INTRODUCTION TO THE DRAGONFLIES

The insect order Odonata (Greek for "toothed jaws") contains the groups of insects known in English as dragonflies and damselflies, but "dragonflies" is often used to refer to the whole order. "Odonates" is another name that is gaining popularity. The Odonata contains about 5,500 named species in 33 families worldwide. For comparison, there are roughly the same number of mammal species in the world and almost twice as many birds. Most dragonflies live in the tropics, but a few have adapted to the cooler temperatures of higher latitudes; even in our region there are many more species living in the south than the north.

Dragonflies are large and abundant insects and, because of this, the order forms one of the predominant groups in standing freshwater communities in the northern regions of British Columbia. We have recorded 87 species in the province. In the western mountains, species are less abundant in running water than they are in standing water habitats, becasue mountain streams are often too cold for dragonflies to develop in. Dragonflies live around most types of fresh water. Certain kinds prefer lakeshores, others are found only along streams, or around springs and in peatlands. Ponds and marshes rich in aquatic vegetation support the most species.

Dragonflies are among the most ancient insects - their ancestral line goes back to the Carboniferous Period, about 300 to 350 million years ago. They have retained many primitive characteristics and developed some specialized features for a successful aerial and predatory lifestyle. Dragonflies share with mayflies the ancient inability to fold their wings flat over the body. They differ from all other insects in their combination of biting mouthparts; their two equal (or almost equal) pairs of long, membranous, net-veined wings; their large, bulging eyes and short, thread-like antennae; and their long, slender abdomen that, in the male, bears secondary genitalia at the base.

A dragonfly leads a dual life - in its immature stage, the larva lives in water, obscure and camouflaged. When it is time to mature, the changing larva emerges from the water and transforms into a colourful, flying adult.

Some early dragonfly-like insects were enormous - fossils from the Carboniferous Period show that one had a wingspan of 70 cm - but the largest North American species found today measures about 14 cm across the wings. The greatest wingspans in modern times - about 17 cm - belong to the giant damselflies of the American tropics.

Many dragonflies around the world are as colourful and flashy as the most spectacular birds and butterflies. Most of our local dragonflies are more subdued, but they are still lovely and striking insects. They come in a myriad of colours, from iridescent metallic green to breathtaking crimson. Their bodies can be boldly spotted or striped, and their wings are often strongly patterned with spots and bands of colour.

The order Odonata is usually divided into three suborders: the Zygoptera (damselflies), the Anisoptera (true dragonflies) and the Anisozygoptera (a tiny group of two rare species from the mountains of eastern Asia). Damselflies are slimmer, often smaller and usually fly more slowly than true dragonflies. At rest they usually hold their equal-sized wings together above the body - Zygoptera means "joined wings". Anisoptera means "unequal wings", because the hindwings of the true dragonflies are broader than the forewings. When perched they hold their wings out away from the body.

The flying ability of dragonflies amazes most people (Fig. 2). Although the wing structure and arrangement of the flight muscles are primitive, the flight performance and efficiency are remarkable. Unlike most insects, dragonflies usually beat their forewings and hindwings separately - when the forewings are up, the hindwings are down. Each wing also has much independent control, accounting for the surprising manoeuvrability of many species, which can fly upwards, sideways, backwards and forwards. A large darner can fly up to 60 km per hour. Aeshnidae (darners), Corduliidae (emeralds), Cordulegastridae (spiketails), Macromiidae (river cruisers) and some Libellulidae (skimmers) are called flyers because they spend most of their active life flying - they even generate additional body heat from their wing muscles. Damselflies, Gomphidae (clubtails) and most Libellulidae are often called perchers, because they spend more time perching than flying. Perchers gain much of their body heat from basking in the sun and make only short flights to catch food or mate.

