October 3, 2011

Fall Dragonflies

By Naturalist Eric D'Aleo

In the past few weeks, presence of late summer and autumn dragonflies have reminded me that fall is rapidly approaching. Many late season species emerge from their aquatic nurseries at this time of year to become adults, and like their earlier brethren, they are amazing to watch.

Dragonflies are among the most ancient of flying insects and have been in the fossil record for 350 million years. They are classified in the order Odonata, along with the similar looking damselflies, with over 5,000 species found around the world. The number of dragonfly species in North America drops to 450, with just over 100 species found in New Hampshire. Although dragonflies found as fossils had wingspans up to 31 inches, today a large dragonfly has a 5 to 6 inch wingspan. Even at this size, these colorful, acrobatic flying insects with their bulging eyes, heavily veined wings, long slender abdomens, and curious habits seem as creatures from the distant past.

Adult dragonflies have a reputation of being voracious predators, capturing other insects in flight, earning the nickname of “mosquito hawk”. Dragonflies use their sharp eyesight and impressive flying ability to catch their prey on the wing including insects such as flies, mosquitoes, black flies and gnats. By holding their bristle covered slender legs in a curved funnel, they form a basket that allows them to scoop a flying insect out of the air and trap them there until the dragonfly grabs its prey with its jaws. Dragonflies eat large amounts of insects that humans find problematic, and a single dragonfly may eat as many as 600 insects in a day.

An interesting behavior I’ve observed is the formation of a swarm of 50 darner dragonflies that were feeding in my yard over Labor Day weekend. The large insects were cruising at various heights catching winged ants, mostly males and some potential queens, which were emerging from the ground in large numbers. My family and I gathered on the back steps to watch for over an hour as the dragonflies swooped, hovered, and dove. They grabbed each ant, allowing none to escape, until the numbers of ants became so numerous that several were able to flee undetected by their aerial predators. Although dragonflies are usually solitary hunters, some types of dragonflies have a habit of feeding in swarms at this time of year and are often seen flying over fields, pastures, and lawns.

As the daily temperatures in autumn cool, dragonflies become easier to observe and watch because of their inability to maintain a constant body temperature. I’ve noticed several dragonflies clinging to the side of my house or perched in a nearby sapling early in the morning or late in the afternoon, absorbing the heat of the sun on their wings like solar panels as they attempt to keep warm. The flight muscles of a dragonfly account for just over 60 percent of their body weight and allow certain species to reach flight speeds of 35 miles per hour. Other types of dragonflies, such as the green darner, have some populations that migrate to warmer locations in the fall.

People are sometimes concerned about dragonflies that are flying near them thinking they might “sting”. That idea might come from the historical name “horse stinger” that was given because people observed dragonflies darting around the horses believing that they were stinging the animals. Actually the dragonflies were snatching horseflies flying around the animals. A recent experience I had on an early September evening suggests that some species may be curious. I went out to close my greenhouse for the night when I discovered a fawn dragonfly, a species I hadn’t seen before, trapped in a corner. I was able to free it by carefully taking hold of the two pairs of membranous, netlike wings to remove it from its perch and set it free, but not before it landed on me and flew into my home. It seems that this species explores many objects that it encounters, including dragonfly watchers.

Although it may seem too late in the year for dragonflies to breed, many species that emerge in the fall as adults are breeding. Most dragonflies breed around permanent bodies of water which may include ponds, marshes, shallow areas of streams, slow parts of rivers, and on lake margins. Males establish a territory and aggressively defend it from other males, while attempting to breed with every female that enters his territory. Mating dragonflies are often observed in tandem in a head to tail position before the female lays her eggs in the water, on floating vegetation, in the mud, or above or below the waterline. Depending on the species of dragonfly, a female may lay 100 eggs in a day and over 1000 during the few weeks or months they have to breed.

Over the winter and during next spring the eggs will hatch and the larvae will feed and live in an aquatic environment going through 8 to 15 gradual stages of growth, which they may complete in less than one year or as many as almost five years. Once the last stage of transformation approaches, the dragonfly nymphs will crawl out of the water onto plant stems, sticks, or other structures above the water to complete their emergence as adult dragonflies and the process will begin again. Keep a lookout for these late autumn insects this fall.

For those interested in identification of dragonflies in NH check out the following sites.

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