Meet the Beetles!

Meet the Beetles posterI wanted to let everyone know about an event that’s coming up this weekend at the Arizona-Sonora Desert Museum west of Tucson.  During the summer, the Museum is open late on Saturday nights so that people can visit when the temperatures are reasonable and see some of the interesting things that happen only at night in the desert.  This Saturday, August 28, there will be a special event: Meet the Beetles!  Dr. Wendy Moore, the new systematist in the University of Arizona’s Department of Entomology, will have live specimens of over 30 Arizona beetle specimens available, as well as a whole hoarde of professional entomologists (grad students, postdocs, and professors) to share what’s known about these beetles with the public.  It’s a fantastic opportunity to learn a lot about Arizona’s beetles from some very knowledgeable people – and we have some truly spectacular beetles!  The event runs from 7-9PM and will be located in the Desert Garden.  Admission for the Museum is only $7 after 4PM and only $2.50 for kids 6-12.  Kids 5 and under are free.  If you’re in the Tucson area on Saturday, this is a great event and one I highly recommend!

Advertisements

Migratory Dragonfly Species – Common Species

In most of the dragonfly swarm reports I’ve been getting, I’ve had people tell me that they know nothing about dragonflies or how to identify them, though several of you have an expressed an interest in doing so.  So, I thought I would do something to help all of my ambitious reporters out!  Today, I’m posting images and descriptions of the most common of the American migratory dragonfly species, including males and females when they’re different, so that you can try to identify some of the dragonflies you’re seeing in those swarms.  The migratory species are all dragonflies, not damselflies (see my past post on the subject if you aren’t sure how to tell them apart!), and fall into two families: the darners (Aeshnidae) and the skimmers (Libellulidae).  You’re probably not going to get close enough to these to see some of the wing structures that make it very easy to tell these two families apart (I’ll be posting on this eventually), but the most common migratory species all look different enough that it should be pretty easy to distinguish them on the wing.

Let’s start with the darners.  The family Aeshnidae contains the biggest species of dragonflies in the world, so if you’re thinking, “Wow!  That’s a REALLY BIG dragonfly!” it likely belongs to this group.  The most commonly observed darner is this:

Anax junius adult

Common Green Darner (Anax junius) male

anax junius female

Common Green Darner (Anax junius) female.  Image taken from http://www.dragonflies.org/catalog.htm.

This is the common green darner or Anax junius.  This species is the most commonly reported migratory dragonfly in the U.S. and will be seen very often in swarms.  Things to look for: 3 inches long.  Bright green thorax in all individuals.  Bright blue markings on abdomen in mature males, green markings on brown bodies in females and immature males.  Some immature males might also have red abdomens.  “Eyespot” on top at front of head (clearly visible in image of male – click on the image if you wish to enlarge it).  Clear wings or with only minor duskiness, no distinct spots or markings.  Green darners are found throughout the United States and in southern Canada.

The other common migratory darner is this one:

epiaeschna heros

Swamp Darner (Epiaeschna heros).  Image taken fromhttp://www.dragonflies.org/catalog.htm.

This is the swamp darner or Epiaeschna heros.  If you live west of central Texas, you’re not seeing this one in your swarms!  Anywhere else, this is a darner commonly found in swarms.  Things to look for: Very large dragonfly, up to 3.5 inches long.  Dark brown body with narrow, bright green stripes on abdomen, thick green strips on sides of thorax.  Bright blue eyes.  Males and females similar, though female may lose some of the blue as she ages.  This dragonfly is found only east of Ohio in the north and east of Nebraska and Texas further south.

That’s it for the common migratory darners!  Most of the time, if you’re seeing what you think are very large dragonflies in a swarm, it will be one of the two species above.  Most of the Libellulidae (also known as skimmers) are smaller than the darners, though they’re still pretty big.  Most of the migratory skimmers are also very common species of dragonflies with wide ranges in the U.S..  These include this gorgeous skimmer:

libellula pulchella

Twelve Spot Skimmer (Libellula pulchella) male. Image taken fromhttp://www.dragonflies.org/catalog.htm.

libellula pulchella

Twelve Spot Skimmer (Libellula pulchella) female. Image taken fromhttp://www.dragonflies.org/catalog.htm.

