From the Literature: Jocks and nerds in the damselfly world (The Dragonfly Trilogy, Part Three)

Welcome to the third and final segment of the Dragonfly Trilogy – and another installment of From the Literature!  If you don’t know anything about dragonfly territoriality, I recommend reading part two of my trilogy for more information on how dragonflies and damselflies set up and defend territories.  You’ll get more out of this post if you know something about territoriality before diving in.

Last time, I discussed how odonates benefit from being territorial, why they set up territories and defend them from other males.  Like many things in biology, it boils down to sex: males that defend high quality territories generally mate with more females than males with low quality territories.  Likewise, males that defend territories generally mate with more females than males that do not defend territories.  It is usually better to be a male with a territory than a male with no territory, but there are often many more males than there are available territories and some males are inevitably left out.  Presumably the stronger, better males (the most fit males) end up successfully claiming and holding territories while the weaker, wimpier males are left without territories and become wanderers.

Calopteryx virgo male

Calopteryx virgo male. Photo from Wikipedia, by Michael Apel.

This is the situation that a group of researchers  in Finland recently investigated.  They chose to study the damselfly Calopteryx virgo, a European damselfly also known as the beautiful demoiselle.  This gorgeous creature is pictured at left and is one of the damselflies known to be territorial.

The researchers asked a simple question: are C. virgo males that defend territories larger than males that do not defend territories?  They wanted to know if the damselflies that were able to protect a territory from other males were somehow better suited to being territorial than the damselflies doomed to be wanderers and unable to gain their own territory.  They also wanted to know if this changed over time, whether the non-territorial males eventually became territorial.

To answer these questions, the researchers captured mature C. virgo males arriving at their study stream in Finland, then marked them (so they could tell them apart), measured their right hindwings, weighed them, and released them back into the study area.  They observed the damselflies on two different days ten days apart and determined whether the males were territorial (they stayed within a small, 2 meter area for at least three hours) or non-territorial (they did not remain within a 2 meter area).  Males were considered wanderers if they moved more than 100 meters during the observational periods.

The team discovered that there was a significant difference in size between territorial and non-territorial males.  Territorial males had longer wings and were heavier than wandering males, so the bigger males were the ones claiming and holding territories.   The researchers also discovered that time didn’t have much to do with whether a damselfly male was territorial or not.  The wing length and weight of the wandering males was about the same and wandering males were consistently smaller than the territorial males on both days.  Wandering males made up about the same percentage of the population both days too.  So, the smaller males weren’t ever getting territories and were consistently being excluded.

The data that Koskimaki and colleagues presented in their paper suggest that the bigger, more physically impressive males get more mates.  To better understand the significance of these results, let’s consider a similar situation in humans that many people will recognize.  Think about what you know about stereotypical high schoolers.  Who gets more dates: the jocks (usually the bigger, more physically impressive males) or the nerds (usually the smaller, less physically impressive males)?  When I was in high school (I myself was firmly rooted in the nerd category!), the jocks got most of the girls while the nerds usually only admired the girls from afar.  The jocks outcompeted the nerds physically, and because they were generally more attractive, they excluded the nerds from finding dates by hogging all of the available girls.  If you ignore the possible confounding affects of wealth, intelligence, and overall personality that come into play in human mating behaviors, almost the same thing is happening in the high school students that we see in the damselflies that Koskimaki and colleagues studied!  In effect, the jocks among the damselflies were getting all the girls because they were better suited to protecting territories, and thereby attracted more mates, than the nerds who were unable to gain a territory.

I’ll end with two important questions: if it is so much better for males to be bigger so that they can more successfully hold valuable territories, 1) why are any males small and 2) why aren’t the damselflies getting bigger and bigger over time?  Koskimaki and colleagues suggest that that territorial and non-territorial males might form two distinct subgroups within the damselflies, each with their own strategies and goals for mating.  Even males without territories are able to mate with some females.  They could end up with the same number of offspring, thus ensuring the continued existence of smaller males in the population, if they have means for compensating for their relative lack of mating opportunities.  The team cites several other studies that suggest that this is happening in several territorial damselfly species, that non-territorial males are equally successful in producing offspring compared to territorial males.   It is likely that there are some benefits to being smaller or some costs to being larger that have not yet been accounted for.  Further studies in this area would be a great avenue for future research!

