Suborder Anisoptera
dragonflies

Dragonflies (Anisoptera) belong to the order Odonata. Odonata means "toothed,” and refers to the toothed mandibles of these carnivorous insects. There are two suborders within Odonata: Anisoptera and Zygoptera (damselflies). Currently there are eight recognized families and 124 genera of dragonflies, but this may change as many species are still undescribed. (Corbet, 1980; McGavin, 2001; von Ellenrieder, 2003)

Dragonflies are carnivorous insects with a lifestyle that is closely tied to freshwater aquatic habitats. Eggs are laid in or near water, and immature dragonflies have adaptations such as gills that allow them to spend the first few months to several years as aquatic nymphs, preying on other small aquatic organisms. As adults, dragonflies are agile fliers that prey on other insects. (Corbet, 1999)

Some species of dragonfly are territorial, with males that defend territories of high-quality egg-laying sites. In these species, males are often brightly colored and use aerial displays both to ward off other males and to lure females. Other species are not territorial. In these species, males do not attempt to attract females, but rather seize them as they pass by. Whichever mating strategy is employed, sperm competition is an important factor in determining how many eggs a male dragonfly will fertilize. In order to ensure that their own sperm will fertilized the majority of eggs, males remove previously deposited sperm from the sperm storage organ of the female before depositing their own sperm. Most males further ensure their paternity by guarding the female as she lays her eggs. (McGavin, 2001; Silsby, 2001; von Ellenrieder, 2003)

Dragonflies are found worldwide, except in polar regions. The greatest diversity of dragonflies is found in tropical areas. (Silsby, 2001; von Ellenrieder, 2003)

Dragonflies are nearly always found near freshwater. They lay their eggs in or close to water, and the immature dragonflies, called nymphs, are fully aquatic. Dragonflies are most abundant and diverse in and around slow-moving freshwater with submerged and emergent vegetation, for example small streams and ponds. However, they can also be found in many other freshwater habitats, including small water-filled holes, vernal pools, ditches, dikes, marshes, swamps, rivers, waterfalls and lakes. Adult dragonflies spend the majority of their time near water, but may travel miles away while hunting. They are agile fliers, and tend to hunt in open areas rather than amongst thick trees or other vegetation. (Borror, Triplehorn, and Johnson, 1989; McGavin, 2001; Silsby, 2001)

Anisoptera (dragonflies) is one of two suborders in the order Odonata, the other suborder being Zygoptera (damselflies). The division between Anisoptera and Zygoptera is widely accepted and supported by a number of synapomorphies. However, the relationship between Anisoptera and Zygoptera is unclear. In some phylogenies, Anisoptera and Zygoptera are sister groups. In others, Anisoptera (dragonflies) evolved from within the Zygoptera. (Corbet, 1999; Gullan and Cranston, 2000; McGavin, 2001)

Odonata was previously considered to comprise an additional suborder, Ansiozygoptera, which contained a single genus, Epiophlebia. Most scholars now lump Epiophlebia within Anisoptera. However, this more recent classification is not universally accepted. (Corbet, 1999; Gullan and Cranston, 2000; McGavin, 2001)

Characteristics of dragonflies that are shared with damselflies include biting mouthparts, spiny legs, two sets of long membranous wings, an elongate, 10-segmented abdomen, secondary genitalia in males, and a fully aquatic nymphal stage with gills and an enlarged, hinged labium. (Needham, Westfall, and May, 2000; Silsby, 2001)

Distinguishing characteristics of dragonflies include large eyes that touch or nearly touch on top of a rounded head, a broad base on the hind pair of wings, wings held horizontally when at rest, division of the disoidal cell of the wing into a “triangle” and “hypertriangle,” males having three rather than four anal appendages, females having a non-functional or vestigial ovipositor and larvae that breath through three caudal gills in the abdominal cavity. (Silsby, 2001)

Wing venation, color patterning and structure of the genitalia are often used to classify dragonflies into genera and families. (Borror, Triplehorn, and Johnson, 1989)

Dragonfly nymphs have large eyes, six legs, and a stout body that is only a few times longer than it is wide. They are typically brown or greenish and sometimes have algae growing on them. Nymphs occupy different habitats within aquatic ecosystems, and their shape reflects their habitat. Bottom dwellers are slow moving and have flat, stout, hairy bodies with short strong legs for digging. Weed dwellers are more active, and therefore have long, smooth, streamlined bodies. (Silsby, 2001)

Dragonfly nymphs have a modified mouthpart called a facial mask. The facial mask is a modified lower lip (labium) that is long and hinged and armed with a set of grasping, claw-like lobes. When a nymph senses prey nearby, it thrusts the labium rapidly forward to capture the prey between the two lobes. When folded, the facial mask covers the majority of the underside of the head, hence the name “facial mask.” (McGavin, 2000; McGavin, 2001)

