Using publicly collected data to study dragonfly swarms: Part 2

dragonfly swarm banner

Whew!  Last week ended up being really busy!  But it’s time now for another summary of the dragonfly swarm data I collected this summer on my blog.  Today’s topic: distribution of dragonfly swarms and the maps!

If you read my first summary post, you know that I learned that dragonfly swarms were very widespread across the US this year.  It actually became a sort of game for me to collect reports from all the lower 48 states as the summer went on.  I was thrilled that I got reports from nearly every state.  WAY too thrilled actually.  And, it irked me to no end that I didn’t get any reports from Louisiana.  I should have.  There were swarms in every neighboring state.  There have been dragonflies recorded flying across the Gulf of Mexico.  There should have been at least one report from LA!  Alas, the summer ended with my dream of collecting the 48 states unfulfilled.  Maybe next summer…

Part of the excitement of this project for me this summer was mapping out all of the reports I collected on Google Earth.  When I inputted the swarms on the map, it was easy to see when and where massive swarm activity was occurring.  It was also easy to see the patterns in the distribution of the swarms.  And, I learned that, though a lot of the work on migratory dragonflies has been done on the east coast, the total dragonfly swarms in the midwest vastly outnumbered the eastern swarms, nearly 20 to 1!

The best way to illustrate the distribution of the swarms is to show you my maps.  Now I could put up a ton of maps and have you scroll down through the longest blog post of all time, but I think video captures the activity much better.  As you watch the video below, you’ll see a series of maps, one for each week starting with the week of 7/4/2010 and ending in mid-October.   A few things to keep in mind as you watch:

  • The maps are cumulative.  The pushpins for a week are added to all of the pushpins from the weeks previous.
  • I’ve zoomed out sufficiently to see all of the areas from which I received reports, including Belize down at the bottom right of the frame.  Every report is represented by its own pushpin on the map, but with this level of zooming you can’t see all of them.  Remember that this map represents about 640 reports!
  • The colors of the pushpins have meaning, which is explained in the video.
  • The viewer is tiny, so the pushpins are hard to see.  This is a high definition video, so I highly recommend that you push play, the click on the resolution button at the lower right of the viewer (it’s the one that says 360p) until 1060p is displayed.  Then make the video full screen by clicking the button at the very lower right.  Press the ESC key to exit full screen mode when you’re done.

On to the video!

I think these maps are fascinating and I’ve thought of a hundred different ways I can display the information they contain.  I’ll discuss what I think this all means in the third and final post in this series, but I think you can take away a few ideas from simply looking at the maps.

First, the swarming activity in the Great Lakes region was intense this summer!  I have a few hypotheses for why this might be that I’ll share next time, but it’s obvious that there was some major swarming happening in the north central US this year.  However, if you look at the colors of the pushpins, you’ll notice that the vast majority of the swarms in the midwest were static feeding swarms.  Some migratory swarms were reported in the midwest (most notably in Nebraska), but most of them were reported along coastal areas.

And speaking of coastal areas, there were reports of dragonfly swarms on both coasts!  Western areas do see both migratory and static feeding swarms.  But, take a look at the video again if you missed it: the dragonfly swarms reported in the western US were much fewer than in the eastern half of the country.  In fact, in most western states, I only received a single dragonfly swarm report.  Some of these were bizarre reports too.  The report from Colorado described thousands of dragonflies sleeping on a ranch until disturbed when the rancher returned in his truck at 10PM.  The dragonflies burst into frantic activity in the dark in what sounded a lot like the climax of Hitchcock’s The Birds.  So, swarms might occur in many different regions and I may have gotten to check off nearly every state on my US map, but it does look like swarms are more common in the midwest and east than they are in the west.

I feel obligated to caution readers about reading too much into these data though.  Consider the person who sends in a report.  This is a person who happened to see a swarm in his or her area, then felt interested enough in the event to look it up online, then spend a few to several minutes making a report.  However, my blog had hundreds of hits a day from people looking for information about dragonfly swarms while I maxed out at about 30 daily swarm reports.  This means that for every person who saw a swarm and was interested enough to look up more information about it (and I think most of these people had seen a swarm or they wouldn’t have thought to seek more info in the first place), there were about 20 others who didn’t submit reports.  The data is somewhat subjective because it is depends on the individual’s level of interest in the behavior and that’s going to vary a lot from person to person.  We don’t all love this subject as much as I do!

