Common green darners are one of the larger dragonfly species, growing up to about 7 or 8 cm in length. Like all insects, they have three body sections: the head, the thorax, and the abdomen. Their eyes are large, their thorax is short and thick, and their abdomen is long and thin. Their abdomen gives this type of dragonfly the name "darner", as it resembles a darning needle. They have four large outstretched wings that attach at the thorax. Both males and females have a green thorax. On the back of their abdomen, there is a black stripe. Males have dull green eyes, their face is green, and their thorax is bright to dull green. The first segment of their abdomen is green, the middle part is bright blue, and the end of their abdomen fades to a dull green and then mostly dark. Females are polymorphic, meaning in this species, there are two different types of females. On most females, the first two abdominal segments are green; the rest is brown on top with gray-green on the sides. The other type of female, which is less common, has the same colors and patterns as the male. When common green darners have newly emerged as adults, they are not yet ready to mate (called the teneral stage). The last 7 segments of their abdomen are red-purple during the teneral stage, while their wings are uncolored or orange-tinted. Their wings become an amber color as they grow older, particularly in females. Nymphs have a long, oval abdomen, large eyes, and a large lower lip (labium) that can be shot out to capture prey. Darner nymphs have flat labiums with pointed palps that are used to stab prey. They are typically brown or green in color. Older nymphs have prominent wing pads, and can grow up to 3.5 to 5 cm in length. (Fraker and Luttbeg, 2012; Paulson, 2011)
Common green darners (Anax junius) are a dragonfly species found across most of the Nearctic region. Their range covers all of the United States, and extends north into southern Canada, and south into parts of Mexico. Common green darners that have migrated from the north can be found during the winter in Central America, including the Yucatan, Belize, and Veracruz. There are also populations in Hawaii, Tahiti, the West Indies, China, and the Kamchatka peninsula in Russia. Occasionally, common green darners will turn up in Bermuda, the United Kingdom, France, Alaska, and parts of northeastern Asia, but these individuals are carried there by storms or strong winds, and there do not seem to be any populations living in these regions. (Corbet, 1999; May, 2013; Paulson, 2011)
Adults are found in a large variety of habitats. They spend much of their time in habitats close to bodies of water, including the water from which they emerged and the water where they will lay their eggs. Adult common green darners often perch in low plants and shrubs, especially during the night. Since some populations migrate thousands of kilometers, they are often spotted in many different habitats during the trip. Migrations are very common along coastlines and other landmarks, so dunes and beaches are popular habitats. They also live and forage in forests, grasslands, and deserts, though they need to have water sources nearby. Nymphs are aquatic, and are found in lakes, ponds, and slow streams. They typically stay close to the shore, in waters less than 0.5 m deep. (Corbet, 1999; May, 2013; Paulson, 2011; Polcyn, 1994; Russell, et al., 1998)
Common green darners develop through incomplete metamorphosis, which includes egg, nymph, and adult stages. Eggs are laid in aquatic habitats, and hatch after at least 6 or 7 days. They hatch as nymphs, and go through 9 to 13 stages, called instars. As they reach the end of nymphal development, the nymphs climb out of the water and search for a place to molt into adults. Common green darners can actually travel quite far from the water before climbing a vertical surface, such as a plant stem. After which, the nymphal skin splits and the adult climbs out. The wings uncurl, as the wings and body expands. At this time, they are in the teneral stage, as adults that are not yet able to mate. They leave their nymphal skin, and quickly gain body weight, as they spend much of their time foraging for food. Their skin hardens, and their color pattern becomes more noticeable as they develop into adults that are able to mate. There are two different population types: resident and migratory populations. Residents remain where they were as nymphs. Residents in the north mate and lay eggs in late July to August. The offspring hatch and then go into hiding for the winter when temperatures drop. In the spring, the nymphs become active again, and complete development into adults in June or July. Total development time from egg to adult can take 11 to 12 months for residents. Adults that migrate to the north tend to arrive in the northern regions in the spring, before the residents emerge. Migratory adults mate and lay their eggs in June. Unlike resident populations, development time from egg to adult takes only 3 to 5 months for migrant offspring. They emerge as tenerals typically around late-August and September. They often begin their migration as tenerals, feeding along the way and developing into adults. They mate and lay eggs during the migration and when they reach their southern destinations. The offspring produced develop during the warm winter in the south, before becoming adults and migrating to the north. (Corbet, 1999; Crumrine, 2005; Hopkins, et al., 2011; May, 2013; Paulson, 2011; Russell, et al., 1998)