This is the twelve spot skimmer or Libellula pulchella.  This is a very common skimmer found throughout most of the U.S. (except for the southwest) and southernmost Canada.  Count the number of black spots on the wings and you’ll know where it got its common name. Things to look for: Medium-large dragonfly, 2 inches.  Body dark brown, males often with white prunescence on thorax and abdomen.  Females similar, but lack most of the prunescence.  Three dark spots on each wings, including one at the base, middle, and tip.  White spots may be present between the black spots.  The eight spot dragonfly is similar, but the dark spots do not reach the edges of the tips of the wings.

This species is very closely related to the 12 spot:

Painted Skimmer (Libellula semifasciata). Photo copyright Darrin O’Brien and taken from http://bugguide.net/node/view/218767/bgimage.

This species is the painted skimmer or Libellula semifasciata.  It’s range is similar to the swamp darner above and is restricted to the eastern half of the U.S. Things to look for: Medium-large dragonfly, just under 2 inches.  Pattern on wings is distinct: amber coloration at base and tip, brown spot at center and near tip.  Abdomen with a subtle orange-yellow tint in males, tending more toward brown in females.  Sexes very similar otherwise.

Now we have some gliders, my favorite dragonflies!

Wandering glider (Pantala flavescens)

Wandering Glider (Pantala flavescens)

This is my favorite dragonfly, the wandering glider or Pantala flavescens.  Considering my long-standing interest in dragonfly responses to weather patterns, there’s a good reason for this dragonfly to be my favorite.  But that’s a story for another time.  For now, you’ll want to look for these characteristics: Medium-large dragonfly, just shy of 2 inches.  Wings long relative to body, broad.  Body yellow brown, but can appear golden in flight.  Abdomen tapered, thicker at base than at tip.  Males and females similar.  This species is common across the U.S. and southeastern Canada.

The wandering glider’s close cousin is this species:

Spot-winged glider male (Pantala hymenaea)

Spot-Winged Glider (Pantala hymenaea) male

The spot-winged glider, or Pantala hymenaea, is very similar in shape to the wandering glider, but bears some distinctive differences.  Things to look for: Medium-large dragonfly, just shy of 2 inches.  Abdomen grey and black, appearing mottled, and tapering to a point.  Wings long and broad, mostly clear with a distinctive dark round spot at the base of the hindwings (look for the spot in the center of the hindwings right next to the body in the image above).  The location and shape of the spot spot is only seen in this species.  Males and females similar.  Found throughout most of the U.S. and southeastern Canada.

And finally, we come to the saddlebags.  The first saddlebags is this:

tramea carolina

Carolina saddlebag (Tramea carolina)

This gorgeous dragonfly is the Carolina saddlebags or Tramea carolina.  This group of dragonflies is rather closely related to the gliders, which should be obvious if you compare the shape of the wings. Things to look for: Medium-large dragonfly, 2 inches.  Long, broad wings.  Hindwings with wide deep red-brown spot at the base.  Body bright red, with two black abdominal segments the near tip of the abdomen.  Males and females similar.  Common across most of the eastern U.S.  Note: this species may be replaced by the red saddlebags (Tramea lacerata), which looks nearly identical, in swarms if you are in the western part of the country.  The red saddlebags hasn’t ever been officially declared a migratory species, however.  You can distinguish the two by looking at the color of the face (Carolina has a violet forehead while red does not) and the black abdominal segments (the black doesn’t wrap all the way around the sides of the segment in the reds as it does in the Carolinas), but these characteristics may be difficult to distinguish in the field.  Luckily, the ranges of these species don’t overlap very much except in eastern Texas and Oklahoma.

And finally, we have the other very common saddlebags:

Tramea lacerata

Black Saddlebags (Tramea lacerata)

This is the lovely black saddlebags or Tramea lacerata.  I think this one is gorgeous!  Sleek black dragonfly, very common in Tucson.  Love it!  Things to look for: Medium-large dragonfly, just over 2 inches.  Long, broad wings with broad brown-black spot filling the 1/4 of the hindwing closest to the body.  Abdomen black, often with white spots on the upper surface near the base and the tip of the abdomen.  White spots darken over time.  (You can see the white spots in the image above – just click on the image to enlarge it, and look halfway between the end of the wings and the tip of the abdomen.)    Males and females similar.  Common throughout much of the U.S. except in North Dakota, Montana, and Wyoming.