I hope you’ve enjoyed the dragonfly trilogy!  It’s been a lot of fun delving into the dragonfly literature for a few weeks and sharing information about my favorite group of insects.  I’m sure to post more odonate research in the future, but next time I’ll be telling a story of a centipede and a woman who is very, very scared of them – me!

Literature Cited:

Koskimaki, J., Rantala, M.J., & Suhonen, J. (2009). Wandering males are smaller than territorial males in the damselfly Calopteryx virgo (L.) (Zygoptera: Calopterygidae). Odonatoligica, 38 (2), 159-165.

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Text copyright © 2010 DragonflyWoman.wordpress.com

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Color Polymorphisms in Dragonflies

In my last post, I talked about the dragonflies that my fiancee and I saw on our recent photographic trip to Sweetwater Wetland in Tucson.  We took a lot of photos of dragonflies and as I went through them, it occurred to me that there is an important topic related to identifying dragonflies that I have not covered so far in my blog: odonate polymorphisms.

If you’re like most people (including me until I’d been an entomology grad student for almost a year), you’re now thinking to yourself, “what is a polymorphism?”  You probably already know more about polymorphisms than you might think!  First, let’s take apart the word to define it.  The root “poly” means many.  Think back to geometry and polygons – a polygon is a shape with many sides.  The root “morph” means shape or form.  So, the word polymorphism effectively means “many forms.”  Form can refer to shapes, colors, sizes, and other characteristics of biological organisms that might vary between stages, sexes, or individuals within the same species.

You probably know about a few species that exhibit polymorphisms already.  Birds are a classic example of sexual dimorphism (di = two, so dimorphic species exhibit two forms): males take one form while females take another.  In birds, the males are often one color while the females are another.  In most cases, the males attract the females, so the males are the more colorful, showy individuals.  In general, the more brightly colored males are healthier and better able to produce strong offspring with a high chance of survival, so females will choose mates that are brightly colored over less colorful males.

Peacock and peahen

Peacock and peahen

As an example, consider peacocks.  Peacock males are VERY showy with their long, elegant tails while the female peahens are much less colorful and have much shorter tails.  Peacocks with the biggest, baddest tails get all the girls while the less showy males have to settle for leftovers (sometimes younger or less healthy females) or simply cheat their way into getting a mate.

Other animals that show dimorphisms are deer and elk.  Again, the males are trying to attract the females and are willing to fight other males for them.  Elk or deer with big racks are generally better able to successfully fight other males.  As in birds, the buck with the biggest rack is likely healthier than the bucks with smaller racks – they have to be getting enough food and other resources to grow those antlers in the first place.  The doe chooses her mate from the available males and usually selects the one that is best able to win fights, the one with the best antlers.  Because the females are not fighting amongst themselves and are not trying to attract the males, a doe doesn’t need antlers.  So, some elk and deer have horns (the males) and others do not (the females).  They are also sexually dimorphic.

Now let’s get back to the dragonflies.  Like in birds, elk, and deer, it is the role of the male dragonflies to attract female dragonflies if they wish to produce offspring.  Thus, male dragonflies are often much more brightly colored than the females and many species are sexually dimorphic.  Take, for example, the blue dashers, or Pachydiplax longipennis.  If you follow my blog, you’ve seen this one before, but here he is again in all of his elegant glory:

Blue dasher (Pachydiplax longipennis) male

Blue dasher (Pachydiplax longipennis) male

Blue dashers are very common mid-sized dragonflies across a big section of the United States, including Arizona.  The males are easy to identify based on their bright green eyes and the bluish coloration of their bodies.  The abdomen is covered in a waxy substance, or prunescence, which can give them a bit of the whitish look you see in the male pictured here.  Blue dashers are perchers, so you’ll commonly find the males sitting on emergent vegetation or on bushes and/or other plants alongside lakes and ponds.  They sit and guard their territories from their perches, waiting for females to come into their areas so they can mate.  In contrast, the females are known to spend a much greater part of their time away from water and only come to the water to mate and lay eggs.  I found this female sitting on a tree branch far from the water:

