Odonates – dragonflies and damselflies

Empty larval skin or exuvia

All larvae or nymphs are voracious predators, eating pretty much anything moving that they can subdue, including aquatic insects, tadpoles, worms, crustaceans, small fish and sometimes each other. Once fully grown, the larva crawls out of the water up a plant stalk or other support. The skin on its back splits open, and an adult, which has been packed in like an accordion, emerges, expands and dries its four wings. Until the wings have hardened, they fly only weakly and many succumb to birds. Once their wings have hardened, they are usually too fast to be caught.

The empty larval skin is usually all we see of the largest part of an odonate’s life. Unlike butterflies and many other insects, there is no pupal stage.

Masters of the air

Their life as adults often lasts only a few weeks or months, but is equally predaceous. They terrorize flying insects including other odonates. All their prey is taken from the air (they can swoop down to take insects perching on vegetation) and often eaten in the air. The pair of damselflies you see flying locked together, are not discussing tactics but mating. With so much of their life conducted while airborne, there is great advantage to being an agile flier. They have wonderful adaptations to that end.

Their four wings move independently which permits exceptional maneuverability. They can even fly upside down – try that, hummingbird. Intentional asynchrony between the pairs of wings allows the hind wings to exploit the lift of the front wings, so that they attain high speeds while beating only 30 times per second. Species they capture, like flies, honey bees and mosquitoes, beat their wings 200 to 600 times per second. Bees, like butterflies, synchronize their front and back pairs of wings on each side with tiny hooks. That is helpful, but not nearly as effective as independent movement.

Shadow darner	Western pondhawk feeding on damselfly

Nearly all of their freely movable head is eye, with 20,000 to nearly 30,000 facets each, permitting acute vision. Hemispherical eyes encompass almost every angle except directly behind them. About 80% of an odonate’s brain is devoted to vision. They are aerial masters of the insect world, successfully completing about 95% of their strikes, much higher than any bird. Their high speed requires high energy intake. No fears. They can take 30 to 100 mosquitoes in a day. That alone should endear them to us. They take anything else that flies and is small enough to subdue.

They often catch prey in flight using their feet and hind legs covered in wee hairs. The downside to this ability is they do not walk or climb well, but they don’t need to. Adults can devour their prey while in flight, holding it in their legs as they fly. The name of the Order, Odonata, was inspired by the serrated teeth on their mandibles (actually true of many insects) which allow them to chow quickly through their prey. The larger, more robust dragonflies manage larger prey than do damselflies, including eating damselflies and other dragonflies.

Symbols and beliefs

Blue-eyed darner

Their appearance and ability as killers has inspired different views in different cultures. For Samurai they were symbols of power, agility and victory; for Chinese, prosperity, harmony and good luck; for some Native Americans, happiness, speed and purity. Purity because the dragonfly eats from the wind itself. In Britain and Europe, odonates often were more frightening. The prevailing early notion was that Satan set them on earth to cause chaos and confusion. They were called the Witches’ Animal, Ear Cutter, Devil’s Needle, Adderbolt and Horse Stinger (dragonflies around bucking and stamping horses were likely killing the smaller parasitic insects that inspired the bucking). Later, poets were moved by their beauty, but darners acquired their common name because children were warned to keep quiet or else the dragonfly’s “darning needles” would sew their mouths shut. They have been around far longer than we have, giving us ample time to conjure images.

The mating game

Odonates attract our attention with the speed and agility of their hunting. Far more is going on that is intriguing to watch – like finding a mate, defending a mate and mating. Two things are happening. The female is choosing a suitable spot to oviposit her eggs; males are seeking a mate. What is an appropriate egg-laying site depends on species and may be a mat of algae, open water or a stand of cattails. A good site attracts the females. Females attract males intent on mating. The trick for males is to find a favourable site to attract females, then be ready to fend off intruding males. Mature males may perch, watching for both breeding females and male intruders, or patrol prime breeding habitat to intercept females and run off other males. Patrollers, flying around the territory are usually dragonflies; damselflies are generally perchers. Once a defending male spots a female, he is off to grab her. The grip must be firm, because other males will attempt to steal her. When the visitor is male, there is much high speed zipping around and circling. The combat is more ritualistic than physical, intended to demonstrate strength and speed. Actual contact is dangerous, simply because wings are fragile and adult odonates are completely dependent on their wings. That doesn’t mean these meetings are not intense; you are witnessing aerial masters showing off their prowess.

