I work out on a lake once a week as part of my job.  We’re managing the water quality of the lake, which involves a lot of hot, hard work and heavy lifting in the sun on a metal boat.  Sometimes the job is really awful, especially during the summer when it’s over 100 degrees in the shade and super humid thanks to the evaporation from the lake.  A few days ago, my coworker/friend and I got out to the lake as early as we could to avoid the forecasted 108 degrees.  I’m glad we got there early because we were treated to the most spectacular dragonfly display I’ve seen so far!  Take a look at the video of this behavior I posted on YouTube so you can see what it looked like:

Isn’t it fun to see so many dragonflies flying together at one time?  The swarm was made up hundreds of male dragonflies, all flying over the grass on the hill adjacent to the lake.  There were also several different species making up the swarm.  My friend (also a biologist) and I weren’t sure what the dragonflies were up to, so we speculated about what they might be doing.

One possibility was that the dragonflies were patrolling the area over the grass, protecting a territory they had set up.  Dragonflies, especially the males that made up the swarm, tend to be highly territorial.  You would usually see only one male in an area at a time, unless a male from another area is challenging the resident male.  It would thus be very unusual for so many male dragonflies to be in a single area at the same time, especially a position away from the water.  Dragonflies are known to be tricked into thinking they have found water when they have not on occasion (see my post about dragonflies patrolling over cars), but there was no reason to suspect they were being tricked en masse.  If dragonflies commonly mistook grass for water, they wouldn’t have been around for as many millions of years as they have!  The grass hadn’t been recently watered either, so they likely didn’t mistake it for water because it was wet.

When dragonflies patrol their territories, they tend to exhibit distinct flight patterns, making the same motions over and over again until something (food, mates, competitors) enters their territory and they veer off path to investigate.  The dragonflies in the swarm exhibited erratic, jerking flights, not the controlled, fluid flights of typical patrolling males.  Dragonflies of different species also tend to fly at different heights while patrolling and/or exhibit different flight patterns.  All the bugs in the swarm were  flying at the same height and using the same motions.  With all of these facts in mind, we ruled out patrolling over the grass.  The members of the swarm clearly weren’t patrolling.

We ruled out the possibility that the males were looking for mates for many of the same reasons.  A female dragonfly usually needs water to be able to lay her eggs.  She will fly out over the water, find a place that looks safe for her offspring, and mate with the male that controls that area.  If the dragonflies weren’t mistaking the grass for water, they wouldn’t be looking for mates.  Females wouldn’t come to the area over the grass to mate because the area wasn’t suitable for egg deposition.  So, mating probably wasn’t the reason for the swarm.

We eventually decided this had to be a feeding behavior.  If you looked just right, you could see thousands of little insects flying above the grass.  These little insects were buzzing around all over the place.  If the dragonflies were chasing little bugs that were flying erratically, they would be flying erratically themselves.  Also, there really wouldn’t be any reason why they should be over the grass instead of the water unless there was some benefit to their being there.  Because they find mates at the water, each moment a male dragonfly spends away from the water represents a lost chance for mating.  However, if there were a ton of food available on the hill that wasn’t available over the water, there was a benefit to flying over the grass.  If the food was particularly abundant, i.e. there was enough to go around, it would also explain why the dragonflies were tolerating one another and not chasing their competitors away.  It had to be a feeding swarm.

We collected a few dragonflies from the swarm (MUCH easier to catch in the swarm than as individuals over the water!), watched the swarm for a while, and then got to work.  It was a hot, miserable day on the lake, but my friend and I agreed it had been worth taking the time to watch the dragonflies.  It ended up making up for the discomfort of the work.

I looked into the swarming behavior when I got home from work and learned that it is indeed common for some dragonfly species to fly in large feeding swarms like the one we saw.  Looking into it further, I learned that the four species I was able to identify within the swarm are the ones most commonly known to make these sorts of feeding aggregates.  I hadn’t ever read about dragonfly swarms or seen one before, but it was gratifying to know that my friend and I were able to figure it out on our own, simply by thinking about what we knew about dragonflies and their behaviors.  It just goes to show that the more you know about an insect’s behavior, the better you are able to explain new behaviors you haven’t seen before!  Regardless, I was ecstatic to see the swarm!  It was an amazing experience and one I will remember forever.

In my next post, I’ll go over the species we found and how to identify them so you’ll know some of the species to look out for if you ever come across a dragonfly swarm yourself.  The dragonflies that made up this swarm are easy to tell apart.  In another post, I’ll talk about what we think the dragonflies were eating and the evidence we found for it.  Stay tuned!

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Have you seen a dragonfly swarm?

I am tracking swarms so I can learn more about this interesting behavior.  If you see one, I’d love to hear from you!  Please visit my Report a Dragonfly Swarm page to fill out the official report form.  It only takes a few minutes!

Thanks!

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Want more information?

Visit my dragonfly swarm information page for my entire collection of posts about dragonfly swarms!

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