Blue dasher (Pachydiplax longipennis) female

Blue dasher (Pachydiplax longipennis) female

Can you see how different the female looks compared to the male?  The males are a whitish bluish color while the females are largely black!  The female blue dashers are about the same size and of similar shape compared to the males, but they have very different body colors, making this a dimorphic species.  Female blue dashers are also very easy to identify.  Just look for a black abdomen with yellowish or brownish stripes on each section of the abdomen.  The abdomen tends to be a bit stumpy compared to the male abdomen, so the wings look disproportionately large.  In fact, this is the origin of the species name longipennis, which means “long winged.”

Blue dashers are considered dimorphic because there are two main forms, but they are not exactly sexually dimorphic either.  Odonates are not sexually mature when they molt from nymph to adult and require a period of a few days to complete their maturation.  Immature individuals, including the males, look like the females.  So, the mature males are blueish and the immature males, immature females, and mature females all tend to look like the picture of the female.

Many dragonflies and damselflies follow similar patterns.  Green darners (Anax junius) are actually polymorphic:

Anax junius mating pair

Green darner (Anax junius) pair

As you can see in the photo, the male (the dragonfly in front) is green and blue while the female is green and green-brown.  Immatures of both sexes can have a reddish abdomen and the females can have brown sections on their abdomen.  Sometimes the females even have about the same blue on their abdomens as the males!  Because the green darners have so many different color patterns, they are considered polymorphic.

Not all dragonflies exhibit dimorphism or polymorphism.  Some dragonflies are monomorphic (mono = one) such that all indviduals look about the same, regardless of sex or age.

Next time I’ll post some more photos from the Sweetwater trip and go over how to identify them.  I hope you’ll stay tuned!

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

A Trip to Sweetwater Wetlands

Before I get back to the water bugs, I want to continue my detour for a few posts and talk about some dragonflies I saw yesterday.  I needed to take some dragonfly photos, so my fiancee and I went to a constructed wetland in Tucson called Sweetwater to shoot.  Sweetwater is part of one of the wastewater treatment plants in Tucson and is fed entirely with reclaimed water and secondarily treated wastewater.  It can smell pretty bad at times (it IS made up of treated wastewater after all), but if you can overlook the scent it’s gorgeous and wholly worth a visit.  All those nutrients in the water do wonders for the plant life that grows in the water:

Sweetwater Wetland

Sweetwater Wetland

As you can see, the nutrient-rich water of the wetland is able to support a wide variety of aquatic plants.  Yesterday, the wetlands were full of cattails (the dark green bushy looking things on the right side of the picture), rushes, sedges, and duckweed.  Most of the bright green stuff floating on the water in this picture is duckweed, NOT algae, though there was definitely algae there as well.  The wetland also suports a variety of trees and shrubs and many different species of wildlife.  All those dark spots in the picture are ducks.  I’ve also seen several snakes and lizards, many other birds, and even a bobcat once!  And then there are, of course, the dragonflies.  Thousands of them.  I think Sweetwater is the best place in Tucson to see dragonflies.

We saw several dragonfly species, including green darners (Anax junius), arroyo darners (Aeshna dugesi), blue-eyed darners (Rhionaeshna multicolor),  flame skimmers (Libellula saturata), roseate skimmers (Orthemis ferruginea), blue dashers (Pachydiplax longipennis), Mexican amberwings (Perithemis intensa), variegated meadowhawks (Sympetrum corruptum), and black saddlebags (Tramea lacerata).  We also saw one species of damselfly (though we weren’t really looking hard for the damselflies either – there may have been more), the desert firetail (Telebasis salva).  Most of the dragonflies were flying above the cattails or were resting on them in various positions.  Some were even in the obelisk position, which I have talked about before in another post on dragonflies.