Mating Red-waisted whiteface	Mating common green darner dragonflies (the male is blue)

The mating itself is unique. When he is poised, a male grasps a female by the front of the thorax (damselflies) or by the top of the head (dragonflies) with the appendages at the tip of his abdomen. The female loops the end of her abdomen up to the base of the male’s abdomen where sperm is stored, forming a kind of wheel. Odonates are the only insects that mate in this wheel position.

Weaknesses

The most apparent weak point in the odonate life cycle is that stage they live in longest – as aquatic nymphs. The nymphs are not good swimmers, so are typically found sitting on aquatic vegetation, or climbing on plants or rocks in ponds and streams where water is still or slow-moving, or in the substrate below water. Still water is often shallow, so prone to drying. Both damsel and dragonflies are exposed, but dragonfly nymphs can be exposed to three or more summers of drying that can eliminate a generation.

Swarm from thedragonflywoman website

Swarming also may reflect vulnerabilities. Sometimes, hundreds of dragonflies of different species gather in swarms, apparently for feeding or migration. Because it is ephemeral, we know little about swarming behavior. In static feeding swarms, the dragonflies fly repeatedly over a localized, well-defined area near the surface, usually feeding on clouds of smaller insects. In what are believed to be migratory swarms, hundreds to perhaps millions of dragonflies fly in a single direction, usually 15 to 30 m above the ground. What is worrying about swarming behavior is that it may reflect drying of small water bodies critical to odonates. Both swarm types reveal ‘organizational’ skills we do not normally attribute to damsel or dragonflies. Because we know so little, the Dragonfly Swarm project was initiated. The Swarm Project is one more example of the contributions citizen science can make. Click here to submit your own observations.

Migration

Migrations of dragonflies have been known since biblical times. We now know the Globe skimmer, a dragonfly, makes the longest migration of any insect, about 17,000 km back and forth across the Indian Ocean. Closer to home, Common green darners frequently make small north-south migrations. These have been tracked by tiny transmitters attached to their wings. By migrating and laying eggs in more southerly ponds, the green darner’s offspring can emerge earlier the following spring and migrate north to occupy ponds still too cold to allow local dragonflies to emerge. Migration is much like that of many birds – travel days interspersed with rest. Travel days averaged 12 km, although one determined individual covered 160 km in a single day. They typically exploit tailwinds. Southern Canada appears to have two populations — one seems to migrate north in the spring and lay eggs, their offspring develop rapidly and fly south in August and September; the other population is resident all year round, the adults taking flight in June and July.

Both the common green darner dragonfly (left) and the variegated meadowhawk (right) migrate. Around A Rocha’s Brooksdale Centre in Surrey, BC the green darner is seen regularly and may be resident. The variegated meadowhawk is seen irregularly in nearby areas and is likely simply passing through.

To better understand and conserve North America’s dragonfly migration, academic institutions, nongovernmental programs and federal agencies from the United States, Mexico and Canada have formed the collaborative Migratory Dragonfly Partnership (MDP). The MDP works to better understand North America’s migrating dragonflies and to promote conservation of the wetland habitat on which they rely. A major part of the MDP is developing an international network of citizen scientists to monitor the spring and fall movements of the main migratory species in North America. The Xerces society, based in Portland, Oregon, assists in coordination.

The Xerces Society collates spring and fall movements of five focal species: common green darner (Anax junius), black saddlebags (Tramea lacerata), wandering glider (Pantala flavescens), spot-winged glider (Pantala hymenaea) and variegated meadowhawk (Sympetrum corruptum). All but the wandering glider have been reported from southwestern British Columbia. Two of these have been seen at (or nearby) A Rocha’s Brooksdale Environmental Centre (common green darner and variegated meadowhawk).

Click here for a field guide to migrating dragonflies
Click here for details on citizen participation