Here be dragonflies

Yes, yes, I know.  “More insects?” you say?  Well…uhm…yes.  I was out along a creek near the Catawba River again yesterday.  As I was looking around for salamanders (and listening to Red-eyed Vireos in the trees), I got distracted by something in the stream.  I had actually walked to a little hollow to peer in and turned around to re-trace my steps when I saw a stick-like thing bounding in a shallow pool in the stream.  While it was the middle of the afternoon, at the stream, the light was all dappled due to the tree cover.  It took a minute for my eyes to figure out what I was seeing.

When my eyes finally came around, I tried to figure out what I was observing.  My first thought was that there was a branch bouncing in the breeze.  There were two problems with that hypothesis:  1.  No breeze; and 2.  The bouncing thing wasn’t attached to anything.  My second thought was that there was something caught in a spider’s web.  I moved in closer and saw a black and white (or yellow) pattern on the animal.  It wasn’t a bee, though, as the “stick” was probably 4 or 5 inches long.  It sure began to look like a dragonfly.

When I got within about 4 feet of it, I could definitely see a dragonfly.  What I couldn’t see was any webbing.  The wings were also working so hard that they were a blur.  This animal was bouncing up and down over the water, the abdomen tip just hitting the top of the water repeatedly.  That’s when it hit me (a realization, not the dragonfly)!  She was laying eggs.  I’ve seen a lot of dragonfly mating (no, I’m not a pervert), but I’ve never actually watched a female lay eggs, but that’s exactly what she was doing.  Well, at least that’s exactly what I thought she was doing.  I had to come home and double-check to see if there were any other possible explanations for that behavior.  What I found was pretty cool.  First of all (and here’s a new one for me), I found a youtube video that shows a dragonfly in Ohio doing the same thing that I saw.  It’s short and worth seeing.

Youtube Spiketail Ovipositing.

As I was digging around trying to identify the dragonfly (she was quite large), I came across some other interesting information on the factors that are important for oviposition sites in dragonflies.  Females apparently prefer areas with shallow, warmer water.  The benefit to that is that warmer temperatures speed up development of the larvae.

Wolf, L.L. and E.C. Waltz.  1988.  Oviposition site selection and spatial predictability of female white-faced dragonflies (Leucorrhinia intacta) (Odonota: Libellulidae).  Ethology 78:  306-320.

There were also some strange, surprising papers that described some of the other factors that are important for dragonflies making their oviposition decisions- factors that have been used to explain why the sometimes oviposit on cars and highways.  Dragonflies do have a preference for horizontally polarized light.  Light from the sun bounces off substrates (like water and oil) in very particular ways.  This reflection creates a particular polarization, or angle to the light.  Anyone who has ever worn polarized sunglasses and looked at the surface of water can understand what this concept does.  Polarized sunglasses filter out some angles of light and allow the user to see past the reflections from the surface of the water and to see into the stream.  Well, dragonfly eyes are keen on a particular pattern of horizontal polarization and they cue in on that pattern when making their egg-laying decisions.

Wildermuth, H.  1998.  Dragonflies recognize the water of rendezvous and oviposition sites by horizontally polarized light: a behavioral field test. Naturwissenschaften 85:  297-302.

Unfortunately, water isn’t the only thing that produces this kind of polarization.  Oil and even the protective coating on car paint also produce this kind of polarization pattern and those things do attract dragonflies.

Horvath, G., B. Bernath and G. Molnar.  1998.  Dragonflies find crude oil visually more attractive than water:  multiple-choice experiments on dragonfly polarotaxis. Naturwissenschaften 85: 292-297

As the spring and summer move on and more dragonflies are hanging around, just keep this post in mind.  When you’re in the mall parking lot and they’re diving around your car, they’re not angry or attacking you (contrary to their name, dragonfly adults are quite nice), they’re just trying to lay eggs on your car.  Isn’t that a comforting thought?




About thomasbiology

I'm an Associate Professor of Biology at Queens University of Charlotte with a background in animal behavior with an emphasis in bird song. I've got two secret goals with this blog (well, since I'm sharing them, they're not so secret): 1. To encourage people to look at the natural world around them- not just as a hiking destination, but to notice all the little things moving around them all the time; and 2. To show some of the science that relates to these little things moving around. There's some really fascinating research out there that so few people get to see.
This entry was posted in conservation, insect, invertebrate, mating, reproduction. Bookmark the permalink.

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