That does it for the dragonfly species you’ll commonly find in migratory swarms!  I suspect that many of the same species are present in static swarms as well (please see my post Dragonfly Swarms Revisited for more information about the two types of swarms), so hopefully this post will be handy for those of you who wish to ID the dragonflies you’re seeing in swarms.  Next time I’ll cover the less common species of migratory dragonflies and comment on some of the other species that have been popping up in swarms around the country.  Until next time!

_______________

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!

_______________

Unless otherwise stated, all text, images, and video are copyright © 2010 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

_______________

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!

_______________

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

Changes to The Dragonfly Woman

I finally took the plunge and made some of the updates to my blog that I’ve been putting off.  If you’ve been keeping up with my posts, you should immediately notice that I changed the theme.  Although I loved my previous theme’s color scheme and I’m a huge sucker for walling things off in boxes with rounded edges (it is my preferred design for conference posters!), it just wasn’t functional enough.  The theme didn’t allow for anything but parent pages to appear at the top of the website, which meant a lot of extra work if I wanted to include subpages.  This irritated me more than I can possibly say and I’ve wanted to change the theme for a while.  It was only a matter of time before a theme came along that had the functionality I craved without sacrificing too much in the way of the design.  This is the result!  I hope it will work well, though there’s a good chance I will change it again if a theme that better fits my design preferences comes along.

The other thing I’ve updated is the gallery page.  There’s actually a gallery there now!  The gallery will contain all of the images you see on my blog that I have taken myself, i.e. those that I have the right to distribute as I wish.  The gallery is in part a tool for myself (so I can easily see which photos I’ve used – slogging through my media gallery is an incredibly slow and painful experience), but I think it will be handy for people who wish to look at photos of a whole group of insects or easily find a photo without having to remember the name or the date of the post on which it was featured.  The main gallery page (the one you’ll arrive at if you click Gallery above) is arranged by taxonomic order in order of ascending evolution according to Grimaldi and Engel.  The galleries for individual orders are arranged alphabetically by family.  For those of you who have no idea what this means, not to worry!  I have also included the common names of the orders and families for your convenience.  I’ll be adding images to the gallery as I create posts, so it will serve as a repository for all of the photos that I’ve made available on my blog.  I think it will be a useful tool and will improve the educational value of my blog.

I continue to be very busy with research projects and these changes took quite a bit of time to make, but I hope to have a new post up in the next few days.  Thank you all for your continued readership and support!

_______________

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

From the Literature: Mass Migrations in Dragonflies

dragonfly swarm

Dragonfly swarming behavior. Photo copyright Steven Young and taken from http://www.flickr.com/photos/steven-young/2893876500/.

Wow!  I’m running way behind on getting a new post up.  I’ve been getting a ton of dragonfly swarm reports in the last week, so I’ve been scrambling around trying to keep up with that.  Couple that with some writer’s block and the start of my field season, and Ms. Dragonfly’s life has been a bit hectic!  I’ll try to get something up a bit more quickly next time…

I’m going to do a short series on dragonfly swarming behavior since this seems to be a topic that a lot of people are interested in this summer – and one that I’ve become rather obsessed with over the last few months.  (Seriously – ask my husband, my Ph.D. advisor, or my coworkers how many times I’ve brought up dragonfly swarms in conversation over the last few weeks!)  After combing through the literature, I’ve discovered some great information on dragonfly swarming and I think I have some good explanations for why we see these swarms in nature.  Today’s post will cover a paper that came out in 1998 about the mass migratory swarms observed in dragonflies.  For those of you out there who have seen hundreds of thousands or millions of dragonflies all flying together in a single direction, a sort of river of dragonflies, this paper describes and attempts to explain this behavior.  And what an amazing behavior it is!  I would dearly love to see one of these giant migratory swarms.  If any of you happen to get this behavior on video, I strongly encourage you to upload it to YouTube so that others and I can see it!  But let’s get to that paper.