The data is also somewhat subjective based on some simple demographics of the states.  The state of Wyoming has a total state population less than that of the city I live in (about 544,000 people)!  It’s also the 10th biggest state in the country.  The lack of people and the space between them in Wyoming means that there is a huge potential for swarms to occur without human witnesses.  Swarming almost surely occurs in WY, but I didn’t receive any reports.  I think this may have occurred in many areas of the western US.  Note that there are several pins in my area in southern Arizona.  There are likely lots of reports in my area simply because I live here!  People in my entomology department tell me about how they saw this interesting dragonfly behavior and wonder if I know anything about it.  When I hear anything about dragonflies that suggests “swarm” to me, I question that person.  My friends are on the lookout for swarms – and they find them – because it’s something I’m deeply interested in!  Hence, lots of pushpins in my area, in spite of its western location.  As a result, I think western swarms may be much more widespread than the maps lead me to believe.  If we can find this many swarms in the Sonoran Desert, there are likely many more swarms in areas with more water.

Because I think it’s interesting, I want to finish this post up with an image of my final dragonfly swarm map and this year’s monarch migration map:

dragonfly swarm map

Dragonfly swarms

monarch map

Overnight roosting sites reported for 2010 monarch southern migration. Image from http://www.learner.org/jnorth/maps/monarch_f10_roosts.html.

I’m not going to say too much about this because I don’t know what it means, but I would like to point out a few things:

  • The distribution of dragonfly swarms is nearly identical to that of monarch roosting sites.
  • Monarchs were reported flying with dragonflies in migratory swarms in the majority of the reports I received.
  • Both sets of data were collected via submission of reports by people who chose to make reports online.

The similarities between the two data sets surprised me, so I am putting them out there for you to make your own tentative conclusions.  I intend to continue collecting data on dragonfly swarms on my blog for several more years and think pursuing the links between the dragonfly and monarch activity could be prove very interesting.

In spite of my cautions, I still think the swarming activity in the midwest this summer was something special.  I also think that swarming is much less common in the west than in the midwest and east.  Next time, I’ll share why I think these things and make some tentative conclusions based on the data I collected.  Check back soon for the final installment!

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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!

_______________

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

Dragonfly Swarms: Static Feeding Swarms

At long last, I am finally getting to the information about static swarms in dragonflies!  Apparently posting once a week is a bit ambitious for my current schedule and even my weekend became one giant black hole of work.  I’ve had no time to blog!  Better cram this post in quick while I have a few free minutes…  :)

Before I jump into what’s known to science about static dragonfly swarms, take a moment to ponder the amazing swarm depicted in this image I found online:

Isn’t that glorious?!  I can’t be sure that this is a real photo, but based on what others have reported and what I’ve observed myself, this is certainly not outside the realm of possibilities!  The people in the photo aren’t enjoying the experience, even though the dragonflies in the swarm were completely harmless.  I wish I were in their place!  I would run out and stand in the middle of the swarm, soaking up the sound of the dragonfly wings fluttering against one another and the sight of several thousands of my favorite animals flying in a single location.  This is what my heaven looks like!  (And yes, you read that right: I did just say that my heaven involved massive numbers of flying insects.  What can I say?  I’m an entomologist!)

Now for the science!  If you’ve read any of my swarming series, you know that dragonflies migrate, sometimes in mass migratory swarms, and that migrating dragonflies have patterns of behavior similar to migrating birds.  I have also speculated, based on my own observations and those of people who reported swarms to me early this summer, that the non-migratory swarms (what I call static swarms) are feeding swarms.  Today, I’m going to go over what’s known about these swarms.  Unfortunately, most of this information is either buried in the scientific literature where it’s largely inaccessible to the public or in a $100 dragonfly book that, while truly brilliant, is also largely inaccessible.  And let’s face it.  Apart from college libraries, only odonatologists (scientists who study dragonflies) and other entomologists are going to shell out that kind of cash for a dragonfly book.  For those of you who don’t LOVE DRAGONFLIES SO MUCH that you’re willing to run out and fork over 100 bucks for a book (i.e. you’re normal), I’ll summarize what is known about static swarms here!

