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What is a dragonfly?
Whether delicately perched on a cattail or hovering over a pond, dragonflies are a sure sign of summer.
Living on every continent but Antarctica, these insects are instantly recognizable by their large bodies; four long, horizontal wings; and the way they hover and zip around. Dragonflies can reach speeds of up to 35 miles an hour and fly just as gracefully backward by lifting off vertically, helicopter style. Their compound eyes are so large they nearly touch, and each one has about 28,000 single eyes, or ommatidia.
Around 7,000 species of true dragonflies are alive today, and, together with the closely related damselflies, they form the group Odonta, which goes back 325 million years.
Dragonflies begin their lives in the water. Around midsummer, females hover over bodies of fresh water and dip in their abdomens, laying eggs that hatch in about seven to eight days. Called naiads or nymphs, these larvae stay in the water for up to three years, hunting aquatic insects, mosquito larvae, or small fish with a lower jaw that reaches out and opens up sideways, snatching prey into their mouths. They may even cannibalize other dragonfly nymphs.
Nymphs propel themselves quickly by sucking water into their abdomen and then spitting it out. They undergo between six and 15 molts before emerging as fully formed adult dragonflies, a process that takes about 12 hours. Adults live only about a month, hunting flies, mosquitos, and midges. Their arms slant upward, forming a type of basket and enabling them to scoop up prey in flight.
Adults also look for opportunities to mate. First the male goes through what’s called self-insemination, curling his body to transfer sperm from his primary genitalia, at the tip of his abdomen, to the secondary genitalia, just below the thorax.
When he finds a receptive female, he grasps her behind her head with claspers at the tip of his abdomen, and the two fly off together.
When they land, the female curls her abdomen under to meet his secondary genitalia, forming a “mating wheel” that can resemble the shape of a heart. (Sometimes they will mate in the air.) Mating takes just a few seconds, after which the female can immediately lay her eggs.
Dragonflies are important to their environments both as predators (particularly of mosquitos) and as prey to birds and fish. Because these insects require stable oxygen levels and clean water, scientists consider them reliable bioindicators of the health of an ecosystem.
In 2009, the first comprehensive assessment of insect species showed that 10 percent of dragonfly species were under threat of extinction. The animals are imperiled by destruction of freshwater habitats—particularly ponds, bogs, and fens—by pollution, and non-native vegetation.
For example, the yellow presba, a dragonfly native only to South Africa, is losing its grassland habitat to the Australian black wattle, a commercially introduced tree. The fast-growing invasive species blocks out sunlight that sustains the grasses. The International Union for Conservation of Nature now considers this dragonfly vulnerable to extinction.