Because dragonfly nymphs are entirely aquatic, they breathe by pumping water over tracheal gills in their rectal chamber. When necessary, they can rapidly expel water from the rectal chamber, moving themselves forward by jet propulsion. Dragonfly nymphs can be distinguished from damselfly nymphs by the location of their gills. Dragonfly gills are located inside the rectal chamber, and are not visible. Damselfly nymphs have three external gills at the end of their abdomen. (McGavin, 2001)

Adult dragonflies are easily recognized by their long thin bodies, two pairs of long wings and very large eyes that seem to cover most of their head. They can be distinguished from the closely-related damselflies by the round shape of their heads, their disproportionately large eyes, and the difference in shape between their two pairs of wings, which are held horizontally at rest. Unlike damselflies, the rear wings of dragonflies have a broad base. Other characteristics that distinguish dragonflies from damselflies include three anal appendages (one less than damselflies) and females with non-functional or vestigial ovipositors. (McGavin, 2001; Silsby, 2001)

Dragonflies have a short thorax, with three pairs of long, spiny legs that are slanted forward. Bristles on the legs interlock to form a basket that is used to scoop prey out of the air. The abdomens of adult dragonflies are very long, with 10 segments, and are often marked with colorful spots, bands and lines. The long wings of dragonflies can also have distinctive color patterns, as well as venation patters that can be used to sort them into families and species. The abdomens of some tropical species can change color with temperature, from bright red or blue in warm weather to a dull dark gray at lower temperatures. This may be an adaptation against to avoid detection from predators. (McGavin, 2001; Silsby, 2001)

In most species of dragonflies, the two sexes are similarly colored, though the male may be slightly brighter. Females may also have a slightly more stout abdomen. In a few species in the family Libellulidae, the two sexes have different patterns on their wings. (Borror, Triplehorn, and Johnson, 1989)

The smallest wingspan of an extant dragonfly species is 34 mm. The largest wingspan, 162 mm, belongs to the Australian species, Petalura ingentissima. (Silsby, 2001; von Ellenrieder, 2003)

Dragonfly eggs are laid in water, on the surface of the ground near the water or inserted into submerged vegetation. The dragonfly larvae, called nymphs, spend at least a few months (sometimes several years) as aquatic predators. They have gills that allow them to remove oxygen from the water. (Gullan and Cranston, 2000; McGavin, 2001)

As they grow, dragonfly nymphs molt 8 to 15 times. After their final immature stage (instar), nymphs crawl out of the water, usually early in the morning, and attach themselves to a plant stem or rock. Here the adult stage frees itself from the nymphal exoskeleton. The adult spends the first half hour or so after emerging drying out and expanding to its full size, including expanding and hardening its wings. After this initial expansion as an adult, dragonflies do not grow, though their exoskeleton may harden some and they may change color over the first few weeks as an adult. (McGavin, 2001; Silsby, 2001)

The lifespan of dragonflies varies by species. Once in the adult stage, some species survive for a few weeks and some for several months. The average dragonfly probably has a lifespan of 6 to 8 weeks, once they reach the adult stage. However, there is quite a bit of variability. The amount of time that dragonfly nymphs spend in the aquatic stage can also range from weeks to years. In general, tropical species spend less time in the aquatic stage and more time in the adult stage than their temperate counterparts. (Borror, Triplehorn, and Johnson, 1989; von Ellenrieder, 2003)

Dragonflies are solitary, rather than social creatures. The males of many species are territorial. However, individuals may congregate where resources are abundant, such as where a swarm of flying ants is emerging, or behind large animals that stir up prey. (Silsby, 2001)

Dragonflies are generally excellent fliers. Their two sets of wings beat out of phase with one another (one set is up while the other is down), and dragonflies can control both the frequency and amplitude of their wingbeats. They can also control the angles of the two sets of wings independently, and so can hover or fly forward, upward, backward or sideways. Because they can control their two sets of wings independently, dragonflies are agile fliers, and can even turn around within the space of one body length. Most species of dragonflies require a certain minimum temperature in order to fly. Dragonflies can control their temperature by stretching out on a sunny perch to warm up and changing their orientation to the sun (tipping abdomen up or down) to adjust the amount of solar heat they absorb. Some larger dragonflies can also cool themselves by gliding through the air and by circulating large amounts of blood through their abdomen. Because they require a minimum temperature in order to fly, most species are diurnal. However, there are a few species of dragonflies that hunt at night. (McGavin, 2001; von Ellenrieder, 2003)

A few species of dragonflies are migratory. However, a single generation of dragonflies does not complete a roundtrip migration. Rather, the individuals that arrive in the north each year are the offspring of individuals that flew south the previous year. For example, in North America, a generation of Pantala flavescens that spends its time as nymphs in the south flies north to breed. When their offspring mature, they fly south to breed. (Silsby, 2001)