First of all, the static swarms are almost always feeding swarms!  I’ll go over the reasons why you might see these feeding swarms in a particular area in a moment, but first a few interesting facts.  First, this behavior appears to be exclusively anisopteran.  This means that the behavior is observed in dragonflies only, not their close relatives the damselflies.  This is probably because dragonflies exhibit vastly superior flight compared to damselflies. However, among dragonflies, many species of both perchers and fliers will take part in the swarms and fly for extended periods of time.  Both males and females swarm, though males are more commonly observed.  Swarms can be made up of several different species, and can even include other organisms, such as the vertebrate birds and bats.  If birds or bats are present, they will usually be found just above the dragonflies, feeding on the same insects the dragonflies are eating rather than on the dragonflies themselves.  And for those of you who are shocked at the number of dragonflies in the swarms you’ve see, you likely only saw the tip of the iceberg!  In one of the only studies looking at the number of individuals swarming within a population, only 16% of the dragonflies in the area participated in the swarm.   Just think: if you see 1000 dragonflies in a static swarm, that means that there were likely 6250 total dragonflies nearby!

You’ll most often see dragonfly swarms near dusk or dawn, and it’s thought that the dragonflies can see flying prey (most often mosquitoes or non-biting midges, but also termites, ants, and honey bees)  better when the sun is close to the horizon.  During these times, you may notices that dragonflies appear very suddenly, fly in circular patterns over a very specific area for some time, and then disappear as quickly as they arrived.  This is because the dragonflies are attracted to large groups of prey organisms.  Once the prey numbers drop or they become less active (e.g. as it get darker), the dragonflies move on.  If the prey return the next day, the dragonflies likely will too.  In some particularly productive areas where prey are consistently available, you might even see a swarm form nearly every day for months.

There are several reasons why dragonflies might congregate in one area versus a similar area nearby, but in nearly every case there is an abundance of prey species present in the area containing the swarm.  Dragonfly swarms will form where other insects are swarming.  Most people have seen a big swarm of gnats at one point.  Those swarms are like a dragonfly buffet!  The dragonflies will swoop in and out of the fly swarm, picking off flies and eating them on the wing before going back for more.  This is likely what happened in August when the dragonflies descended on Milwaukee.  Massive numbers of mosquitoes in the area drew dragonflies into the city and large swarms formed where the mosquitoes were congregating.  Dragonfly swarms often form when there is a seasonal emergence of ants or termites as well.  This was the case in the swarm I witnessed.  The ants and termites were emerging out of the grass and the dragonflies were catching and eating them as they emerged.

Dragonflies might also be attracted to objects that attract other insects.  If prey insects are consistently attracted to a particular object, dragonflies can learn to associate that object with a good meal.  In one study, dragonflies were found over a set of traps intended to attract other insects, feasting on the insects flying in toward the trap.  The prey insects eventually stopped coming to the traps, but the dragonflies returned for several more days.  In this case, the dragonflies were attracted to the traps and not the insects themselves because they’d learned to associated the traps with an abundance of prey.

Dragonflies might also be observed swarming in areas where a weather front has just passed through.  Insects often get trapped in fronts and are pushed along with the winds for some time before being deposited somewhere else.  When they finally free themselves from the front, they might find the dragonflies ready!  Dragonflies take advantage of these windfalls of prey by forming swarms and eating the insects as they arrive.  This sort of feeding also happens during migratory flights.  The same fronts that deposit large numbers of prey insects in an area help the dragonflies fly long distances, so prey is readily available when the dragonflies stop to rest and feed.

In wooded areas, many insects will congregate in sunny, open patches.  Lots of dragonfly swarms form in small sunny patches to take advantage of the other insects that are attracted to the spaces.  The swarms of prey insects will move as the sun changes position, os the dragonflies will move too.