Figure 2. Dragonflies are amazing fliers. Aeshna palmata male.
Photo: George Doerksen, RBCM

For millennia, dragonflies have instilled superstitious fear in humans, even though they do not sting or bite people. Maybe their boldness takes us aback, or their speed startles us. To the uninitiated, their strange appearance up close can make them seem fearsome. The English name "dragonfly" echoes the feelings these insects sometimes arouse - they are the fanciful "devil's darning needles" that sting venomously or sew up the lips; they are "snake doctors" with the power to bring dead snakes back to life. These legends and folktales are groundless - dragonflies are harmless to humans.

Dragonflies spend their youth as aquatic larvae preying on other underwater animals. Dragonfly larvae - sometimes called nymphs - have an enormous (for their size) hinged labium (a sort of lower lip armed with pincers) that they use as an extendible grasping organ for capturing prey. They are voracious predators, eating small aquatic insects, crustaceans and even fish and tadpoles.

Figure 3. Aeshna interrupta larva.
Photo: Robert A. Cannings, RBCM

Biologists place dragonfly larvae into three categories, according to their feeding behaviour: Claspers (Zygoptera and Aeshnidae) stalk their prey while using their clasping legs to hold onto vegetation (Fig. 3); their colour patterns of green and brown help camouflage them among the water plants. Sprawlers (Macromiidae, Corduliidae and most Libellulidae) lie spread-eagled on the bottom mud and debris or on vegetation, waiting to ambush prey; they often keep hidden under a coating of mud and algae. Burrowers (Gomphidae and Cordulegastridae) dig into the sand and silt, where they await their prey.

Damselfly larvae, like the adults, are slender animals. The tip of the abdomen bears three leaf-like gills, richly laced with the fine tubes that carry oxygen and carbon dioxide throughout the body. The stouter larvae of the true dragonflies do not have external gills; instead, they pump water in and out of the gut and breathe through gills lining the rectum. Damselflies use their gills to help them get around, sweeping them back and forth like swimming fins. Larvae of true dragonflies also use their breathing mechanism to help them move: they can blast pressurized water out the anus, jet-propelling them through the water - an effective tactic for escaping predators or attacking prey.

Dragonflies, like grasshoppers and many other insects, develop without a pupal stage. After the larva pops out of the egg, it eats, grows and moults 8 to 17 times (usually 10 to 14), depending on the species and the conditions. The developing wingbuds get larger with each moult. For many species in British Columbia, the life cycle takes about a year. Some Lestes (spreadwings) and Sympetrum (meadowhawks) that live in temporary ponds overwinter as eggs, hatch in the spring, grow rapidly and emerge as adults in the summer. Many species overwinter as larvae and emerge the following spring or summer; others spend two years in the larval stage. For some dragonflies (especially certain Aeshnidae and Corduliidae), the larval life may last six years or longer. Development time depends on the species and also on altitude, latitude and amount of daylight. Growth slows with the shorter summers and colder temperatures of northern habitats and high altitudes.

In British Columbia, dragonflies live only a short time as adults - about one to two months. A dragonfly begins its adulthood when the fully grown larva metamorphoses into an adult inside its last larval skin, then crawls out of the water, up a plant stalk or some other support. Gomphidae and Coenagrionidae (pond damsels) can emerge horizontally on rocks, floating logs and plants, or the shore.

Now exposed to air, the dragonfly begins its final moult: the top of the thorax splits open and the adult dragonfly squeezes out of the larval skin (Fig. 4). It pumps blood into its wings and abdomen, which expand slowly, and gradually, the body hardens. After an hour or two the dragonfly can fly, weakly at first, on fragile, glistening wings. It leaves the empty larval skin, the exuvia, clinging to the support. Once its body has hardened, the adult dragonfly will not grow larger even though it eats a lot.

Figure 4. The mature larva crawls out of the water and the
adult dragonfly emerges from the larval skin. Aeshna interrupta male.
Photo: Robert A. Cannings, RBCM

Emergence can occur by day or night. Most Zygoptera, Gomphidae and some Libellulidae emerge during the day. Many darners emerge at night, but in cool weather or in the far north, may transform during the day. The newly emerged adult, called a teneral, is vulnerable to predators and bad weather.