I got some good shots of several different species, which I’ll go over in my next post.  However, I was most thrilled with all of the large flying dragonflies, the darners.  I like taking pictures of flying dragonflies the best because they are the hardest to shoot.  This is the same reason I got interested in dragonflies in the first place – catching dragonflies for my collections for 4-H was the biggest challenge.  I have yet to get a really, really good shot of a flying dragonfly, but I keep at it, and I got some pretty decent shots today.

This is Anax junius, the green darner:

Anax junius mating pair

Anax junius mating pair

This is a mating pair.  The male, the more brightly colored one with the blue abdomen, had just grabbed his mate in preparation for mating, and then they promptly fell out of the tree onto the sidewalk, right next to me!  They sat there in this position for almost a minute while I snapped away with my camera.  Then they flew off to mate and lay eggs.  Anax junius was very abundant at Sweetwater yesterday, so I ended up getting a lot of pictures of them.  They are fliers, so they don’t land very often.  That also makes them fairly hard to photograph – they don’t sit still for very long.  So, most of the photos I got were of these insects flying, and most of them looked like this:

Dragonfly photo

Typical dragonfly flight photo

The little smudge in the photo at the tip of the big blue arrow is the dragonfly!   I took about 90 photos of Anax in flight, and these were the best:

green darner in flight

green darner in flight

green darner in flight

green darner in flight

green darner in flight

green darner in flight

What I particularly liked about these images, aside from the fact that they are actually mostly in focus, is that you can see the legs of the dragonflies folded up underneath their thoraxes.  Having the legs folded up under the body likely helps the dragonflies fly more efficiently.  If they let their legs dangle down underneath them, they are likely to slow the dragonflies down, get snagged on the vegetation when they fly low, and otherwise cause problems.  So, they fly with them tightly folded under their bodies and only stretch them out when they grab food in midair.  It’s fun to be able to see that they do this, but it would be hard to see by observing them directly because they fly so fast and so erratically.  It’s easy to see in the photos.

Like in my post on the species from the dragonfly swarm, you can clearly see from these photos that dragonflies are able to move all four of their wings independently from one another.  For example, in the top photo, you can see that the forewings are both moving down while the hindwings are moving up.  This ability to move their wings independently contributes significantly to the amazing agility that dragonflies exhibit.  If you’ve ever seen dragonflies flying, you know how fast and agile they are.  They can stop in midair, make 180 degree turns, fly backwards, hover, and do all sorts of other things that are nigh impossible for most flying animals.  They dart all over the place, which is why it’s hard to get photos of them in flight.  If you’re dealing with a flier species, such as Anax junius, they are going to be moving constantly too.  Another reason it’s hard to get good photos of dragonflies in flight is the behavior you see in this photo:

green darners in flight

green darners in flight

Dragonflies are highly territorial and protect their territories from other dragonflies that might be trying to steal their access to prime egg laying habitat or other valuable resources.  In flier species, the males typically patrol, or fly within the boundaries of their territories, looking out for females to mate with, food, and males who might want to try to claim the territory for themselves.  In this photo, the dragonfly at the top tried to steal the territory that belonged to the dragonfly on the bottom.  The owner of the territory, the dragonfly on the bottom, successfully chased the would-be thief out away from his territory.  In the photo, the territory holder is returning to his territory to continue patrolling while the loser is flying away to find another spot, hopefully one guarded by a wimpy male.  Because there are usually fewer territories available than dragonflies at a pond, these battles are constantly occurring.  When a male sees another male that might enter his territory, he will immediately change directions and charge the tresspasser in an attempt to protect his territory.  This means that the dragonflies are darting back and forth constantly.  You might track one dragonfly and just be ready to snap a photo when he stops, turns around, and zips off in a completely diferent direction.  It’s very hard to predict where a dragonfly is going to be at any given time, so it is difficult to get good photos of them in flight.

Next time I’ll go over how to identify some of the species that we saw at Sweetwater, then it’s back to the giant water bugs for a while!