In high winds, insects will congregate in lee areas (areas protected from the wind).  Lees promote dragonfly swarming behavior because of the high abundance of insects found in these areas.

Finally, and I think most remarkably, some dragonflies will swarm in areas where insects are being stirred up due to some sort of disturbance.  Mowing your lawn?  It disturbs the small insects living in your grass and cause them to fly around more than they usually would.  The prey draws the dragonflies in and swarms form.  Similarly, some dragonflies have learned to follow large, slow moving objects (these could be people, bicycles, cows, cars, etc) because they disturb prey insects as they move and encourage the prey to fly – often into the eager grip of a hungry dragonfly.

So all of this boils down to one simple concept: any time you have an abundance of dragonflies in an area as well as a significant prey population, you are likely to see dragonfly swarms.  The behavior is thus fairly common in many different species of dragonflies.  That said, the chances of a single person seeing more than one or two swarms in their lifetime in a single area can be quite low.  The conditions have to be just right for swarms to occur, perfect for both a large number of prey insects AND a large number of dragonflies to exist in the same area at the same time.

Next time (and I’ll get the post up much more quickly this time), I’m going to discuss some of the references available for identifying dragonflies, both in print and online.  In the meantime, keep sending me swarm reports!  I am beyond thrilled with the participation in my swarm project, so keep ’em coming!

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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!

_______________

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

Dragonfly Swarms Revisited

Pantala flavescens

A wandering glider (Pantala flavescens) from the swarm I witnessed last summer.

Since I put out my request for reports of dragonfly swarm sightings a month ago, data have been streaming in!  I thought it was time to give a brief report of the dragonfly swarming activity in North America this summer so that everyone who’s sent a report in can see how much I appreciate your sending me reports.  I couldn’t be happier with the information that’s been coming my way, so thank you all so much for helping me track this behavior.  Keep the reports coming!

First things first.  In my next post, I’m going to cover a paper I came across on dragonfly migratory swarms in North  America from 1998.  I think it will give everyone a better idea of why these swarms might be forming and what they’re doing.  For now, just know that there are two types of swarming behavior:

  1. Migratory swarms. These are effectively rivers of hundreds of thousands of dragonflies all flying in a single direction and covering large distances.  These types of swarms are like bird migrations or the migrations of monarch butterflies – lots of individuals traveling together between habitats and usually made up of a single species or with one dominant species and a few other minor players.  These swarms move very quickly and may appear and disappear in a matter of minutes.  The dragonflies in these swarms typically follow significant waterways and fly high above the ground (20-100 feet).
  2. Static swarms. This is the type of swarm I reported on last summer and the type that prompted my interest in this behavior.  These swarms contain far fewer individuals than migratory swarms (20-1000 instead of tens of hundreds of thousands) and are highly localized.  Individuals in the swarm will remain restricted to a very small area (like one field or yard or hill) and fly in a circular or figure-8 pattern about 1-20 feet off the ground, usually over a grassy area.  These swarms are likely feeding swarms and may contain one to several species of dragonflies in about equal proportions.  (Please read the post from last summer linked above for a more detailed description of the behavior of the swarm I witnessed and a video.)

These two types of swarms, though very different, might be related to one another.  There are some striking similarities between the two types that suggest that this might be the case.  For one, the known migratory dragonfly species are the same species that appear to be making up the static swarms.  Also, both types of swarms occur during the same part of the year.  Last, both types of swarms seem to be weather related.  Almost everyone who has reported a swarm has also reported a recent storm or an incoming one, especially after a long period of hot, dry weather.  Weather is thought to play an important role in the migratory swarms as well.

When I wrote my initial posts on dragonfly swarms, I got a few reports from people who had seen them in other locations.  I also got several people stumbling onto my blog after searching for dragonfly swarms on the internet.  Last summer I got maybe 30 hits a week on my swarm pages and none at all for most of the rest of the year.  This year, I’m getting 500 hits a week!  Clearly, something is happening this summer that is making these swarms much more abundant and visible than they have been in the past.  I can’t, of course, say for sure why this is, but I suspect it has something to do with the weather pattens we’ve seen this year.  I saw a report on the National Geographic website a few days ago that said that this is the hottest year on record in the US – and we started recording weather data in the late 1800’s.  Perhaps the hot weather and the recent rains have something to do with the huge number of swarms (and the large size of some of these swarms) that have cropped up this year.

