In this episode of the I Can't Sleep Podcast, fall asleep learning about Dragonflies. I learned a lot more about these insects than I probably ever wanted to know, but the information is a snooze-fest. Happy sleeping!
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Transcript
Welcome to the I Can't Sleep Podcast, Where I read random articles from across the web to bore you to sleep with my soothing voice. I'm your host, Benjamin Boster. Today's episode is from a Wikipedia article titled, Dragonfly. A dragonfly is a flying insect belonging to the infra-order 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 characterized by a pair of large, Multifaceted compound eyes, Two pairs of strong, Transparent wings, Sometimes with colored patches, And an elongated body. Many dragonflies have brilliant, Iridescent or metallic colors, Produced by structural coloration, Making them conspicuous in flight. Adult dragonflies' compound eyes have nearly 24, 000 ommatidia each. Dragonflies can be mistaken for the closely related damselflies, Which make up the other odonton infra-order 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 flyers, While damselflies have a weaker, Fluttery flight. Dragonflies are predatory insects both in their aquatic nymphal stage, Also known as naiads, And as adults. In some species, The nymphal stage lasts up to five years, And the adult stage may be as long as ten 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 flyers, Capable of highly accurate aerial ambush, Sometimes migrating across oceans and often live near water. 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 750 millimeters, Or 30 inches, Though they were only distant relatives, Not true dragonflies. Dragonflies are represented in human culture on artifacts such as pottery, Rock paintings, Statues, And Art Nouveau jewelry. 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 colors and agile flight are admired in the poetry of Lord Tennyson and the prose of H. E. Bates. The infra-order Anisoptera comes from Greek anisos, Unequal, And teron, Wing, Because dragonflies' hind wings are broader than their forewings. Dragonflies and their relatives are similar in structure to an ancient group, The Meganisoptera or griffinflies, From the 325 million years ago upper Carboniferous of Europe, A group that included the largest insect that ever lived, Meganuropsis permiana, From the early Permian, With a wingspan about 750 millimeters. The Protanisoptera, Another ancestral group that lacks certain wing-vein characters found in modern Odonata, Lived in the Permian. Modern dragonflies do retain some traits of their distant predecessors and are in a group known as the Palaeoptera, Ancient-winged. They, Like the gigantic griffinflies, Lack the ability to fold their wings up against their bodies in the way modern insects do, Although some evolved their own different way to do so. The forerunners of modern Odonata are included in a clade called the Panadonata, Which include the basal Zygoptera, Damselflies, And the Anisoptera, True dragonflies. 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 for Chordylidae, And the Australopithelidae are sister to the Echinoide. About 3, 012 species of dragonflies were known in 2010. These are classified in 348 genera in 11 families. 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 lives all across North America and in Central America. Emperors Annex live throughout the Americas for 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 is probably the most widespread dragonfly species in the world. It is cosmopolitan, Occurring on all continents in the warmer regions. Most Anisoptera species are tropical, With far fewer species in temperate regions. Some dragonflies, Including libellulids and hyschnids, Live in desert pools, For example in the Mojave Desert, Where they are active in shade temperatures between 18 and 45 degrees Celsius. These insects were able to survive body temperatures above the thermal death point of insects of the same species in cooler places. Dragonflies live from sea level up to the mountains, Decreasing in species diversity with altitude. Their altitudinal limit is about 3, 700 meters, Represented by a species of ishna in the Pamirs. Dragonflies became scarcer at higher altitudes. They are not native to Iceland, But individuals are occasionally swept in by strong winds, Including one native to North Africa, And an unidentified darter species. In Kamchatka, Only a few species of dragonfly, Including the treeline emerald and some ishnids, 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. 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. Echna interrupta has 22, 650 ommatidia of two varying sizes, 4, 500 being large. The facets facing downward tend to be smaller. Petulor gigante has 23, 890 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 ostoli. 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 hind wings are broader than the forewings and the venation is different at the base. The veins carry hemolymph, 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. A 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 joint bears rows of spines, And in males one row of spines on each front leg is modified to form an eye brush for cleaning the surface of the compound eye. The abdomen is long and slender and consists of ten segments. Three terminal appendages are on segment 10, A pair of superiors, Claspers, And an inferior. Dragonfly nymphs vary in form with species, And are loosely classed into claspers, Sprawlers, Biters, And burrowers. The first instar is known as a prolarva, A relatively inactive stage from which it quickly molts 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 into the body at rest, And struck out at great speed by hydraulic pressure created by the abdominal muscles. Only dragonfly nymphs have internal gills called a bronchial 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 club tails 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 colors produced by structural coloration, Making them conspicuous and flight. Their overall coloration is often a combination of yellow, Red, Brown, And black pigments with structural colors. 