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

Dragonflies from the swarm

I’ve already posted twice about the dragonfly swarm my friend and I came across last week on the lake where we work, Lakeside Lake in Tucson.  While we watched the swarm, we also collected some of the dragonflies to add to our insect collections.  Because dragonflies tend to lose their colors VERY quickly in collections, it has become common practice for dragonfly enthusiasts of all levels of expertise to scan their specimens on standard flatbed scanners to preserve the colors as they are in life.  If you would like more information how to make your own dragonfly scans or would like to see some amazing images made using this process, I recommend taking a look at the website Digital Dragonflies.  (This is a great website to consult if you want to try to identify a dragonfly you’ve seen and you don’t have a field guide.)  Or check out the book written by the Digital Dragonfly creators, A Dazzle of Dragonflies. This book is absolutely gorgeous and I recommend it to anyone with even a passing interest in dragonflies, even if you only look at the pictures.

We found 4 species of dragonfly in the swarm, all of which are common in the Tucson area.  We’ve seen all of these at Lakeside many times, just not in the numbers we saw in the swarm.  I only managed to catch three of the four species (and if you know anything about catching dragonflies, especially these particular species, you know that’s pretty good!), so I’ll only go through the ones I have images for.  First, let’s meet the wandering glider, Pantala flavescens:

Wandering glider male (Pantala flavescens)

Wandering glider male (Pantala flavescens)

This handsome dragonfly is my favorite of all the dragonflies.  (If I ever get a tattoo, this is what I’m getting!)  This is a fairly common dragonfly in the Tucson area, though I find they’re much more abundant during the monsoons than at other times of the year.  This might have to do with a behavior they exhibit – they sometimes travel very long distances in front of storms!  I’ll post more about Pantala flavescens in a future post, but for now just know they’re amazing insects.  For a relatively boring looking dragonfly, it’s certainly got some fantastic behaviors.

How can I tell this is a wandering glider?  There are several ways.  This is a fairly big dragonfly with a wingspan of about 2 inches.  When they fly, they tend to appear brightly yellow-orange. There aren’t  many dragonflies with this sort of coloration, so it’s distinctive.  They are also fliers and rarely perch.  When they do perch, they tend to rest vertically (perpendicular to the ground) instead of horizontally (parallel to the ground) like other species.  If you manage to be lucky enough to catch a male, like the one above, and look at it head on, his face will be bright orange.  His eyes, as you can see in the image above, will be reddish.  His abdomen will be yellow to orange with black spots that widen the further down the abdomen you look.  And finally, the cerci (those little pointy bits sticking off the back end) are black.  This dragonfly gets its common name of glider from the shape of its wings.  See how the hindwings are much more broad than the forewings?  This is a flight adaptation that helps make them one of the strongest fliers of all the dragonflies.  There aren’t all that many dragonflies with wing structures like this, so if you see it, you can narrow down your options quickly.

This dragonfly, the spot-winged glider (Pantala hymenaea), is closely related to the wandering glider:

Spot-winged glider male (Pantala hymenaea)

Spot-winged glider male (Pantala hymenaea)

Compare the wings of this dragonfly to the image of Pantala flavescens above and you’ll see they’re about the same shape.  That makes this a glider as well.  These are very easy to tell apart from their other Pantala relatives, even though they do rest vertically, are also fliers, are about the same size, and have very similar shapes.  In flight, these dragonflies will look reddish or brown instead of yellow.  The eyes are grey with a reddish spot on the top, so they’re not red all over as they are in P. flavescens.    Their faces are red instead of yellow.  The easiest way to tell this dragonfly from the previous one, however, is the dark brown, round spot at the base of the hindwings.  It’s visible in the image above and if you look closely, you will be able to see it when these dragonflies are in flight.  In fact, you can see it in one of the photos I posted in my last post.  If you see a dragonfly with very broad hind wings with a dark, rounded spot at their base, you’ve got a spot-winged glider.  However, sometimes in flight they can be difficult to tell apart from the saddlebag dragonflies, the group to which the last species I’ll discuss here belongs, at least to the untrained eye.  This dragonfly is Tramea onusta:

