Dragonflies

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Dragonflies can be seen at IslandWood in the spring, summer and fall.  They are most frequently seen at Mac’s Pond but can also be found cruising across the various fields on campus.  They will land on people at times and also like to perch on the railing of the floating classroom. 

Present on earth for about 300 million years, today’s dragonflies have an average wingspan of about 2-3 inches.   Fossil dragonflies have been found that were much larger, with wingspans of up to 24 inches. 

Dragonflies belong to the Order Odonata, which means “toothed one”.  They have serrated teeth that they use to kill and eat their prey.  Dragonfly prey include a variety of flying insects as well as pond macroinvertebrates. I have personally seen them pluck macroinvertebrates from the surface of Mac’s Pond.

Dragonfly larvae can be found in Mac’s pond.  In their larval stage, which can last for several years, they eat just about everything in the pond, including other dragonfly larvae.  When they reach the adult stage the larvae climbs out of the water and sheds its exoskeleton.  The long abdomen and four wings “unpack” and harden over the next couple of hours.  Dragonflies are extremely vulnerable during this time.

Prominent features of dragonfly morphology are a long abdomen, four transparent wings and extremely large eyes on the head. The long abdomen is not a stinger and dragonflies are harmless to people.  In fact, dragonflies are extremely helpful to humans, eating many times their weight in mosquitoes, flies and other bugs each day.

Dragonfly wings are capable of independent operation which allows the dragonfly to hover in mid-air and move in any direction, including backwards and flying upside down.  Dragonflies can fly up to 30 miles per hour.

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Dragonfly eyes are made up of many individual facets and the dragonfly is able to see in a radius of 300 degrees.  Although the vision at the distal margins is less than that of the medial line, the dragonfly really does have “eyes in the back of its head”.  Furthermore, its vision includes three red, green and blue light plus ultraviolet and polarized light, all of which makes it an extremely effective hunter in flight.

Ferocious eaters, it is believed that dragonflies capture up to 95% of their prey.  Unlike many animals which merely chase down their prey, dragonflies actually engage in three-dimensional calculations and intercept their insect prey in mid-air. 

Males are territorial, defending prime territory near water sources because females select males based on whether their territory contains a water source. Dragonfly mating is a rough, aerial affair and frequently the male has to first scrape off the sperm of another dragonfly before depositing its own.  If you see two dragonflies that appear to be attached to one another in a roughly circular shape they are probably mating mid-air.

Dragonflies are ectothermic and require a certain amount of heat energy to fly.  They can sometimes be spotted sunning themselves to build up energy.  When they get too hot, they turn their abdomen almost straight up towards the sun and appear to be almost standing on their head.  This is known as the obelisk pose and designed to minimize their body surface area in contact with sunlight.

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Dragonflies have been featured on many stamps.  17th-century Japanese warriors viewed them as a symbol of strength.  Dragonfly shows up as a character in several Navajo, Hopi and Zuni legends. While most people react with revulsion or disgust to insects, this instinct does not normally apply to the dragonfly.  The Green Darner dragonfly is the state insect of Washington State.


Japan and England have created dragonfly sanctuaries and the City of Albuquerque in New Mexico hosts a dragonfly sanctuary pond.

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Brian Carpenter, EEC '17

Sources:

http://www.pnas.org/content/110/2/696.abstract?sid=f4fdddf7-bf52-4ecc-9e7f-1142102e0f8c

http://www.nytimes.com/2013/04/02/science/dragonflies-natures-deadly-drone-but-prettier.html?_r=0

http://www.smithsonianmag.com/science-nature/14-fun-facts-about-dragonflies-96882693/

http://www.cabq.gov/culturalservices/biopark/garden/exhibits/dragonfly-sanctuary-pond

http://integraonline.com/~pondhawk//odonata/ips_odonata.html