Friday 5: Dragonflies Getting Busy

It is still cold in many parts of the US.  In fact, it even snowed in Tucson last weekend!  Having moved to Arizona from somewhere much colder, I know that winters can be dreary and sometimes it seems like the spring will never come.  So, let’s think about the good things to come in the spring and summer instead!  Today’s Friday 5 is all about something that’s coming soon to an area near you: dragonflies getting busy and making babies!

Step 1: The male finds and defends a territory.

defending territory

A male blue dasher (Pachydiplax longipennis) defends his territory from a perch.

Like many animals, male dragonflies attract females by showing off.   To have any chance at success in convincing a female to mate with him, a male dragonfly has to show how amazing he is by finding a good place for egg laying, setting up a perimeter around that area, and defending the space within that perimeter from other males.  This is hard work, but the males who defend the best territories tend to get the most girls, so it’s worth the effort.  Some dragonflies fly within their territories nearly constantly (these are “fliers”).  Others find perches within their territories (the “perchers”) and only fly when their territory is threatened by another male, or when they progress to Step 2.  (For more info on dragonfly territoriality, check out part II of my dragonfly trilogy for a whole post on the topic!)

STEP 2. The female flies into the area.

And all hell breaks loose! Dragonfly males may spend hours each day flying around in their territories, and it’s all in preparation for this moment.  It must be terribly exciting. When a female flies into his territory, the male will fly out toward her and try to grab her. If he is successful, he moves to Step 3.

Step 3: The male grabs the female.

male grasping female

A pair of desert firetails (Telebasis salva) shortly after the male has grabbed the female.

Success!  As you can see in the photo of the damselflies above, the male grabs the female behind the head.  Males need to get a firm grip because other males might try to steal the female from him and this is probably the best place to hold on.  This means that the male is grabbing the female with the section of his body where his sperm are produced, so he has to transfer sperm from the genitalia at the back end to a secondary set of genitalia up near the thorax before he grabs a female.  (Scroll down to the lower part of my post on why dragonflies are the best insects for more info on this and a photo of the two sets of genitalia.)  Females can refuse to mate with a male, even after he’s grabbed her.  If she’s unwilling, the male will eventually let her go her own merry way.  If she is willing to mate, they progress to step 4.

Step 4: The pair assumes the “wheel position” and mates

mating dragonflies

Dragonflies mating (Pachydiplax longipennis). The male is the blue dragonfly on top and the female is the brown-black dragonfly on the bottom.

To transfer sperm to the female, the male has to bring the genitalia at back end of the female into contact with his secondary genitalia near his thorax, all while maintaining his grip on her head.  He curls his abdomen under so that the female is held mostly parallel to his body.  She then curls her abdomen up toward his secondary genitalia.  In the process, they form the wheel position, this awkward looking position that looks like a circle drawn by a toddler.  Some dragonflies, such as the dragonflies in the photo, continue to fly in the wheel position and others land while in the wheel position.

Males are super competitive with one another when it comes to females, even at this stage.  If he is to be the biggest, baddest male at the pond (or stream), he needs to father the most children.  If a female has mated with another male before him, there’s a chance that the other male’s sperm is still stored in her sperm storage organ and might fertilize some of his gal’s eggs, even after he’s mated with her himself.  No problem!  Many male dragonflies have genitalia designed to scoop sperm out of females before he deposits his own.  Others do some really amazing things.  My advisor shows a video in his insect behavior lecture that shows a dragonfly in Germany that grabs the female and immediately whips her whole body around in a somersault, flinging the sperm from her body in the process!  Basically, males are selfish.  They want their offspring to be the only offspring in their territories and removing sperm deposited by other males from their mates before transferring their own sperm into the female helps ensure that this is realistic.

So the dragonflies get it on for a while.  Then they move on to Step 5.

5.  The female lays her eggs.

Anax laying eggs

A pair of common green darners (Anax junius) laying eggs. The males guards his female from usurpers by maintaining his hold on his mate.