To begin mating, a male usually just grabs a female, but the female chooses whether to mate. The male grasps the female by the top of the head with the end of his abdomen. The pair flies away from water together, and typically lands on vegetation. To mate, the female swings the end of her abdomen up to the front part of his abdomen, called the "wheel position". Both males and females mate many times with different mates. Females often have marks on their heads from being grasped by the males, and males may also have marks as well. Non-migratory populations in the north mate during the late summer, from July to August. In migratory populations, those that have migrated to the north mate in June, while those that have traveled south mate in late fall. Mating also takes place during the southward migrations. Male common green darners do not necessarily defend their territory, but they will act aggressive toward other males. Single males often attack mating pairs. Single males try to land on mating pairs, and mating pairs respond by fluttering their wings. Mating males also quickly shake their abdomen. Sometimes single males ram, pull on, and bite mating males. Mating males defend the pair by beating their wings, holding on to plants, and biting back. In these instances, the mating pair often separates or drowns. (Corbet, 1999; May, 2013; Paulson, 2011)
Once mating is complete, common green darners stay connected and lay eggs in pairs. The pairs fly low over the water, with the male still grasping the female, and they land in the open to lay the eggs. They lay eggs on floating stems and leaves, including alive and dead material, as well as on woody branches. Large female dragonflies can lay huge clutches of eggs. Females can lay multiple batches of eggs from multiple matings. Resident populations take about a year of development before they are able to mate, while migratory populations mature after 3 to 5 months. (Corbet, 1999; Paulson, 2011)
Adult common green darners provide nutrients in the eggs for the offspring to grow and develop. They also lay the eggs in a body of water where the offspring can successfully live and hunt. Otherwise, the adults provide no parental care. (Paulson, 2011)
Adults can often be seen flying over open areas, sometimes in small groups. They are active during the day, and at dawn and dusk. To rest, common green darners perch in low plants, grasping the perch with their legs and hanging. They also spend the night in vegetation. Common green darners are strong fliers, with all four wings moving separately, giving them the ability to fly forward, backward, and to quickly change directions. This allows them to be very successful hunters of other flying prey. However, their flight is affected by strong winds and unfavorable weather conditions. Some are blown off course during migrations and end up across the ocean. They can often be seen hovering in place, facing into a strong breeze. Common green darners actively regulate their body temperature. As ectotherms, their body temperature is determined by their environment. By contracting and vibrating their wing muscles, these dragonflies can warm themselves, and they can also transfer heat by controlling the flow of fluids in their body. This allows them to be active at lower temperatures in colder regions, without having to rely on the sun or other heat sources. However, they use heat sources when available, as they have been seen sunning themselves in the morning on migrations to warm up. Nymphs are very active, and are constantly moving in their aquatic habitat, spending much of their time hunting prey. They are solitary, and avoid other green darner nymphs, likely because cannibalism is common. They also avoid other dragonfly and damselfly nymphs, as these nymphs often prey on each other. Nymphs are active during the day and night. Resident populations in the north hibernate during the winter as nymphs, while migratory nymphs do not go into hiding for the winter. (Corbet, 1999; Fraker and Luttbeg, 2012; Paulson, 2011; Polcyn, 1994; Russell, et al., 1998)
Some populations of common green darners make huge migrations every year. In mid-August to October, large groups move south from Canada and the northern United States to Mexico and other southern regions along coastlines and other areas. They can gather in huge swarms on the coast when migrating south. In these swarms, common green darners can often be found with other dragonfly species, such as variegated meadowhawks. Migrating birds are usually nearby as well. During the migration, they make many stops to find food and build up their energy. Between the foraging flights are long migration flights. Migrations can take several weeks due to this alternating short flight, long flight behavior. There is also a return migration to the north in the spring, though this trip is taken by the offspring of the southern migrants. Northern migrations have not really been studied, as it seems fewer dragonflies migrate north, or that the migrations take place over a longer period of time. Swarms do not usually form on the northward migrations. Northern migrants also do not return to the same bodies of water their parents emerged from, and also likely do not return to even the same area. Weather systems are important for migrations. Autumn migrations often begin after cold fronts in the north, while northern migrants in the spring often arrive into the north on warm fronts. Dragonflies use the wind and air movement in these weather systems to help them fly. They also use the weather in their daily activities. During southern migrations, adult spend the night in warm, west-facing surfaces in vegetation. In the morning, after moving their wings to heat their body, they move to spots in the sun on the east side of the plants to warm themselves and gain energy for the day. (Corbet, 1999; May, 2013; Russell, et al., 1998)
After developing into adults and leaving the water, tenerals may travel far from the water, before becoming mature adults that are able to mate and return to a different body of water. Resident adult populations likely remain in the same general area, staying near the water. Migratory populations have a huge range they travel through. Some travel from southern Canada all the way into Mexico and perhaps even farther. Migrants can move up to 140 km per day. The maximum total migration is 3,000 km, with most averaging a total of about 900 km. Once they reach their destination, they also likely stay in the same general area, near a body of water to lay their eggs. (May, 2013; Paulson, 2011)
