Thursday, August 16, 2012

Invasion of the Blue Dasher Dragonflies

Blue dasher dragonfly.  Are those aviator sunglasses?
The outdoor classroom and back field of our school has been a-buzz with these miniature blue helicopter-like insects.  They are a type of dragonfly called blue dashers (scientific name Pachydiplax longipennis, which means double-thick long wings, by the way), and they are beautiful!  The one above is an adult male, which you can tell from the bluish color of his abdomen.  Females and young adult males have brownish-black abdomens.  Both males and females have brown and yellow wavy stripes on their mid-section, or thorax.  Blue dashers all have a white head with bluish-green eyes.  In the picture above, the white patch on the head is the upper and lower mandible, which the dragonfly uses to bite its prey.

Dragonfly eyes are amazing.  They cover most of the head and provide the dragonfly with a 360 degree view of their world.  Dragonflies need excellent vision of their surroundings because they can fly and maneuver so quickly.  If they couldn't see in all directions, they'd constantly be smashing into things.  Dragonflies fly quickly because they need to catch their prey, which consists of insects that can also fly.  Blue dashers specialize in mosquitoes, gnats and flies, but they also eat butterflies and grasshoppers.  

Blue dasher demonstrating its helicopter shape, with a long abdomen to balance the head.
Since dragonflies are so big and fast, sometimes people are afraid of them.  The good news is that dragonflies are harmless to humans.  They do not bite or attack humans, and the long, pointy abdomen does not sting.  The shape of the abdomen serves as a counter-weight to the head, keeping the body of the insect level as it is suspended by its wings in flight.  If you look at the picture above, you can see how the head and tail appear to balance each other out under the wings.  Blue dashers are particularly useful on the school grounds because they keep other biting insects away from the kids' outdoor areas.

All dragonflies lay their eggs in water, and we probably have blue dasher nymphs in our pond at school.  After being deposited in the water, dragonfly eggs hatch into nymphs (underwater larvae), which grow and eat other aquatic insects and tiny fish until they are big enough to metamorphose into adults.  When a dragonfly becomes an adult, it crawls out of the water onto a stem or rock, splits its exoskeleton, and drags itself out of the shed skin as a fully-formed adult.  Blue dasher nymphs are particularly tolerant of poor water conditions, which makes them ideal for living in an urban environment.  They are widespread around the United States and in southern Canada in all variety of habitats. 
Dragonfly nymph exoskeleton. (Photo: Mary Entrekin Agee)

No comments:

Post a Comment