Red saddlebags male (Tramea onusta)

Red saddlebags male (Tramea onusta)

It’s also called the red saddlebags.  All of the saddlebags (we have 6 species of Tramea in the United States) have broad hindwings like the gliders and are likely close relatives.  However, they have dark, broad bands on their wings that run along the base of the wings from the top to the bottom.  These bands can be narrow or wide and the width will help determine which type of saddlebag you have.  For example, here in Arizona, we have four species of saddlbags.  I can tell this is a red saddlebags because the body coloration is distinctly reddish and the band on the wings is wide.  The Antillean saddlebags and striped saddlebags, though reddish, both have narrow bands.  And it’s very easy to tell this dragonfly apart from the black saddlebags, a dragonfly you commonly find in the same locations as the red saddlebags in Tucson, because the red saddlebags are red and the black saddlebags are black.  (Bet you didn’t see that coming!)  The saddlebags are pretty easy to tell apart from most other dragonflies based solely on the dark band on their hindwings.  If you see a dark stripe running along the base of the hindwings, it’s a good bet you’re looking at one of the saddlebags.

The last species we saw was the black saddlebags, Tramea lacerata.  They’re the only black saddlebag species, so they’re very easy to tell apart from the others!

Since I wrote about the swarm last week, I’ve received several comments and e mails about swarms that other people have seen.  The four species we saw in this swarm are known to do this swarming behavior and it’s common for them to swarm in mixed groups like the one we observed.  I’ve been hearing reports of many other dragonfly species forming swarms though.  If you see a swarm, it might include these speices (and hopefully you can identify them now if it does!), but it could include other speices as well.  Regardless, if you happen to see one of these swarms, consider yourself lucky!  It’s an amazing thing to see so many dragonflies flying around together at one time.

I’ve been on a big dragonfly kick recently, but next time I’ll be shifting gears to my own research.  I’m making a trip up to Phoenix next week to do some research in a lab at Arizona State University, so it’s time to introduce you to my research study subjects, the amazing giant water bugs!

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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!

_______________

Want more information?

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

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Text and images copyright © 2009-2010 DragonflyWoman.wordpress.com

Dragonfly swarm!

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!

_______________

Want more information?

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

_______________

Text and video copyright © 2009-2010 DragonflyWoman.wordpress.com

Dragonflies and Damselflies – What’s the difference? (Adults)

A couple of posts ago, I went over the difference between dragonflies and damselflies in the nymph stage.  Today I’m going to cover the difference between dragonfly and damselfly adults.  They’re very easy to tell apart once you know what you’re looking for, so let’s drive right in!

This is a dragonfly:

Anax junius adult

Adult dragonfly (Anax junius, male)

A few thing to notice about the dragonfly:

1) The eyes are broadly rounded and lie mostly flat against the head
2) The thorax (the green part the wings are attached to in the picture above) ismore broad than the abdomen (the blue part in this dragonfly)
3) The forewings and hindwings are different shapes
4) Body is quite large (The dragonfly in the picture, a green darner, is about 3 inches long!), though there is a lot of variation in size

All dragonflies share these characteristics.  Also, if you saw this dragonfly sitting on a plant or on the ground, it’s wings would be held in the same position you see in the picture, spread out flat and to the sides of its body.

Now compare the dragonfly picture to this picture of a damselfly:

damselfly adult

Damselfly adult (unidentified sp.)