Over the years, several dragonfly researchers have witnessed massive swarms of dragonflies flying together in what appeared to be migratory swarms.  Each time, they consisted of several hundreds of thousands or millions of dragonflies, enough to nearly blacken the sky and make themselves really obvious – if you happened to be looking up at the right time to see them!  In 1998, Robert Russell, Michael May, Kenneth Soltesz, and John  Fitzpatrick combined their eyewitness accounts of these migratory swarms and wrote one of the first papers describing this behavior in detail.  At the time of the paper’s release, very little was known about dragonfly migratory patterns.  25-50 species of all dragonflies were thought to be migratory and only 18 migratory species had been reported in the eastern United States.  In their paper, the authors set out to describe the massive swarm migrations they’d observed in dragonflies, but they also wanted to review the literature concerning migrations of dragonflies to see if any patterns emerged.  They were hoping to determine why these swarms formed and understand how they worked.

Anax junius adult

Green Darner (Anax junius) male – notice the large wings and huge eyes!

First the authors described three swarm migrations in the United States that three of the four authors had witnessed themselves: one swarm each in Chicago, Illinois; Cape May, New Jersey; and Crescent Beach, Florida.  In each place, the observers reported the same things:

  1. Nearly all of the dragonflies in each swarm were moving together in the same direction, though not all swarms moved the same direction.
  2. The swarms were largely made up of green darners (Anax junius – pictured to the left above), though several other species were present in the Cape May swarm to a much lesser extent (black saddlebags, twelve-spotted skimmers, swamp darners, Carolina saddlebags, wandering gliders, spot-winged gliders, and blue dashers).
  3. The swarms were very large, at least 200,000 dragonflies (Florida) and up to an estimated 1.2 million in the Chicago swarm!  (They compared the dragonfly swarms they observed to those observed in locusts and found that the number of swarming dragonflies was comparable to swarming locusts.  If you’ve heard about locust plagues, you have a good idea of what one of these swarms should look like!)
  4. The swarms tended to be very compact, that is they flew within a rather confined corridor rather than spreading far apart either horizontally or vertically.
  5. The swarms tended to follow very obvious landmarks such as shorelines, ridges, and coasts.
  6. All of the swarms took place soon after a cold front passed through the area, so they appeared to be distinctly weather-related.

The authors also reviewed the literature and collected field observations from other researchers to better determine how, where, and when these massive swarms were taking place.  They gathered 41 reports spanning the years 1881 through 1995 and found that these swarms generally shared the same characteristics as the swarms witnessed by the authors.  They further discovered that the swarms took place between July 30 and October 13, though the majority (28 swarms) occurred in September.  This was clearly a highly seasonal event.  All of these reports were also made in locations in the eastern United States, so it appeared that these swarms only occurred in half of the U.S.

Based on all of the reports together, the authors made some speculations about the massive migratory swarming behavior observed in several species of dragonflies.  First, their data suggest that at least some of the dragonfly species are migratory and are using these swarms to travel from north to south during the fall.  Where exactly they’re going is much less certain, but there are some reports of dragonflies crossing the Gulf of Mexico and massive swarms suddenly appearing in locations in Mexico.  The final destination could be as far south as Central America or as far north as the Gulf coast.  Regardless of where they end up, the dragonflies appear to be flying south for the winter, much like birds and monarchs do.

Second, the authors suggest that this type of swarming behavior might be unique to the United States and the regions where the dragonflies overwinter.  There are similar swarms in Europe, but the timing and recurrence of the swarms are very different than the yearly migrations observed in the US.  This also led the authors to suggest that the wandering glider, the only dragonfly species naturally found on all continents, might not form these migratory swarms.  While they have been observed in American migratory swarms, they typically only make up a small percentage of the total individuals within the swarm and are very uncommon in swarms reported in other locations.

Third, the authors proposed that weather may be the most important factor regulating the mass migrations of dragonflies.  Indeed, in nearly all of the accounts they collected, a cold front had passed through the area just prior to the appearance of the swarm.  A combination of season (autumn), north-westerly winds, and cold fronts may be necessary to entice dragonflies to fly from the cooler northern regions to warmer southern climates for the winter.  The timing of this behavior also suggests that these dragonflies are relying on aerial plankton for food as they migrate.  Aerial plankton is abundant during the fall, allowing the dragonflies to feed along the way.

