humans. A dragonfly sees around 200 images per second, compared with the lower rate of people. It can see through 360 degrees, and nearly 80 percent of its brain is devoted to sight.

  Adults hunt on the wing and are almost exclusively carnivorous. Their menu runs from midges and mosquitoes to butterflies, moths, damselflies and smaller dragonflies. A large victim is bitten on the head, carried to a perch, stripped of its wings and eaten, usually head first. A dragonfly may consume as much as a fifth of its body weight in a day.

  A study of Libellula cyanea chasing Drosophila melanogaster fruit flies in a netted outdoor enclosure suggested a remarkable success rate. The dragonflies caught up to 95 percent of the prey they pursued. Success rose with prey density, while erratic flight let more fruit flies escape when they were scarce.

  Male Australian emperors, Hemianax papuensis, use a trick in their territorial fights. The attacker picks a path that keeps him on a line between the rival and the start of his attack. To the rival he looms larger but never seems to move. Researchers found motion camouflage in six of 15 encounters.

  Dragonflies face their own enemies. Falcons such as the American kestrel, the merlin and the hobby are fast enough to catch them, and some wasps provision nests with captured adults. Nymphs are eaten by ducks, herons, newts, frogs, fish and water spiders.

  Parasites add a second threat. Water mites can kill smaller nymphs, and gregarine protozoa infect the gut. Trematode flukes reach a nymph through a snail host and form a cyst in its body. If a frog eats that nymph, the frog becomes infected with the adult fluke.

  One predator may hunt in mid-air on a vast scale. Amur falcons cross the Indian Ocean at the same time that globe skimmers migrate, so they may be feeding on them while still flying.

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• 6. Territory and Courtship

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  The blue dasher, Pachydiplax longipennis, jostles rivals for the right to a single perch. A good perch gives a view over an insect-rich feeding ground, and males of many species defend one fiercely. Others guard a stretch of sunlit shallow water, a special plant or a preferred spot for laying eggs. A territory may be held for a few minutes or for several hours.

  Some species signal ownership with striking colours. The common whitetail, Plathemis lydia, dashes at intruders with its white abdomen raised like a flag. Black saddlebags, Tramea lacerata, appear to notice landmarks that help mark the boundaries. Other species fight in aerial dogfights and high-speed chases.

  A female must mate with the territory holder before she lays her eggs. Males can harass females so heavily that it cuts into their feeding. Some dimorphic species have evolved female forms that resemble males, and some females feign death to escape attention. In Hine's emerald, Somatochlora hineana, males use wetlands while females stay in dry meadows and visit the wetlands only to lay eggs or find mates.

  The mating itself is precisely choreographed. The male first moves a packet of sperm from his opening on segment 9 to the secondary genitalia on segments 2 and 3. He then grips the female behind the head with the claspers at the tip of his abdomen, and the pair flies in tandem. She curls her abdomen forward to collect the sperm, forming the heart or wheel posture, a pairing also described as in cop.

  Sperm competition then takes over. Sperm can stay viable in the female's spermatheca for at least 12 days in some species. The male uses his genital structures to compress or scrape out the sperm of earlier matings, and this takes up much of the time the pair spends in the heart posture. Afterwards he guards her while she lays eggs, and he may help pull her out of the water if she submerges.

  Egg-laying takes two forms. Females in families such as Aeshnidae and Petaluridae use a sharp-edged ovipositor to slit a stem or leaf. Skimmers and emeralds tap the water with the abdomen, shake the eggs out in flight, or place them on vegetation.

  In a few species the eggs are laid on emergent plants above the water, and development waits until those plants have withered and sunk.

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• 7. Omens and Amulets

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  A blue-glazed faience dragonfly amulet found by Flinders Petrie at Lahun dates to the Late Middle Kingdom of ancient Egypt. Dragonflies have appeared on human artefacts ever since. For the Navajo they symbolise pure water. Stylised in a double-barred cross design, they are a common motif in Zuni pottery, Hopi rock art and Pueblo necklaces.

