Read an Excerpt
Dragonflies
Magnificent Creatures of Water, Air, and Land
By Pieter Van Dokkum Yale UNIVERSITY PRESS
Copyright © 2015 Yale University
All rights reserved.
ISBN: 978-0-300-19708-2
CHAPTER 1
Beginnings
The light of the full moon reflects off the water on this warm summer night. Amid a chorus of cicadas and frogs, an inconspicuous creature that has spent its entire life under water is dreaming of wings. The dragonfly nymph slowly clambers up a reed. Inside its hard shell everything has been prepared for this moment: wing buds have appeared on its back, a network of tubes for breathing air has been put in place, and a new set of compound eyes has developed behind the larval ones. After the nymph reaches the top of the reed, it is time for its great transformation, which will take the rest of the night. By morning, birds and other daytime predators will find nothing but a lifeless, empty shell still clinging to the reed as if to remind its former occupant of its dark and shadowy youth.
The life cycle of dragonflies is superficially similar to that of butterflies: they begin life as eggs; the eggs hatch into nymphs; and the nymphs transform into their adult form by way of a metamorphosis. Adults lay eggs, and the cycle starts again.
Whereas caterpillars are as cute as insect larvae can be, no one would use that word to describe dragonfly nymphs. They live in water, where they are among the most ferocious and (presumably) feared predators in the community of small fish, amphibians, and insects that live in ponds and streams. Dragonfly nymphs have large eyes and a huge, extendable lower lip that they use to capture prey: essentially anything that moves and is not too much bigger than they are. Unlike butterflies, dragonflies have no pupal stage: the adult emerges directly from the nymph in a span of hours. This amazing transformation is a good place to start this book.
CHAPTER 2
An Excellent Design
I the course of the earth's history many different types of animals have come and gone, responding to changes in climate, geology, the availability of appropriate food sources, and the rise and fall of other species. However, some body plans are so successful that they have endured virtually unchanged for millions of years. The shark is a famous example, having similar-looking ancestors dating back 400 million years. The dragonfly is in the same illustrious group and is thought to be one of the first flying animals, appearing in the fossil record some 300 million years ago. The wingspan of some species reached an astonishing two and a half feet, probably owing to higher oxygen levels in the atmosphere and the absence of predators such as birds. These ancient giants were the largest insects that ever lived. If their nymphs were still around today we would probably think twice before going in the water!
Like all insects, dragonflies and damselflies have an external skeleton and six legs arranged in three pairs. Their bodies are composed of three parts: the head, which is dominated by a pair of huge eyes; the thorax, the power center to which the legs and two pairs of wings are attached; and the abdomen, which can bend, as it is made up of ten segments. The head is mostly used for seeing and eating, the thorax for movement, and the abdomen for breathing, processing food, and reproduction.
Dragonflies and damselflies are both members of the order Odonata, or "toothed ones," referring to their tooth-like mandibles. They have a similar body plan, but there are some important differences. Dragonflies are generally larger and stockier than damselflies. While resting, their wings remain spread, whereas many damselfly species hold them over their backs. Their eyes and head are also different: dragonflies have huge eyes that in many species touch at the top of their heads; damselflies have widely separated eyes. It is easy to overlook the humble damselfly as it slowly goes about its business inches above the water, while its more robust cousins are zooming overhead. Nevertheless, those who make the effort to observe these delicate, slender creatures will find they have a beauty—and a fierceness—all their own.
CHAPTER 3
Dew-clad Dragons
The photographs in this and the following chapters show dragonflies as they go about their daily activities. Because this is not a field guide, no attempt is made to cover all 5,500 dragonfly and damselfly species, or even the 450 species that occur in the United States. Instead, the photos aim to convey the intimate and subtle magic of dragonflies, and their brilliant colors, delicate wings, and spectacular flight. They offer a glimpse into a world of air and water and movement, occasionally interrupted by a short rest on a thin reed swaying in the breeze.