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 tenerels are often pale and obtain their typical colors after a few days. Some have their bodies covered with a pale blue waxy powderiness called prunosity. Some dragonflies, Such as the green darner, Have a non-iridescent 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, However, Many genera have areas of color on the wings. For example, Groundlings have brown bands on all four wings, While some scarlets and dropwings have bright orange patches at the wing bases. Some eschnids, Such as the brown hawker, Have translucent pale yellow wings. Dragonfly nymphs are usually a well-camouflaged blend of dull brown, Green, And gray. Dragonflies and damselflies are predatory both in the aquatic nymphal and adult stages. Nymphs feed on a range of freshwater invertebrates, And larger ones can prey on tadpoles and small fish. One species, Gomphos militaris, Even live as parasite, Feeding on the gills of gravid mussels. Adults capture insect prey in the air, Making use of their acute vision and highly controlled flight. Adult males vigorously defend territories near water. These areas provide suitable habitat for nymphs to develop, And for females to lay their eggs. Swarms of feeding adults aggregate to prey on swarming prey, Such as emerging flying ants or termites. Dragonflies as a group occupy a considerable variety of habitats, But many species, And some families, Have their own specific environmental requirements. Some species prefer flowing waters, While others prefer standing water. For example, The Gomphidae, Club tails, Live in running water, And the skimmers live in still water. Some species live in temporary water pools and are capable of tolerating changes in water level, Desiccation, And the resulting variations in temperature. But some genera, Such as dargers, Have eggs and nymphs that can resist drought, And are stimulated to grow rapidly in warm, Shallow pools, Also often benefiting from the absence of predators there. Vegetation and its characteristics, Including submerged, Floating, Emergent, Or waterside, Are also important. Adults may require emergent or waterside plants to use as perches. Others may need specific submerged or floating plants on which to lay eggs. Requirements may be highly specific, As in greenhawker, Which lives in swamps with the water soldier. The chemistry of the water, Including its trophic status, Degree of enrichment with nutrients, And pH, Can also affect its use by dragonflies. Most species need moderate conditions, Not too eutrophic, Not too acidic. A few species, Such as black darter and four-spotted chaser, Prefer acidic waters, Such as peat bogs, While others, Such as scarce chaser, Need slow-moving, Eutrophic waters with reeds or similar waterside plants. 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. Males of many species, Such as the blue dasher, Jostle other dragonflies to maintain the right to alight there. Defending a breeding territory is common among male dragonflies, Especially in species that congregate around pools. The territory contains desirable features, Such as a sunlit stretch of shallow water, A special plant species, Or the preferred substrate for egg-laying. The territory may be small or large, Depending on its quality, Time of day, And the number of competitors, And may be held for a few minutes or several hours. Dragonflies, Including black saddlebags, May notice landmarks that assist in defining the boundaries of the territory. Landmarks may reduce the costs of territory establishment, Or might serve as a spatial reference. Some dragonflies signal ownership with striking colors in the face, Abdomen, Legs, Or wings. The common whitetail dashes towards an intruder, Holding its white abdomen aloft like a flag. Other dragonflies engage in aerial dogfights or high-speed chases. A female must mate with the territory holder before laying her eggs. There is also conflict between the males and females. Females may sometimes be harassed by males to the extent that it affects their normal activities, Including foraging, And in some dimorphic species, Females have evolved multiple forms, With some forms appearing deceptively like males. In some species, Females have evolved behavioral responses, Such as feigning death, To escape the attention of males. Similarly, Selection of habitat by adult dragonflies is not random, And terrestrial habitat patches may be held for up to three months. A species tightly linked to its birth site utilizes a foraging area that is several orders of magnitude larger than the birth site. 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 1, 500 eggs, And they take about a week to hatch into aquatic nymphs, Or nyads, Which molt between 6 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. Some nyads hunt on land. The nymph stage of dragonflies lasts up to five years in large species, And between two months and three years in smaller species. When the nyad 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 molts, Anchoring itself firmly in a vertical position with its claws, Its exoskeleton begins to split at a weak spot behind the head. The adult dragonfly crawls out of its nymph exoskeleton, 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 hemolymph 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, 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 early ones complete their lifespans. Dragonflies are powerful and agile flyers, Capable of migrating across the sea, Moving in any direction, And changing direction suddenly. In flight, The adult dragonfly can propel itself in six directions, Upward, Downward, Forward, Backward, To left, And to right. They have four different styles of flight. 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. Synchronized stroking, With forewings and hindwings beating together, Is used when changing directions rapidly as it maximizes 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 certain dragonflies, Such as Darners, When in-crop with a male, The female sometimes simply glides while the male pulls the pair along by beating his wings. The wings are powered directly, Unlike most families of insects, 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, And creating and shedding vortices. 