Fourth, landmarks appear to be very important in directing dragonfly migration paths.  Mass migrations are almost always reported along major waterway, coasts, or prominent land features.  These “leading lines” help dragonflies orient themselves and follow the correct path south.  However, they also seem to reorient themselves under certain conditions, especially when they have to cross significant stretches of water.  This will be the subject of an upcoming post, so I won’t go into more detail here.

And last, there is some evidence that the dragonflies that fly south for the winter are not the same individuals that return north.  Dragonfly adults typically have a life span far shorter than the length of time the dragonflies overwinter, so it is very likely that migratory dragonflies fly south, mate, produce young, and die.  The offspring then make the return trip north in the spring.  There is a lot of uncertainty about what happens in the spring, however.  The route the dragonflies take on the return trip remains mysterious.  The migration north also does not appear to involve huge migratory swarms and may be comparatively inconspicuous, making it difficult to determine when and where the northward bound dragonflies are flying.

The authors ended their paper with a long list of questions that remain unknown about dragonfly migration.  12 years later, we still don’t know most of the answers to these questions!  For example, why exactly are these dragonflies forming these massive swarms when they migrate?  If it were only to protect the dragonflies as they fly (safety in numbers), then why do so few dragonfly species exhibit this behavior?  It doesn’t appear to be solely related to timing of adult emergence from the water either as emergence is not synchronous in the green darners and they are the most commonly observed migratory species.  Still, this paper went a long way toward explaining mass migrations in dragonflies.  Due to the ephemeral nature of swarms – a million dragonflies might fly over an area in a matter of minutes – we’re lucky there have been any studies of this behavior at all!

Over the next few posts, I’ll go over another paper about dragonfly migrations (one using radio transmitters!) and talk about what I’ve discovered in the literature about what I’m calling static swarms.  With these three posts, I hope to summarize most of what’s known about dragonfly swarming behaviors so that the information is available outside of the scientific community in an accessible way.  I am also going to post photos of all of the known migratory dragonfly species in the US.  Many people reporting dragonfly swarms to me have expressed an interest in identifying the dragonflies in their swarm, so I want to make that possible.  And, as always, keep sending me dragonfly swarm reports!  I have a huge collection of them going this summer and I couldn’t be happier with the level of participation in my project.

Literature Cited:

Russell, R., May, M., Soltesz, K., & Fitzpatrick, J. (1998). Massive Swarm Migrations of Dragonflies (Odonata) in Eastern North America The American Midland Naturalist, 140 (2), 325-342 DOI: 10.1674/0003-0031(1998)140[0325:MSMODO]2.0.CO;2

_______________

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!

_______________

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

Aerial Plankton

aerial plankton

Aerial plankton. Image taken from npr.org.

Okay okay.  I know I promised a post about a dragonfly swarming paper this time, but I decided I should talk about a related subject first: aerial plankton. Don’t worry – I’ll get to the dragonfly paper next time!   I am nearly wholly engrossed by the dragonfly swarming information that’s been coming my way and I will definitely go right back to it.  But this topic is likely an important component of the migratory swarms seen in dragonflies and I thought I should discuss it before I do the dragonfly paper.

If you’re like most people, you probably don’t know what aerial plankton is and may never have considered the possibility that it exists.  But most people know at least something about the plankton that live in oceans, marine plankton.  If you do, the concept is very similar.  The definition on Wikipedia for plankton is pretty good: Plankton are any drifting organisms (animals, plants, archaea, or bacteria) that inhabit the pelagic zone of oceans, seas, or bodies of fresh water.  Most of what we think of as plankton are little crustaceans such as krill or amphipods.  These are the things that are eaten by whales using baleen and several other large marine mammals.  Though small, they’re very important in marine habitats, both as a food source and for the many other services they provide.

Aerial plankton is similar in that it is made up of small creatures drifting along on currents.  However, instead of drifting in water currents, they drift through the air!  There are many, many species of insects, spiders, and other small organisms that make up the aerial plankton community and these creatures rely on wind currents to carry them from one place to another.  Basically, any small animal that can catch and updraft or find another way to get high enough into the air to get caught up by an air current becomes part of the aerial plankton.  Different things will use different methods to launch themselves into the currents.  Most insects fly.  Many spiders are known to “balloon.”  They extend strands of silk into the air that are caught in the wind, carrying the spider up into the atmosphere and away on the wind currents.  Ever read Charlotte’s Web?  The spiders leaving the egg sack at the end of the story were doing this exact thing.  Charlotte’s children became part of the aerial plankton!

