The common green darner has an unusual breeding strategy, being one of very few dragonflies to migrate in spring and autumn. Some individuals behave like other dragonflies, overwintering as larvae before emerging as adults in the spring. However, large proportions of the population move south in the autumn, often flying in large swarms, and are believed to breed during the winter in Mexico, the Caribbean and the far south of the United States. There, the larvae develop over winter, and it appears to be the new generation of adults that travels north again in spring to breed in northern areas over the summer. The offspring produced in the summer then undertake the next southward migration as immature adults (1) (4) (6).
Like other dragonflies, the common green darner is an agile and opportunistic predator, with excellent vision (8) (10). Adults of this species feed during the day or at dusk, often foraging in swarms over rivers, lakes or fields when prey is abundant (3) (4) (6). The diet consists of a variety of flying insects, including butterflies and even other dragonflies (5) (6). The larvae of the common green darner are also opportunistic predators, taking a variety of aquatic invertebrate prey, as well as fish eggs and tadpoles (5) (11) (12). Prey is caught by the larva shooting out its lower jaw, or ‘labium’, which is armed with hooks that impale the victim (8) (10).
Adult male common green darners patrol stretches of water, chasing intruders and pursuing potential mates (4) (5). After mating, the female lays the eggs in aquatic vegetation (4) (5) (6). Uniquely among North American darners, the male and female common green darner often fly in tandem pairs while the female is laying the eggs, although rival males will often try to break up these pairs in an attempt to mate with the female (4) (6).
The larva of the common green darner is long and smooth and usually marked with green and brown (13). Dragonfly larvae, also known as nymphs, are aquatic and pass through a number of developmental stages, often over a relatively long period, before emerging from the water and moulting into the adult form. The newly emerged adult then spends some time feeding and maturing before it is ready to breed (5) (8) (10).