So what patterns have I been able to identify from the data I’ve collected from reports from readers?  There are definitely a few locations that have a lot of static swarming activity recently:

  • Eastern Missouri in the St. Louis area.  There seem to be many swarms within a massive area from slightly north of St. Louis south to the MO border and extending west to the central part of the state.  These swarms are highly localized when they appear and are often in one person’s yard or field and not in the yard/field next door.  The swarms have been made up of several different species and consist of several hundred dragonflies.  It has been quite hot in the area, but they’ve had some recent storms.
  • Northern Illinois/Wisconsin.  These swarms may be part of the Missouri action because they are very similar.  They also cover a large area, consist of mixed-species in highly localized areas, and they’ve been showing up after storms.  It’s been hot in this area recently too.
  • New Jersey.  These sightings seem to be completely unrelated to the swarms in the Midwest, but they’re spread across the state.  The swarms are a bit smaller than the Midwestern ones (20-100 individuals), but they’re often made up of multiple species and have appeared before or after storms.
Pantala hymenaea banking

A spot wing glider (Pantala hymenaea) from the Tucson swarm last summer.

Other sporadic swarms have been reported in Iowa, Ohio, Texas, Tennessee, New York, Connecticut, California, Pennsylvania, and Saskatchewan.

Based on the data I’ve collected so far, it looks likely that in many areas, there are very large, widespread groups of dragonflies.  The swarms people are seeing in their yards may be a subset of these larger super-swarms.  I’m starting to think this because I’ll get 20 different reports from one rather large geographic area all describing the exact same thing over the course of 3 or 4 days.  I’ll get another 20 reports from another area, all with similar descriptions and within a few days of each other.

I did also get several individual reports of a massive migratory swarm in downtown Little Rock, Arkansas earlier this week.  The accounts I heard suggest that the swarm was made up of truly staggering numbers of dragonflies, maybe hundreds of thousands or millions of individuals, that all flew right by the 6th floor windows of several buildings.  One reporter said he couldn’t see the building across the street as the dragonflies flew by!  They were all flying parallel to the river and in a single direction and the whole event was over in less than a minute.  Wow.  I would have given anything to see that!  If anyone else is lucky enough to witness an event like this, I hope you will send a report to me!  However, though I got many reports of this single event, they’re the only reports of anything like migratory swarms I’ve gotten all summer.  It appears that the static swarms are much more common than the big migratory swarms, but I think there’s a good chance that they’re related to one another.

And this is where you all come in!  Keep sending me your reports!  With your help, I might be able to get a better handle on the movements of my proposed super-swarms and determine whether the migratory and static swarms are actually related.  I might also be able to determine whether the Midwestern swarms are all part of one giant swarm or the Missouri and Illinois/Wisconsin swarms are separate.  Plus, I have a feeling this is a special year for this behavior and we might never see another summer like this again.  I’d like to collect as much data as I can now so that I can compare this year to future years and to take advantage of the abundance of swarms that are cropping up throughout the country.

Check back soon for the summary of the migratory swarm behavior paper!  Judging from the number of people who have been searching for information on dragonfly swarms on the internet this summer, it might be quite interesting for people hoping to explain the phenomena they’ve been seeing in their backyards.

(Want more information about dragonfly swarms?  Visit my Dragonfly Swarm Information page for my entire collection of posts on dragonfly swarms!)

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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!

_______________

Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

Unless otherwise stated, all text, images, and video are copyright © 2010 DragonflyWoman.
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Click! Click! Click!

In the summer months in Arizona, it’s common for things to come crawling into your house to avoid the heat.  Leave a door open and your house will soon be full of mosquitoes, geckos, several types of beetles, webspinners (which are insects and NOT spiders), green lacewings, sun spiders (also not spiders), moths, maybe even the occasional bark scorpion or snake.  I happen to have a dog door that doesn’t quite close all the way due to the pressure built up by the evaporative cooler, so all kinds of insectoid creatures make their way into the house that way.  My personal favorites of the things that end up in my house are these:

click beetle

Click beetle, likely Melanotus sp.

