Dragonfly

A dragonfly catches the prey it chases about 95 percent of the time. A lion manages roughly one hunt in four. A great white shark does worse.

This makes the dragonfly one of the most successful predators on Earth, and it does it with eyes that cover almost its whole head and four wings it can move independently.

It also spends most of its life as something else entirely. The flying adult is the final few weeks of an animal that lived underwater for years.

Dragonflies form the suborder Anisoptera, with around 3,000 species, so there is no single two-word name.

• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Odonata
• Suborder: Anisoptera

The order Odonata also contains damselflies, which are the slimmer, more delicate relatives. The quickest way to tell them apart is at rest: a dragonfly holds its wings out flat to the sides, while a damselfly folds them along its back.

Dragonflies are ancient. Their ancestors were flying 300 million years ago, long before dinosaurs, and in an atmosphere with far more oxygen some had wingspans of over two feet.

Three things together, and no single one explains it.

The eyes. A dragonfly’s two compound eyes wrap around its head and contain up to 30,000 individual lenses each, giving it nearly 360-degree vision. It sees colors we cannot, including ultraviolet, and it detects movement at a rate far faster than our eyes manage, so to a dragonfly our world would look slowed down.

The wings. Four wings, each driven by its own muscles and moved independently. That lets a dragonfly hover, fly backwards, fly sideways, turn on the spot and stop dead in the air. Most insects simply cannot do this.

The interception. This is the clever part. A dragonfly does not chase its prey. It calculates where the prey is going and flies to meet it there, adjusting as it goes, which is the same problem a fighter pilot solves. It attacks from below and behind, in the blind spot, and the prey often never sees it.

Researchers studying how a brain the size of a pinhead does this have used dragonflies as models for designing drone guidance systems.

Dragonflies measure 1 to 5 inches long with wingspans from under an inch to about 7 inches, and weigh under a gram.

• Eyes. Enormous, meeting at the top of the head in most species, with up to 30,000 lenses apiece.
• Wings. Transparent and veined, often with a dark spot near the tip that steadies them at speed.
• Legs. Held forward in a basket to scoop prey from the air. A dragonfly cannot walk properly; the legs are for catching and perching only.
• Abdomen. Long and thin, acting as a rudder.
• Colors. Often brilliant, produced partly by pigment and partly by structure in the body wall.

They are fast, reaching around 20 to 35 miles an hour, which is exceptional for an insect.

Dragonflies live on every continent except Antarctica, always near fresh water, because the young stage is aquatic. Ponds, lakes, marshes, streams and ditches all suit them.

One species makes the longest migration of any insect. The globe skimmer crosses the Indian Ocean between India and Africa, a journey of around 4,400 miles, riding high-altitude winds and breeding in temporary rain pools along the way. No single individual completes the round trip; it takes several generations.

Because they need clean water to breed, dragonflies are often absent from polluted sites even when everything else looks fine.

Dragonflies are carnivores at every stage of life, eating mosquitoes, midges, flies, moths, butterflies and other dragonflies.

Adults catch prey in flight with their basket of legs and eat on the wing, and a single dragonfly may take hundreds of mosquitoes in a day.

The underwater young are more alarming. A dragonfly nymph has a hinged lower jaw folded under its head that shoots out in milliseconds to grab prey and pull it back. It takes insect larvae, tadpoles and small fish.

Some nymphs also move by jet propulsion, squirting water out of the rear end to shoot forward.

Adult dragonflies are active by day and need warmth to fly, so they bask in sunshine and are rarely seen in cold or dull weather.

Males are territorial, patrolling a stretch of pond edge and driving off rivals, sometimes in aerial fights.

They can regulate their temperature to a degree: whirring the wings to warm up, and in heat positioning the abdomen to point at the sun so less of it is exposed, a posture called obelisking.

Despite the name and the old stories, dragonflies do not sting. They have no sting at all, and are harmless to people.

The flying adult is the short final chapter of a much longer life.

Eggs are laid in or near water. The nymph that hatches lives underwater for anywhere from a few months to five years, depending on species, molting many times and hunting all the while.

When it is ready it climbs out of the water up a plant stem, its skin splits, and the adult pulls itself free, pumps up its wings and flies. The empty nymph case is left clinging to the stem.

Dragonflies do not have a pupa stage like butterflies. They change gradually through molts rather than reorganizing inside a cocoon.

The adult then lives only a few weeks to a few months. Years underwater, weeks in the air.

Birds, frogs, spiders, fish and larger dragonflies all eat them, and the moment just after emerging, when the wings are still soft, is the most dangerous of a dragonfly’s life.

The main human threat is the loss of wetlands and clean fresh water. Draining ponds, polluting streams and removing water plants destroys the habitat the nymphs need.

Pesticides harm them directly and remove their prey, and introduced fish eat nymphs in ponds that previously had none.

There is no single status across around 3,000 species, and most common dragonflies are doing reasonably well.

Globally, though, roughly one in six dragonfly and damselfly species is threatened, mainly through the loss of wetlands, which have disappeared faster than almost any other habitat type.

Garden ponds genuinely help. A small pond with plants and no fish will attract breeding dragonflies within a season or two, which is one of the easiest pieces of wildlife conservation anyone can do.

Dragonflies are serious controllers of biting insects, at both stages of life. Adults take mosquitoes out of the air in large numbers, and the nymphs eat mosquito larvae in the water, which is arguably the more useful half.

They are also prey themselves, feeding birds, frogs and fish, and they move energy from the water into the air: a nymph that ate underwater for years flies out and is eaten by a bird.

Their greatest practical value may be as indicators. Dragonflies need clean, well-oxygenated water with plants in it, so counting the species at a pond gives a quick and reliable read on its health. Ecologists survey them for exactly that reason.

A pond full of dragonflies is a pond that is working.