Now that the male has successfully held a territory, encountered a female, and encouraged her to mate with him, there’s only one thing left to do: lay eggs!  However, there are a lot of males around the pond (or stream) and very few females, so other males may try to grab the female and mate with her before she finishes laying her eggs.  If that happens, they’ll remove the freshly deposited sperm and replace it with their own.  So a male who just mated with a female often guards her while she lays her eggs.  Guarding takes many forms.  Some males release their hold on the female and fly above them.  Others let the female go, return to their perches, and fly out to fight any potential usurpers.  Many species keep their hold on the female until she has finished laying her eggs, as in the green darners in the photo.

Egg laying habits vary from species to species too.  Some species fly over the water and dip their abdomens into the water several times, releasing eggs each time.  Some stay in one place, holding onto a rock or piece of vegetation, and lay all of their eggs in one spot.  Still others crawl all the way underwater to lay their eggs!  Many species just spray their eggs into the water and let them fall where they may, but some stick their eggs to rocks or vegetation or embed them into emergent plants or algae.  There is a lot of variation in egg laying and mate guarding behaviors, but they all accomplish the same thing: ensuring that the eggs a female lays in a territory are mostly those that have been fertilized by the male holding the territory.

When it warms up and the dragonflies come back out, I encourage everyone to find a pond or stream nearby and settle down to watch a little dragonfly porn.  Dragonfly mating habits are fascinating to watch!  You won’t be disappointed.

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Unless otherwise stated, all text, images, and video are copyright © 2011 DragonflyWoman.wordpress.com

From the Literature: Tracking Dragonfly Migrations

It’s about the time of year for the dragonflies to start moving south!  I’ve already gotten several reports of big migratory swarms headed south from several locations across the eastern and midwestern U.S. and I expect many more – the season has just begun!

A few posts back, I discussed a paper that described mass migratory swarms in dragonflies.  While the authors presented several unanswered questions and got the ball rolling on understanding how and why these swarms form, there has still been surprisingly little done in this field.  As I suggested in my last post, this might have to do with the ephemeral nature of these swarms.  The vast majority of swarm observations are “in the right place at the right time” sorts of observations and it’s extremely difficult to predict exactly when and where a swarm will form and/or travel.  Depending on their location, any given dragonfly researcher might only see one or two mass migratory storms in his or her whole life!  This is clearly a very difficult topic to study, and most accounts of swarms have been buried in the scientific literature.  That means that there is very little information about dragonfly swarms freely available to the public.  I think this is a sad state of affairs, thus today I’m covering another scientific paper on dragonfly migrations.  This one is really fun!

(Okay, okay – I think it’s fun, but I’m also a huge bug geek…  You can form your own opinion!)

dragonfly with transmitter

A darner with its transmitter attached. Photo by Christian Ziegler and taken from http://news.sciencemag.org/ sciencenow/2006/05/ 11-02.html?etoc&eaf

In 2006, one group of researchers decided to answer one of the big unknowns: where do these migrating dragonflies go?  The group, headed by Martin Wikelski of the Department of Ecology and Evolutionary Biology at Princeton, had noticed that though many insect species had been documented migrating, the ultimate destination and migration strategies of many of those species remained unknown.  So, they decided to track swarming dragonflies.  How did they do this, one might ask?  With radio transmitters of course!  Check out the photo to the left.  That’s a green darner (Anax junius), the most common swarming dragonfly, with its radio transmitter attached.  The researchers captured 14 darners in New Jersey between September and October, glued the tiny bug-sized transmitters onto their thoraxes, and released them.  Then they tracked the dragonflies with radio receivers either by car or by Cessna plane for up to the 10 days of the transmitter’s life.  In essence, this qualifies as one of the most awesome research projects ever!  (Pardon me while I drool thinking about how amazing it would have been to track dragonflies from a plane in this study…)

Using this design, the researchers determined how far a dragonfly flew on any given day, how long it rested between flights, and the exact path it took during its migration.  They then put all of their data together to determine how similar dragonfly migrations are to bird migrations and what rules dragonflies follow when making migratory decisions.

