Illinois Natural History Survey - University of Illinois

Common Green Darner Dragonfly

Susan Post

 

By late summer the cicada/katydid droning has reached a crescendo and if you venture out during the late afternoon you might be rewarded with gatherings of large dragonflies, dancing, swirling, and dipping above you. Yet this whirlwind of dragonflies seem to always be just out of reach of an entomologist’s net. These large dragonflies are the common green darners, Anax junius. Their name means “Lord and master of June,” even though these insects may be found from early spring to the first weeks of fall.

The green darner,  one of the largest dragonflies (2.5 to 3.25 inches long), is found in every state, including Alaska and Hawaii. Both sexes have  “bull’s-eyes” on top of their foreheads and unmarked, green thoraxes. Mature males have blue abdomens while females’ and immature males’ abdomens are a rusty brown. Their preferred habitat is any type of slow or still water that is vegetated. Females oviposit their eggs in mats of algae, pieces of rotting wood, in the stems of growing plants at the edge of pond, or in floating vegetation.

Once the eggs hatch, the green darners begin life in the water as dull-colored, predatory larvae, called naiads.  These naiads are varying shades of brown and green, which offer camouflage in the mud and debris as they climb about the submerged vegetation of their watery habitat.  Naiads will tackle any unfortunate small crustaceans, minnows, tadpoles, or their favorite prey, immature mosquito larvae, that get in their path. To capture their prey the naiads’ lower jaws have modified structures, like “folded grappling hooks.” This modification is called a labium.  It can shoot out in a hundredth of a second, seizing its victim with two grasping hooks at the tips. When not in use, it is folded underneath the head. In addition to the labium, the naiads also use jet propulsion to make sure they can make a fast getaway from predators. By expelling water out the ends of their abdomens, somewhat like hydraulic cannons, they can rocket two to three inches through the water and away from any predator.

After one or more years of aquatic life (depending on food availability), and  if the naiads have been able to avoid the attention of fish (their chief predators), they will climb out of the water and metamorphose into adults. A dragonfly specialist quipped that the process of a naiad becoming a flying machine was “somewhat comparable to taking an automobile and transforming it into a small jet airplane.” Emergence from the water usually occurs at night. The naiad climbs out of the water onto a stick, plant, or anything that projects out of the water, and attaches itself. It’s skin splits down the back and the adult emerges soft and glossy with crumpled wings; this is a dragonfly’s most vulnerable stage as it pumps blood into its wings and waits for them to harden.

Adults support two pairs of rigid, transparent wings, which are held outstretched even at rest, and a long, thick body. They do not fold their wings and a comparison would be if humans had eight-foot 2 by 4s strapped to their hands to help them move about! Dragonflies do not give any outward appearance of grace on the wing, however, they are the best flyers in the insect kingdom, with some species having been clocked at over 35 miles per hour as they fly forward, backward, or sideways. The wings are asynchronous—they operate independently. One-third to one-half their body mass is devoted to flight muscles.

Their eyes are the largest in the insect world and are so big that their heads appear to be all eyes. They have three simple eyes (ocelli) and a single pair of compound eyes that gives them a nearly 360-degree field of vision, resulting in excellent eyesight.  They can observe insect prey up to 40 yards away. 

Their forward-directed legs are located in a cluster near the front of the thorax and are arranged in a basketlike way to catch prey (prey basket) and quickly transfer it to the mouth.  Dragonflies are so well-adapted as airborne predators that their legs are nearly useless for walking. 

Names for dragonflies were based on their fierce appearance and the belief of their nasty abilities.  The unenlightened once thought dragonflies were capable of sewing shut the mouths of men who cursed and women who scolded or they sewed up the ears of people who enjoyed gossip.  From these myths a variety of names sprung up such as—snake doctors, horse stingers, sewing needles, and the Devil’s darning needle. The myths and names are in reference to the distinctive shapes of their abdomens and the mistaken belief that dragonflies are capable of inflicting stings and harmful to mankind.  The only harm the insects do, however, is to mosquitoes, gnats,  and flies, some of their favorite foods; thus earning them the name mosquito hawks. 

 

 

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