Look for these things in the damselfly:

1) The eyes are largely spherical and protrude off the sides of the head
2) The thorax (the segment where the wings are attached) is narrow, about the same width as the abdomen
3) The forewings and hindwings are very similar in size and shape
4) Usually fairly small (at least compared to the dragonflies)

If you saw a damselfly resting at a pond, it’s wing would look different from a dragonfly’s.  Rather than holding it’s wings flat and to the sides of it’s body, it holds its wings straight up, pressed together over the top of its thorax.  This is what you would see in the field if you saw one from the side:

damselfly adult

Damselfly adult, side view (Enallagma boreale)

Ultimately, if you’re at a pond or river, the easiest way to tell whether an odonate you’re looking at is a dragonfly or damselfly is to look at how it holds it’s wings while resting.  If they’re lying flat, parallel to the ground, you are looking at a dragonfly.  If the wings are pressed together, held over the bug’s back, you’ve got a damselfly.  So what happens if the odonate you’re looking at doesn’t ever stop flying?  Let’s think back to the difference between perchers and fliers from my last post.  Fliers are almost always dragonflies and damselflies are almost always perchers.  If it doesn’t stop to rest every few minutes, it’s probably a dragonfly.

Okay.  Now imagine you have two pinned odonate specimens, one dragonfly and one damselfly, rather than seeing them in the field.  The wings are spread apart on both.  Can you tell the two insects apart?  We know you should look at the shape and location of the eyes, the width of the thorax, whether the forewings look like the hind wings, and the size of the body.  See if you can tell which one is which in this picture:

dragon and damsel

A dragonfly and a damselfly - can you tell them apart?

The answers are listed below so you can’t cheat!  Scroll down to check your answers.  Hopefully you got them right!

If you have problems remembering the difference when you don’t have a list of their characteristics sitting right in front of you, here’s a good way to remember them.  Think of the names of these insects, dragonflies and damselflies.  What sorts of images do these names conjure in your mind?  I personally think of medieval stories about dragons holding damsels in distress hostage to use as bait for daring knights.  Think of dragonflies as you would the dragon in this image: robust, strong, powerful, and really big.  The damselfly is more like the damsel in the dragon and the damsel image.  They are smaller, softer, and weaker than the dragon.

One final note about dragonflies and damselflies as I finish this up.  The odonates have become very popular with non-scientists recently and dragonfly watching has become a sort of sport similar to bird watching.  With the publication of several excellent field guides which contain all of the species in a region or the country, it is easy for people who are not familiar with insects to identify the dragonflies and damselflies they see without specialized training or equipment.  All you need is a good field guide and a pair of binoculars!  If this is something that interests you, I have two field guides that I haul around with me when I’m out camping, bug collecting, on class field trips, etc, that I would like to recommend.  The first is Dragonflies Through Binoculars by Sidney Dunkle.  The book is about $30 retail (less on Amazon.com) and covers all of the dragonflies in the U.S.  The book contains excellent distribution maps and flight season information, color photos of every species, the common and scientific names for every species, and multiple pictures for species where the males and females (and sometimes the younger males) are different colors.  The descriptions for each species highlights the distinctive characteristics you should look for to tell them apart from other similar species.  It’s a really excellent book.  The downside: no damselflies!  I recently acquired a book that covers dragonflies AND damselflies in the western U.S., Dragonflies and Damselflies of the West by Dennis Paulson, so that I have a field guide for the damselflies too.  It has many of the same features as Dunkle’s book and costs about the same, but includes both dragonflies and damselflies.  The downside to this book: the book doesn’t cover the whole country, so you need another book if you’re going somewhere east of Kansas or Nebraska.  Still, it gets the job done in my area and I find it very useful.  Dragonfly watching is a fun activity and I hope you will give it a try!  There’s nothing quite like the feeling of checking another dragonfly, one you’ve never seen before, off your checklist.

Answer to the dragonfly vs. damselfly quiz above: A is the damselfly (one of the largest in the country!) and B is the dragonfly.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

Dragonfly sighting!

I was planning on discussing the differences between adult dragonflies and damselflies in this post, but I can’t reist talking about a dragonfly I saw on Saturday instead.  This lovely little dragonfly is Pachydiplax longipennis, also known as the blue dasher:

Pachydiplax longipennis

Blue dasher (Pachydiplax longipennis)

Isn’t he gorgeous?!  Pachydiplax longipennis is a very common dragonfly in the Tucson area, medium sized (a couple of inches long) and usually found perching on vegetation near ponds.  As you can see, the males have a sort of whitish waxy coating over blue bodies (they are pruninose) and they have bright green eyes.  The extent of the pruinosity varies a bit from place to place, but here in Tucson it covers most of their bodies.

