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Only a few creatures that we know today also lived when dinosaurs roamed the planet. Dragonflies are one of them – in fact, they existed 100 million years before the dinosaurs.
Our ancient atmosphere contained much more oxygen than today’s air, and giant insects abounded. (Insects do not have lungs like we do. They breathe through holes in their outer shell. When the atmosphere held more oxygen, it was easier for large insects to absorb all the oxygen their larger bodies needed.) The largest of these ancient insects were dragonflies – some with 2.5-foot wingspans. Needless to say, in our comparatively oxygen-deficient atmosphere of today, dragonflies are much smaller.
The eastern amberwing (Perithemis tenera) is one of the world’s smallest dragonflies at only about an inch long. Males have orangish-yellow (amber) wings. Females have clear wings with dark blotches. Both have a red patch on each outer wing that are thicker than the rest of the wing and help with flight: Without the thickened patches, at high speeds the wings would flutter; tests have shown that with these thickened patches, dragonflies’ speed increases by up to 25 percent.
Dragonflies live two very different lives – terrestrial and aquatic. Terrestrial adult fliers live only long enough to mate and lay eggs in water. The larvae that hatch from the eggs are aquatic and live in water for up to several years, growing larger and larger as they molt again and again.
The larvae are called nymphs. They have a pair of fanglike pincers that they use to snatch prey as it swims past. Nymphs do not breathe air like adult dragonflies but instead absorb oxygen from the water using gills inside the anus. The nymph draws water in and out through the anus to breathe. It can also forcibly expel water from its rear end when it wants to move quickly (to escape predators, for example). Nymphs can propel themselves around like underwater Jet Skis.
When a nymph is ready to become an adult flier, it lifts its head out of the water to adapt to breathing air. It climbs onto a plant stem, the skin along its back splits open and an adult dragonfly emerges. The dragonfly swallows air to plump up its body, pumping hemolymph – insect blood – into its wings.
Dragonflies have four wings, which move independently from one another. By changing the angle of each wing, these insects can move forward, backward, sideways, up or down. They can hover or spin. Because of their agility (and amazing eyesight), dragonflies catch 95 percent of the prey they hunt. Unlike damselflies, dragonflies like to catch their prey in the air as they prey upon many insects – but they are especially adept at devouring mosquitoes and midges. (Adult dragonflies prey upon adult mosquitos in the air; dragonfly nymphs prey upon mosquito larvae in the water.)
What makes dragonfly sight so remarkable? Each eye is made up of 30,000 facets, or photoreceptor cells. Each facet sees in only one direction, but there are so many arranged over its bulging eyes that dragonflies can see in almost every direction. Dragonflies can detect light signals coming from all directions better than the best lenses on the best cameras today. Its brain combines the images from each facet into one large image. Because vision is so important to dragonflies, about 80 percent of their brains is dedicated to sight. Good luck trying to sneak up on one. Your pathetic vision is no match for theirs!
If you have ever walked along the edge of a pond, you might see dragonflies perched on vegetation, doing a sort of headstand, with their abdomens sticking straight up. This is called obelisking. It helps dragonflies regulate their body temperature. Dragonflies lower their wings horizontally like an awning to shade their body, exposing just the tip of their abdomen to the sun. This helps them cool themselves. (Damselflies, on the other hand, can fold their wings along the length of their bodies – and by looking at the way wings fold, you can tell a dragonfly from a damselfly quickly.)
Dragonflies have a complex reproductive system involving two sets of genitalia. Male dragonflies produce sperm at the rear of their abdomen. But the penis is at the other end, near the thorax. (Insects have three body parts: a head, a thorax that the wings and legs are attached to, and an abdomen.) Before mating, the male has to curl the tip of his abdomen around almost to the thorax to transfer sperm to his penis. Now he is ready to mate!
The male uses claspers at the end of his abdomen to grab the head of the female, whose genital opening is at the end of her abdomen. She curls her abdomen around to connect with the male’s penis, near his thorax. The two entwined bodies form a circular shape. Dragonfly copulation is brief – a matter of seconds – so it is difficult to recognize what is happening. (Damselfly copulation, on the other hand, can last for hours.)
To ensure he is the father, the male remains near the female after mating, guarding her until she lays her eggs on the water’s surface. He chases off intruding males because he wants to be certain the female does not mate with another guy – who could use his uniquely shaped penis to scrape out the sperm of the first suitor. (According to The New York Times, dragonfly penises are “multijointed contraptions resembling high-tech prosthetic legs accessorized with horns, brushes, hooks, grabbers, spreaders and other implements designed to effect both the deposit and the retrieval of sperm.”)
Dragonflies are a joy to watch as they hover, sprint, perch, chase each other and dart about. Although they have existed for 300 million years, the loss of wetlands in many areas threatens their continued existence. Sixteen percent of dragonflies are now threatened. Let’s hope humans manage the Earth thoughtfully enough to allow them to exist for another 300 million years.
Reader photo
Joshua Preston spotted this black squirrel with a brown tail on Memorial Drive on June 19.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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