Adult dragonflies are visual hunters, and communicate visually much more than most other insects. Because of their large eyes, they also have much better long-distance vision than most insects. Like many other insects, dragonflies can see UV light, which may aid in communication. Male dragonflies frequently employ flight displays to threaten other males or to court females. These displays vary between species and individuals but generally involve showing-off their brightest-colored body part, such as brightly colored patches on the head, legs, abdomen or wings. Males also fight aerial duels for territories, displaying their size and speed to each other. Once paired, mating dragonflies probably communicate by touch and possibly by chemical signal. (McGavin, 2001; Silsby, 2001; von Ellenrieder, 2003)

As larvae, dragonflies respond to mechanical, visual and chemical cues. They sense their prey with their compound eyes and/or with mechanoreceptors. (McGavin, 2001)

Dragonflies are generally insectivores, but they will eat nearly anything they can capture. Dragonfly nymphs have a special hinged labium that is used to capture small aquatic animals. Because the prey that a nymph can capture is primarily limited by the nymph’s size, the variety of prey increases as the nymph grows. Common prey items include aquatic insects, tadpoles, small fish, and other invertebrates. Nymphs will also readily eat other nymphs. (Dunn, 1996; McGavin, 2001)

Adult dragonflies eat flying insects, especially mosquitoes and other true flies, but also aphids, midges, small moths, butterflies and caterpillars, bees, smaller dragonflies and damselflies and just about any other insects they can capture. (Borror, Triplehorn, and Johnson, 1989; McGavin, 2001)

The three pairs of legs positioned near the front of the dragonfly thorax act like a net that allows dragonflies to catch prey in flight. The short front pair of legs allows the insect to feed while flying. After catching a prey item, dragonflies may eat while in flight, or perch to eat. Dragonfly adults are typically active hunters, flying around in search of prey rather than sitting in wait for prey to pass by. (McGavin, 2001)

Predators of dragonfly nymphs include fish, newts, frogs, toads, and larger odonate (dragonfly and damselfly) larvae. The primary defense of nymphs is concealment among vegetation or along the bottom. However, nymphs are also able to move quickly when necessary by rapidly expelling water from their rectal cavity. (Silsby, 2001)

Predators of adult dragonflies include fish, birds such as swallows and martins, frogs, rodents and other small mammals, lizards, spiders, small bees and wasps, and insect eating plants such as sundew. (Silsby, 2001)

Adult dragonflies are agile fliers an may escape predation by outmaneuvering their predators. Some species are camouflaged to escape detection by predators. A few species mimic more dangerous animals such as hornets, wasps or scorpions. (McGavin, 2001; Silsby, 2001)

Both dragonfly larvae and adult dragonflies are voracious predators. The larvae are important predators in freshwater ecosystems and are prey for other species. Adult dragonflies help control populations of other insects, including mosquitoes and true flies. They are also food for many aquatic and terrestrial organisms. (Dunn, 1996; McGavin, 2001)

Dragonflies in both the nymph and adult stages can be hosts to several species of parasites, including trematodes, parasitic wasps and biting gnats in the family Ceratopogonidae. The most common dragonfly parasites are larval water mites. The small, red or green mite larvae attach themselves to the dragonfly nymph, and then to the adult dragonfly when it emerges. They feed on the dragonfly’s blood for 2 or 3 weeks, and eventually leave it to return to the water where they develop into adult mites. In most cases, the parasites do not appear to significantly harm to their host. However, very high numbers of parasites can cause deformation or lowered egg production in some individuals. (Borror, Triplehorn, and Johnson, 1989; Corbet, 1999; Dunkle, 2000)

There are few known adverse affects of dragonflies on humans. Contrary to popular belief, adult dragonflies do not bite or sting. They are, however, host to parasitic trematodes, which can infect economically important animals such as poultry. (Corbet, 1999)

Dragonflies are excellent indicators of freshwater quality. Adult dragonflies may also play an important roll in controlling populations of other insects including mosquitoes. In some tropical areas, dragonfly nymphs are intentionally kept in drinking water storage tanks to control larval mosquito populations. (McGavin, 2001; von Ellenrieder, 2003)

Dragonflies are considered a delicacy in several eastern countries. They have also been used in traditional medicine in China and Japan. (Corbet, 1999; McGavin, 2001)

Dragonflies are eaten several eastern countries. They have also been used in traditional medicine in China and Japan. (Corbet, 1980; McGavin, 2001)

Most dragonfly species are abundant and common, but a few require very specific habitats as immature nymphs. These species are at risk because their habitats are in danger. The most significant threat to dragonfly species is destruction and/or pollution of freshwater habitat by humans. Freshwater quality in many important dragonfly habitats is severely threatened by deforestation. (Silsby, 2001)

Nature preserves for the conservation of dragonflies exist in Japan, Europe and the United States. In Japan, some artificial habitats are also created and managed for the propagation of dragonflies. (von Ellenrieder, 2003)