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. Old and unreliable claims are made that dragonflies, Such as the southern giant darner, Can fly up to 97 kilometers an hour. 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 36 to 54 kilometers an hour, With average cruising speed of about 16 kilometers per hour. Dragonflies can travel at 100 body lengths per second in forward flight, And three lengths per second backwards. In high-speed territorial battles between male Australian emperors, The fighting dragonflies adjust their flight paths to appear stationary to their rivals, Minimizing the chance of being detected as they approach. To achieve the effect, The attacking dragonfly flies towards his rival, Choosing his path to remain on a line between the rival and the start of his attack path. The attacker thus looms larger as he closes on the rival, But does not otherwise appear to move. Researchers found that 6 of 15 encounters involved motion camouflage. The flight muscles need to be kept at a suitable temperature for the dragonfly to be able to fly. Being cold-blooded, They can raise their temperature by basking in the sun. Early in the morning, They may choose to perch in a vertical position with the wings outstretched, While in the middle of the day, A horizontal stance may be chosen. Another method of warming up used by some larger dragonflies is wing whirring, A rapid vibration of the wings that causes heat to be generated in the flight muscles. The green darner is known for his long distance migrations, And often resorts to wing whirring before dawn to enable it to make an early start. Becoming too hot is another hazard, And a sunny or shady position for perching can be selected according to the ambient temperature. Some species have dark patches on the wings which can provide shade for the body, And a few use the obelisk posture to avoid overheating. This behavior involves doing a handstand, Perching with the body raised and the abdomen pointing towards the sun, Thus minimizing the amount of solar radiation received. On a hot day, Dragonflies sometimes adjust their body temperature by skimming over a water surface and briefly touching it, Often three times in quick succession. This may also help to avoid desiccation. Adult dragonflies hunt on the wing using their exceptionally acute eyesight and strong agile flight. They are almost exclusively carnivorous, Eating a wide variety of insects, Ranging from small midges and mosquitoes, To butterflies, Moths, Damselflies, And smaller dragonflies. A large prey item is subdued by being bitten on the head and is carried by the legs to a perch. A dragonfly may consume as much as a fifth of its body weight in prey per day. Dragonflies are also some of the insect world's most efficient hunters, Catching up to 95% of the prey they pursue. The nymphs are voracious predators, Eating most living things that are smaller than they are. Their staple diet is mostly bloodworms and other insect larvae, But they also feed on tadpoles and small fish. A few species, Especially those that live in temporary waters, Are likely to leave the water to feed. Dragonfly vision is thought to be like slow motion for humans. Dragonflies see faster than humans do. They see around 200 images per second. A dragonfly can see in 360 degrees, And nearly 80% of the insect's brain is dedicated to its sight. Although dragonflies are swift and agile flyers, 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 nymphs, 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, May actually be feeding on them while on the wing. Dragonflies are affected by three groups of parasites, Water mites, Gregarine protozoa, And termitode flatworms. Water mites can kill smaller dragonfly nymphs and may also be seen on adults. Gregarines infect the gut and may cause blockage and secondary infection. Termitodes 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. Most odontologists live in temperate areas, And the dragonflies of North America and Europe have been the subject of much research. However, The majority of species live in tropical areas and have been little studied. With the destruction of rainforest habitats, Many of these species are in danger of becoming extinct before they have even been named. The greatest cause of decline is forest clearance, With the consequent drying up of streams and pools which become clogged with silt. The damming of rivers for hydroelectric schemes and the drainage of low-lying land has reduced suitable habitat, As has pollution and the introduction of alien species. In 1997, The International Union of Conservation of Nature set up a status survey and conservation action plan for dragonflies. This proposes the establishment of protected areas around the world and the management of these areas to provide suitable habitat for dragonflies. Outside these areas, Encouragement should be given to modify forestry, Agriculture, And industrial practices to enhance conservation. At the same time, More research into dragonflies needs to be done. Consideration should be given to pollution control, And the public should be educated about the importance of biodiversity. Habitat degradation has reduced dragonfly populations across the world, For example in Japan. Over 60% of Japan's wetlands were lost in the 20th century, So its dragonflies now depend largely on rice fields, Ponds, And creeks. Dragonflies feed on pest insects and 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 it vulnerable to disturbance, Such as from collisions with vehicles on roads built near wetlands. Species that fly low and slow may be more at risk. Dragonflies are attracted to shiny surfaces that produce polarization, Which they can mistake for water, And they have been known to aggregate close to polished gravestones, Solar panels, Automobiles, And other such structures on which they attempt to lay eggs. These can have a local impact on dragonfly populations. Methods of reducing the attractiveness of structures, Such as solar panels, Are under experimentation.