We begin on an early morning, just before sunrise. The pond is still and empty, its surface shrouded in mist. Dragonflies are nowhere to be seen, although they will soon take possession of the air. As the light began to fade the previous evening they sought out protected spots in trees and on plants, sometimes quite far from the water. There they still are, clinging to a stem or a leaf, awaiting the first rays of the morning sun.
Do dragonflies sleep? At night they are motionless, with all bodily functions greatly diminished. Because the physiology of insects is so different from that of mammals, scientists are reluctant to describe this state as sleep, and have settled on the word "torpor" instead—although an observer would be forgiven for thinking that a dew-covered dragonfly on a cold morning is fast asleep. When the sun comes up, the dragonfly will rub its eyes, removing the dew. It will slowly warm up, and as the dew evaporates and the wind picks up, it will take off for its first flight of the day.
CHAPTER 4
Masters of Flight
Watching the flight of dragonflies is mesmerizing. Although they are most easily observed when stationary, dragonflies really are creatures of the air, and their bodies are made for flying. They have a mastery of their element that is unique in the animal kingdom: they can fly upside down, stop, and change direction in the blink of an eye, hover, and suddenly accelerate to speeds of up to thirty miles per hour. Their flight appears erratic, but after a while patterns emerge: a short hover near the edge of their territory; a backward flip to grab a gnat out of the air; a sudden acceleration when a rival is spotted; a furious ball of legs, wings, and jaws when the rival comes too close. With more time, one can learn to follow their flight with a camera, stealing moments of an aerial display that has been honed to perfection over millions of years.
The flight of airplanes and birds is made possible by their curved wing shape, which generates upward pressure as air flows around it. Dragonflies use a different method: experiments have shown that they create and use turbulence in the air to get extra lift. The airflow around the wings is very complex, and its study may one day lead to new types of aircraft. Dragonflies are also able to move each of their wings independently of the other three, giving them great maneuverability. In normal flight the front wings usually lead the back wings, a method of flight called phased stroking. Other flight methods used by dragonflies are counter-stroking, with one pair of wings going down while the other goes up, and synchronized stroking, with the four wings going up and down together.
Most of the actively flying odonates above streams and ponds are male dragonflies, which are patrolling their territory or looking for females. Damselflies have relatively small wings and are much less accomplished fliers than dragonflies; they usually hover close to the surface and spend a lot of time perching on plants near the water's edge.
CHAPTER 5
Portraits
Dragonflies are most easily appreciated while they perch on vegetation during the day. Many species of dragonflies and damselflies take short flights from a favorite stem or leaf that they call home for the day. While dragonflies that are perching may seem to be at rest, they are in fact constantly scanning the sky for potential prey, rivals, and mates. Many of the most easily found dragonflies that live near ponds or slow-moving waters are in the Skimmer family, which includes species with a wide range of sizes, body types, and colors. The large, powerful dragonflies in the Darner family are conspicuous and abundant as well, but they perch only occasionally, spending nearly all their time on the wing.
The photographs in this chapter showcase the variety of dragonflies as they may be encountered in the field. Although dragonflies need water to reproduce, they can be found in forests, mountains, and meadows quite far from a stream or pond. They sometimes perch on flowers; dragonflies have little use for them but are keenly interested in the insects they attract. Most of the species in these pages can be found and approached fairly easily, and many of the details in the photographs can be seen with the unaided eye. In most photos the species and sex are indicated. Males are generally more conspicuous in the field than females: in many common species they have brighter colors, and they are the ones actively patrolling their territories.
CHAPTER 6
Hunters
Dragonflies and damselflies are carnivores and mainly eat other insects. They are not picky and will hunt any bug they can find. Mosquitoes, gnats, and other small insects form a substantial part of their diet, but they also catch larger prey such as butterflies and other damselflies and dragonflies. There is even a report in the scientific literature of a Common Green Darner dragonfly wrestling with a Ruby-throated Hummingbird! They are completely harmless to humans, however. They do not have any tool to sting with, and they have no desire to bite unless they are held. Even in that situation, most dragonflies would be incapable of piercing the skin.