So what, then, did they learn?  First, the dragonflies all migrated within within 4 days of receiving their radio transmitter, so they were still inclined to migrate even with the transmitter in place.  They also learned that the dragonflies tended to move approximately once every three days.  This means that the dragonflies flew one day, rested for two days, then flew again.  Long stopovers were apparently necessary during the migrations.

What about the distance and direction traveled?  The team found that there were three types of daily movements.  Some dragonflies flew a short distance (1-4 km) and in all sorts of different directions.  Others flew 8-12 km in a single direction.  Still others flew 25-150 km (that’s just over 93 miles) in one day!  Clearly these dragonflies were capable of flying long distances under certain conditions, though the average daily flight distance when all flights were combined was only 58 km (36 miles).  As for the direction, some dragonflies flew west and some flew east at times, but the bulk of the movement was southwest.

Weather seemed to be important for determining when the dragonflies flew and when they did not.  They were much more likely to fly in mild winds than in stronger winds, and no dragonflies flew if the wind speed was greater than 25 km/h (that’s just over 15 mph).  They also tended to fly more on days when the wind was blowing from the north than on days when the wind blew in other directions.  The dragonflies apparently depended on the wind to help them travel because the direction of the dragonflies and the direction of the wind on days where they flew were nearly identical.  Curiously, there was no association of temperature and propensity to migrate on flight days: there was no difference in the daily high temperatures of flying days versus non-flying days.  However, all migratory flights took place after a night with a temperature cooler than the previous night.

These data suggest that dragonflies have a set of simple rules they follow when deciding whether to migrate or not.  The dragonflies move with the winds (but not in very strong winds) in response to cool night and take a few days off between flying days, presumably to hunt and/or rest.  This in and of itself is pretty interesting, but it’s also interesting to place this information in the larger context of flying animal migrations.  Nearly everyone is familiar with the annual migrations of birds and know that birds “fly south for the winter.”  The data the dragonfly team collected revealed that the migratory patterns of dragonflies are remarkably similar to those of birds.  Songbirds use the same sorts of weather cues to prompt their migrations, follow coastlines and other prominent landscape features in the direction of the wind, and make frequent stopovers, just like the dragonflies did.  In essence, birds share the same set of rules governing migration that dragonflies exhibit.  It is likely that other migratory flying animals follow the same rules.

The team finished their discussion of the dragonfly behavior by using their data to calculate the maximum migration distance these dragonflies might be expected to fly.  Assuming a modest flight speed and a two month migration season, an individual dragonfly could be expected to fly 700 km, or 435 miles!  This is a long way for what is ultimately a small animal to fly.  Unfortunately, due to the limitations of the transmitters used (i.e. the battery life of 10 days), the team was never able to figure out exactly where the dragonflies ended up.  If the dragonflies are traveling 435 miles, I’ve calculated that dragonflies starting off in New Jersey most likely end up in West Virginia or Virginia.  This is much further north than previously suspected, which leads to at least two possible explanations for sightings of mass migratory swarms reported further south.  1) The dragonflies might fly faster than estimated, which would allow them to travel further during the 2 month migration season.  Or 2) the dragonflies observed in locations such as Florida and further south might be starting off from a more southern location to begin with.  Yet two more questions to be answered about this behavior!  It may be possible to answer these questions using the techniques the dragonfly team developed.  I suspect radio transmitters will play a significant role in answering some of the many outstanding questions about migratory behaviors in dragonflies.

Next time I’m going to post images and descriptions of the most common migratory dragonfly species so that people observing dragonfly swarms can determine which species they’re seeing.  In the meantime, I hope all your easterners enjoy watching the dragonflies that are on the move in your part of the country!  Based on the dragonfly activity in the north this year, it could be downright spectacular.

Literature Cited:

Wikelski M, Moskowitz D, Adelman JS, Cochran J, Wilcove DS, & May ML (2006). Simple rules guide dragonfly migration. Biology letters, 2 (3), 325-9 PMID: 17148394

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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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Unless otherwise stated, all text, images, and video are copyright © 2010 DragonflyWoman.wordpress.com

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