Dragonflies have remarkable vision, with very large, well-developed eyes. Their eyes are so large they wrap around their head and give almost a 360 degree view, though they cannot see well above or below them. They can easily detect movement, which allows them to find prey on land or in the air, as well as to find mates. Dragonflies can see a wide range of colors, including UV light and possibly polarized light. When mating, vision is important in identifying mates, but touch is also important between mates. Throughout mating, males the female's abdomen. (Corbet, 1999; Paulson, 2011)
Nymphs are strong predators, and eat aquatic insects, zooplankton (Daphnia), Amphipoda, and other aquatic organisms, including tadpoles, larval salamanders, and even small fish. They also feed on other dragonfly nymphs, and even cannibalize smaller common green darner nymphs. Dragonfly nymphs have a large, specialized lower lip they use to shoot out and stab prey. Their lips pull back and bring the prey to their mouthparts. Adults are also carnivorous, and primarily eat insects. They usually feed on flying insects including many bees, flies, butterflies, and other dragonflies. They can also snatch insects from plants and the ground including beetles, and true bugs. (Corbet, 1999; Fraker and Luttbeg, 2012; Kerby, et al., 2011; Paulson, 2011)
Common green darner nymphs are the top predators in fish-less habitats, so nymphs have few predators. When they are found in water with fish, they are often eaten. Frogs and aquatic insects are also predators, as well as other larger dragonfly and damselfly nymphs. Larger salamander larvae can also occasionally prey on common green darners. Cannibalism of smaller nymphs by larger nymphs is also a threat. To defend themselves, nymphs use the spines on their abdomen and the appendages on their back end to poke and stab at predators. If stabbing the predator does not work, green darner nymphs can extend their lower lip and use the hooks to stab the predator. However, this can damage the lower lip, which is important for feeding, so nymphs only do this as a last resort, in case they survive and need to be able to feed. Dragonfly nymphs also have a special chamber in the rectum, where the gills are located, which stores water. They can shoot water from their back end to quickly propel themselves forward, away from potential predators. Nymphs may also reduce activity or stop movement altogether in the presence of a predator. Additionally, the brown or green coloration of nymphs serves as camouflage. More green nymphs are found in the spring and summer, when there are more plants in the water to blend in with. Birds, spiders, robberflies, and other large dragonflies are all airborne predators of adult common green darners. Frogs and fish can also prey on adults when they are laying eggs in the water. Adult dragonflies can fly quickly and dodge predators easily. They also have very good vision, which allows them to spot threats. (Corbet, 1999; Fraker and Luttbeg, 2012; Hopkins, et al., 2011; May, 2013; Paulson, 2011)
Common green darner nymphs are considered top predators in aquatic habitats that do not have fish. They play a large role in the ecosystem, and are able to effect the other organisms in their habitats by preying on many insects and other animals. Both adults and nymphs are prey to a variety of other insects, birds, frogs, and even other dragonflies. There are several parasites and parasitoids that live inside common green darners. Parasitic gregarines, Prismatospora cloptoni, a type of protozoa, can be found in the digestive system of nymphs. A larval stage of the tapeworm Schistotaenia tenuicurris, infects nymphs, and can be found in the fluid in their body cavity. The tapeworm kills common green darners during metamorphosis. The main hosts for the tapeworm are grebe species, bird species that hunt and dive in the water, and eat green darner nymphs, which get infected with the tapeworms. Parasitic midges, mainly species of Forcipomyia and Pterobosca, attach to the wings of common green darners and suck out body fluids for a day or two. The parasitoid wasp Aprostocetus polynemae lays its eggs in the eggs of common green darners, which kills the dragonfly embryos. (Corbet, 1999; Crumrine, 2005; Hopkins, et al., 2011; Smith and Cook, 2012)
Common green darners have been seen gathering in large groups to feed on honey bees in bee yards, especially queens and drones on mating flights. This has caused serious financial loses for beekeepers. Huge groups of green darners are able to destroy many bee hives. (Corbet, 1999)
Common green darners often feed on some insects that are harmful to humans, such as mosquitoes, and some agricultural pests, such as Mexican bean beetles. However, these dragonflies likely do not significantly reduce the size of pest populations, so they would not be useful as a method of control. Dragonfly nymphs can sometimes be used as bioindicators of water quality. Being a bioindicator means that by studying the organism, scientists can get a lot of information about the habitat in which it lives. They can look at the chemical levels in the nymphs, which can show the chemical levels present in the water, and whether it is harmful to the nymph. Common green darner nymphs may be useful as bioindicators, though more research is needed. (Catling, 2005; Corbet, 1999)
Common green darners are not an endangered species. (Paulson, 2009)
Angela Miner (author), Animal Diversity Web Staff, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
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