Dragonflies have characteristic flight periods. This is the period during which adults may be seen and does not necessarily represent the adult lifespan of a particular individual. Many species may live in the same locality: some emerge early in the spring and are rare by summer; others appear in mid summer and fly into the fall; still others fly from spring to fall.

After emergence, most adults leave the shoreline to hunt and eat for a few days or even weeks as they mature. They are powerful predators that hunt by sight. They usually capture prey while flying, grabbing it with long, spiny legs and then chewing it with powerful jaws. Adult dragonflies eat mainly flying insects, but some species will pluck insects, spiders and even small frogs off vegetation or the ground.

Figure 5. Mating Enallagma boreale.
Photo: George Doerksen, RBCM

Immature adults are pale in colour, but gradually become darker and often brighter as they mature. Some species produce a waxy, white or pale blue powder, called pruinescence, over parts of the body and wings; this is especially obvious in some male Libellulidae and Lestes and some female Ischnura (forktails).

When they are sexually mature, dragonflies return to the water to breed. Most of the dragonflies you see near water are males aggressively searching for mates. In many species (Zygoptera, Petaluridae (petaltails), Gomphidae, Corduliidae) and many Libellulidae), mature males defend a territory against ther males of the species, patrolling the habitat or sallying out from perches. This territorial behaviour limits aggression by spacing males along the shore and helps prevent undue disturbance of egg-laying females.

Females coming to the water to breed quickly attract mates. With the appendages at the tip of the abdomen, a male grasps a female by the front of the thorax (Zygoptera) or by the top of the head (Anisoptera). This head to tail arrangement is called the tandem position. Before joining with a female or even while in the tandem position, the male transfers sperm from the tip of his abdomen to his penis, which is under the second abdominal segment. The female then loops the end of her abdomen up to the penis so that the male can transfer the sperm to her. The Odonata are the only insects that mate in this circular formation, called the wheel position, which they maintain for a few seconds or several hours, depending on the species (Fig. 5). Female dragonflies usually mate more than once, and in an attempt to ensure that their sperm fertilizes her eggs, males may spend much of the copulation period removing the sperm of other males - the penis is modified to pull another's sperm out of the female or push it aside so that it is inactivated.

Figrue 6. Lestes disjunctus ovipositing.
Photo: George Doerksen, RBCM

The female lays her fertilized eggs by the hundreds. All Zygoptera, Aeshnidae and Petaluridae have a knifelike egg-laying structure with pointed blades, called an ovipositor, at the tip of the abdomen; they lay their eggs in plant tissue (Fig. 6), although some darners and petaltails insert eggs into soil. Cordulegastridae shovel the eggs into a streambed. Other species - without ovipositors, or with simply a scoop-shaped plate called a vulvar lamina - drop eggs into the water, tap the eggs into mud and moss, or simply dip the tip of the abdomen into the water and wash the eggs off.

Competition for mates is usually fierce, and male aggression can prevent females from laying their eggs. Females that lay their eggs alone, especially mosaic darners (Fig. 7), often do so stealthily, flying low among the plants along the shore, wings rustling in the stems as they settle to deposit the eggs. Some Zygoptera actually crawl below the water's surface to escape the attention of males, often remaining submerged for more than an hour - they take a film of air down with them, trapped in the hairs on their body, so they can breathe while they lay their eggs. In many Zygoptera, Sympetrum and Anax junius (Common Green Darner), the male stays in tandem, retaining his hold on the female while she lays her eggs (Fig. 6). In some other species, the male hovers protectively nearby, guarding the egg-laying female from any other males who may attempt to mate with her, and allowing her to lay her eggs undisturbed.

Figure 7. Aeshna palmata female ovipositing in wet log.
Photo: George Doerksen, RBCM