This particular male was perching on the bush next to my fiancee’s car (more about why he was near the car in a moment).  Dragonflies (not damselflies – see my next post to learn how to tell the two apart!) are generally split into two groups based on their behaviors.  Perchers, like Pachydiplax longipennis, sit on vegetation near water.  From their perches, they are able to protect their territories from other males, see females that come into their territories, and spot insects and other animals they might want to eat.  When disturbed for any of these reasons, they’ll dart off their perch and fight, mate, or grab their prey, but they spend a majority of their time sitting on their perch watching and waiting for things to happen.

The other group of dragonflies is made up of the fliers.  These dragonflies fly almost constantly, patrolling their territories for mates, aggressors, or prey on the wing.  They rarely stop moving unless they are mating or wrestling with a particularly large piece of food.  You would be hard pressed to get a photo like the one above if this dragonfly were a flier instead of a percher because they rarely sit still that long.

Back to why this dragonfly was near the car.  Dragonflies have amazing vision which they use to find territories, mates, and food.  See those giant eyes that take up almost the entire head of the dasher pictured above?  These are clearly very visual animals.  Dragonfly nymphs are aquatic, so their eggs need to be laid either in or near water.  So, dragonflies need to be able to find the water.  They do this by looking for certain patterns of polarized light as they fly overhead.  If a male finds the right pattern of light, the one that screams “Water!” at him, he’ll set up a territory and protect his patch of the water from other males in the area.  If he’s a percher, he’ll find a nice place to rest nearby and survey his territory from his perch.  Why, then, was this dragonfly near a car?  The clear coating on many cars gives off the same pattern of light as water in ponds.  As far as this dragonfly is concerned, the car is a pond!  He thinks he must protect it from other dragonflies that might be in the area, so he was perched in the bush nearby and watching the area around the car.

This dragonfly exhibited one other behavior while I watched.  This picture is unfortunately not as clear as it could be because he was having a hard time staying still, but you’ll get the idea:

Pachydiplax longipennis obelisk

Blue dasher (Pachydiplax longipennis), obelisk position

This dragonfly is in what is called the obelisk position, sitting on the antenna of my fiancee’s car.  Have any idea why they might rest in this position?  I’ll give you a hint: this photo was taken around noon on a hot and sunny day.  Dragonflies actually use this position as a way to help cool their bodies down when they are overheated.  Dragonflies, like all insects, are cold blooded (=exothermic) and their body temperature closely tracks the temperature of the air.  When it is very hot or very sunny, their body temperatures get too high and they must use a variety of behaviors to cool down.  Dragonflies are pretty unique in using the obelisk position.  Here’s how it works.  Imagine you are out under a very hot sun.  You have two choices of positions: standing upright or lying flat on your back.  Which position should you use to minimize the amount of sun that hits your body?  If you chose standing upright, you’re correct!  By standing upright, the summer noon sun will only hit your head and shoulders, leaving the rest of your body shaded.  If you laid on the ground, half of your body would be hit by the sun!  Dragonflies use the oblesik position in exactly the same way.  By extending their abdomens upwards, instead of holding it horizontally as they usually do (shown in the first photo), they minimize the amount of sun hitting their bodies.  The sun only hits the tip of the abdomen, part of the thorax and head, and part of the wings rather than the entire top surface of the dragonfly.  So, this dragonfly is in this position because he is hot and needs to cool down.  If the obelisk position isn’t enough, he’ll fly into a shaded area, even if it means abandoning his territory for a while.  Defending a territory isn’t worth risking death by overheating.

This dragonfly flew away shortly after I took this photo.  I disturbed him enough that he gave up on defending his “pond” and flew away to look for a place that had fewer dangerous large mammals.  I hope he found a backyard pond instead of another car!  I always find the car thing a little depressing.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com