Dragonflies grab their prey out of the air, with astonishing accuracy: it is estimated that 95 percent of attempts are successful, making them one of nature's most accomplished predators. The dragonfly attacks from below, tipping upward at the last moment and using its outstretched legs to grab the unsuspecting gnat or mosquito. Some species are nearly continuously on the wing, scanning their surroundings for movement. The large dragonflies lazily flying over soccer fields and picnic areas are usually members of the Darner family, and they are usually hunting. Other species perch on a branch or a leaf, ready to take off when they spot potential prey. They typically return to their perch after the hunt.
Small insects such as midges and mosquitoes are usually an in-flight snack. Once caught, they are held between the legs, which form a little basket, and chewed by the dragonfly's powerful jaws. Dragonflies eat often and fast; they can easily consume fifty or more mosquitoes in a day. Larger prey is carried to vegetation or the ground and consumed there. Although it is rare to see a dragonfly eating, the actual catch can fairly easily be observed: the dragonfly will seem to fly aimlessly, then suddenly change direction, flip, and continue on its flight. Most of the time, this means lunch has been served.
CHAPTER 7
The Next Generation
Just like many other animals, dragonflies spend a lot of time and energy to ensure the survival of their species. Males seem continuously preoccupied with finding a mate and compete aggressively, attacking rivals who enter their territories and keeping a constant eye out for females. Battles over females can be fierce, and even noisy: two males and a female can become a buzzing concentration of energy and motion as wings and legs beat against each other.
Before copulating, male dragonflies and damselflies grab the female behind her head, using claspers on the tip of the male's abdomen. The female then bends her abdomen, bringing the tip in contact with the abdomen of the male just behind the thorax. The couple now forms a wheel, which in damselflies is heart-shaped. Amazingly, many dragonfly species copulate on the wing, continuing their flight as a curious twelve-legged, eight-winged circular creature.
After copulating, females set out to lay eggs, typically hundreds at a time. Many different strategies are employed to enhance the chances that at least some of the offspring will survive. Female dragonflies in the Skimmer family usually deposit their eggs in the water and hope for the best, whereas Darners and damselflies typically carve slits in vegetation and place their eggs inside. Males have a different concern: they want to make sure they were the last to copulate with the female before she lays her eggs. Some males guard the female while she lays her eggs, hovering over her; others take no chances and hold on to the female behind her head. The eggs, floating in the water or neatly tucked away in a plant stem, are now on their own, their parents' focus once again on water, air, and movement.
CHAPTER 8
Endings, New Beginnings
Dragonflies and damselflies do not live long: the odonate life cycle in temperate climates is usually a year. Most of this time is spent underwater as nymphs, whose bodies grow and change in successive moltings. Odonates do not shed their skin in their adult stage, making do with the bodies they obtained in their metamorphosis. These bodies, and particularly the delicate wings, are not made to last. The wings get damaged and torn in fights, and it is not unusual to see a dragonfly whose wings are shredded so much that flight no longer seems possible. The lifespan of an adult dragonfly is typically only a few months. In northern climates the lives of dragonflies can also be cut short by the onset of cold weather in the fall: odonates need warmth to power their muscles.
Old age is, of course, not the only cause of death. Dragonfly nymphs are an important food source for fish, crayfish, and water birds. Many dragonflies perish during metamorphosis, falling victim to spiders and other predators during this time of complete helplessness. Birds are often attracted to ponds when dragonflies are emerging in large numbers, and in the early morning hours they feast on the immature fluttering insects. Although adult dragonflies are superb fliers, they too fall prey to birds, such as Barn Swallows.
Individual dragonflies are ethereal creatures, whose wings beat only a short time in this world. Look closely, however, and just a few feet from the floating body of the dead dragonfly a damselfly is laying eggs. The life cycle turns rapidly for odonates, in the air and unseen beneath the surface of the water. Even in the dead of winter a nymph is scurrying on the bottom of a frozen pond, preparing for a future of light, air, and warm summer wind flowing over gossamer wings.
(Continues...)
Excerpted from Dragonflies by Pieter Van Dokkum. Copyright © 2015 Yale University. Excerpted by permission of Yale UNIVERSITY PRESS.
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