  In Japan, dragonflies are a seasonal symbol of autumn and stand for rebirth, courage, strength and happiness. Japanese children catch large ones with a hair that has a small pebble tied to each end. They throw it into the air, the dragonfly mistakes the pebbles for prey, and the weight drags it down.

  One of Japan's classical names, Akitsushima, can be read as Dragonfly Island, since akitsu is an old word for the insect. A legend credits Japan's mythical founder, Emperor Jimmu, with the link: a mosquito bit him, and a dragonfly ate the mosquito.

  Europe saw the insect very differently. English names such as horse-stinger, devil's darning needle and ear cutter link it with evil and injury, building on the false belief that dragonflies bite or sting people. Swedish folklore says the devil uses them to weigh souls. The Norwegian name Øyenstikker means eye-poker, and the Portuguese tira-olhos means eyes-snatcher.

  Snakes crop up in many names, from the Welsh gwas-y-neidr, adder's servant, to the Southern United States terms snake doctor and snake feeder. Those refer to a folk belief that dragonflies follow snakes around and stitch them back together after an injury.

  People have also used the insect. In China and Japan it has appeared in traditional medicine. In Indonesia, adults are caught on poles smeared with birdlime and fried in oil as a delicacy.

  The watercolourist Moses Harris, who lived from 1731 to 1785, published the first scientific descriptions of several Odonata in 1780, including the banded demoiselle. He was the first English artist to draw dragonflies accurately enough to identify the species.

  Lafcadio Hearn wrote in his 1901 book A Japanese Miscellany that Japanese poets had made dragonfly haiku almost as numerous as the insects themselves in early autumn. Matsuo Bashō, who lived from 1644 to 1694, tied the insect to autumn in a haiku about a crimson pepper pod with two pairs of wings added. Hori Bakusui, from 1718 to 1783, called the red dragonfly dyed with the colour of autumnal days.

  In his 1842 poem The Two Voices, Lord Tennyson pictured a dragonfly leaving its old husk in "sapphire mail". In the 1937 book Down the River, H. E. Bates watched small sapphire dragonflies make "a continuous play of blue gauze over the snowy flowers above the sun-glassy water."

  The insect also went into design. Dragonflies are common in Art Nouveau jewellery and appear on fabrics and home furnishings. The Bristol motorcycle maker Douglas gave the name Dragonfly to its innovative postwar 350-cc flat-twin.

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• 8. Fragile Future

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  In 1997, the International Union for Conservation of Nature set up a status survey and conservation action plan for dragonflies. It proposes protected areas around the world, managed to give dragonflies suitable habitat. Beyond those areas, it urges changes to forestry, agricultural and industrial practices. It also calls for more research, pollution control and public education about biodiversity.

  Most odonatologists live in temperate areas, and the dragonflies of North America and Europe are well studied. The majority of species live in the tropics and have barely been examined. With rainforest being destroyed, many may become extinct before anyone names them.

  Forest clearance is the greatest cause of decline, because streams and pools dry up or clog with silt. Hydroelectric dams, drained lowland, pollution and introduced alien species have all reduced habitat as well. Dragonflies are steadily declining in Africa, which makes them a conservation priority there.

  Japan shows the pattern clearly. Over 60 percent of its wetlands were lost in the 20th century, so its dragonflies now rely largely on rice fields, ponds and creeks. In the rice fields they feed on pest insects and act as natural pest control.

  Their long lifespan and low population density make dragonflies vulnerable to disturbance. Vehicles on roads built near wetlands can kill them, and species that fly low and slow may be most at risk.

  Shiny surfaces create a different trap. Polarised light from polished gravestones, solar panels and cars looks like water, so dragonflies gather there and try to lay eggs. This can affect local populations, and methods to make structures such as solar panels less attractive are under experimentation.

  At the same time, engineers have studied the insect for its steering. A dragonfly was genetically modified with light-sensitive steering neurons in its nerve cord, creating a cyborg-like system called DragonflEye. A small backpack over the thorax carries sensors, a computer chip and a solar panel. Flexible light-pipes called optrodes send steering commands into the nerve cord. Its developers describe the result as a "micro-aerial vehicle that's smaller, lighter and stealthier than anything else that's manmade".

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