It just so happens that this topic is one I’ve been just itching to cover recently thanks to a story by Robert Krulwich that popped up on NPR a few weeks ago.  Robert Krulwich does amazing reports on scientific topics and the animations that frequently accompany his stories are truly brilliant. They’re simple to understand, fun to watch, and get the main points across in a wholly engaging manner. I highly recommend that you check out his work if you haven’t already!  Rather than telling you why animals might want to drift around on air currents and how many organisms are floating around on our heads at any given moment, I’m going to direct you to Robert Krulwich’s story on NPR.  You can read the full article using the link, or you can just watch the animation, which sums it all up in a very succinct way:

I think this little video is quite brilliant.  So there are billions of animals floating through the air at any given time!  And they’re trying to move from one place to another efficiently.  There are certainly downsides to this sort of travel, the most important of which is this: these animals are really at the mercy of the winds and have very little control over where they end up.  It’s rather like hot air ballooning in that way.  As May Berenbaum said in the video, sometimes an animal ends up in a worse spot than when they started out.  It happens.  It probably happens a lot!  But many of those animals also make it to a better place than they started and make a good life for themselves in a new area.

Wandering glider (Pantala flavescens)

Wandering glider (Pantala flavescens)

So why am I bringing up aerial plankton?  Well, dragonflies likely use these same wind currents when they migrate.  They are, technically, becoming part of the aerial plankton when they do.  Some dragonflies are superb fliers, such as the wandering glider pictured at the right.  This dragonfly is known to fly over oceans and is found on all continents naturally.  While it can fly for many, many hours without resting, even these insects are probably getting a boost from the wind as they fly across oceans in search of new homes.  By using the power of the wind to propel them along, they can let the wind do some of the work for them and rest their wings to some extent.

But there’s another reason that aerial plankton and dragonflies are related as well.  Aerial plankton is a very valuable source of food for many animals, including predatory and scavenging insects.  What comes up, must come down, and aerial plankton is no exception.  Eventually, all of those billions of animals fall out of the sky.  Sometimes those things come down alive and other times they come down dead, but they’re important as a food source either way.  Storms are particularly hard on the aerial plankton community.  They can knock vast numbers of animals out of the sky and push them toward the ground, making them easy targets for things that might want to eat them when they do.  Dragonflies are thought to take advantage of this occasional shower of food from the sky and feast after storms.  This behavior is likely related to some of the behaviors observed in the big migratory swarms that I’ll be talking about next time, which is why I wanted to discuss aerial plankton first.

Aerial plankton is a vital dietary component for several insect species, but two come instantly to mind.  There is one insect order that is absolutely coveted by entomologists for collections because they are so hard to find and so rare to collect: the Grylloblattidae.  These insects live on snow fields in the Arctic and on the tops of very high and very cold mountains that are typically covered in snow.  Very few invertebrates can survive in these conditions (indeed, few animals live in them period!), so the grylloblattids rely almost entirely on aerial plankton as food.  Things fall out of the sky, land on the snow or ice, and the grylloblattids go skitting around on the snow collecting them.  These insects likely couldn’t survive at all without aerial plankton.   One of the only truly marine insects also depends on aerial plankton as a food source.  The water striders belonging to the genus Halobtes live on the open ocean, on top of the water like their freshwater relatives.  They’ll eat things from the water, but they also eat things that fall on the ocean’s surface from the sky and become trapped.  Aerial plankton likely forms a large part of the Halobates diet.

The next two posts will focus on dragonfly swarms.  First up is the discussion of the migratory swarm paper.  Then, I’m going to give another update on swarming activity in the US, including a map I’m developing of all of the sightings I’m collecting.  And as always, if you happen to see a dragonfly swarm, I’d love to hear about it!  Head over to my Contact page to submit a report.  I’m averaging about 10 reports a day this month so far, so keep them coming!

_______________

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