Click beetles!  If you haven’t ever gotten to play with a click beetle (and let’s face it – most people don’t squeal with delight and immediately put their hands on random beetles they find the way my crowd does), you’re missing out.  You might not know why they’re called click beetles either.  Rather than explaining it, take a look at this short video I took a few nights ago of a click beetle that made it’s way into my bathroom (where all click beetles seem to end up in my house).  The quality isn’t fantastic as it was taken in a poorly lit inner room at night with a hand held camera, but you’ll get the idea.  And make sure you have the sound on!  Clicking the arrow near the lower right corner and selecting 1080p after you click play will improve the image quality.

Isn’t that wild?!  And did you hear the clicks?  These beetles are called click beetles for a reason: they make clicking sounds when they do their super awesome ninja-like jumps.  Those jumps are possible due to this structure on the underside of the beetles:

clicking mechanism

Click beetle underside. The arrow points to the click mechanism. Pryophorus sp.

click mechanism side view

Click mechanism side view. Pyrophorus sp.

Every entomologist knows this much about click beetles:

1) They click
2) They jump
4) They’re flat, elongate beetles with spines on the middle section of the thorax (you can see them in the pictures)
3) Both clicking and jumping are made possible by the spine indicated by the arrow on the second thoracic segment (the middle section of the thorax) fitting into the groove on the third thoracic segment.

When I decided to write about click beetles, it occurred to me that I didn’t know exactly how this mechanism worked.  The online search was worthless as nearly every site simply copy and pasted the Wikipedia entry on click beetles, and that told me what I already knew – spine fits into groove, makes beetles jump.  Looking through my entomology books wasn’t much more helpful: spine fits into groove, makes beetles jump.  Before I dug into the literature on the subject, I decided to try one final textbook, The Insects: Structure and Function by the late, great Reg Chapman (a very lovely and brilliant British man who taught one of my first entomology classes in grad school).  The book is one of the most dense books I’ve ever read – and I read it cover to cover to prepare for my comprehensive exams for my Ph.D. – but it’s an amazing treasure trove of information about insects.  Reg’s book told me how the mechanism works.

First, the beetle arches the center of its body upward off the ground so that only part of the thorax and the tip of the abdomen are still in contact with the ground.  It then contracts the muscles around the spine.  Normally this would result in the movement of the thorax, but the spine catches on the groove so the thorax doesn’t move.  Instead, energy is stored up as the beetle continues to contract the muscle and the spine remains trapped in the groove.  Eventually, the spine slips off the groove and all of that energy is released.  The front and back ends of the beetle, the parts that were still in contact with the ground as it arched, snap upward off the ground at a high velocity.  The velocity is so great that the rest of the body is pulled up after them, launching the entire beetle into the air.  If this is hard to picture, imagine shooting a rubber band off your finger toward the ceiling.  You pull back on the rubber band with one hand while catching it on a finger of the other hand.  The rubber band represents the spine and the muscles attached to it while the finger is the groove.  Energy is built up as you stretch the rubber band further and further back, similarly to how the beetle stores up energy as it contacts the muscle around the trapped spine.  When you release the rubber band, the part you pulled back launches forward, releasing the energy stored in the rubber band and pulling the entire band off your finger in the process.  (Well, assuming you’re holding it right, otherwise it releases all that energy into your soon-to-be-painful finger instead.)  Now imagine the rubber band and finger setup inside the beetle and it should give you a pretty good picture of how this works.

So why do these beetles do this?  There are two main reasons.  This clicking-jumping behavior is likely primarily a strategy to avoid being eaten by predators.  Most things that try to eat a click beetle will think twice if the beetle launches up into their face as they try to eat it.  It’s a startle tactic: if they can distract the predator for even a moment or two they just might be able to run away.   You can see this in the video.  The beetle clicks when I (a possible predator) touch it, then it runs.  I think this anti-predator mechanism is likely very effective.  Watching one of my dogs messing with them certainly suggests that it is!  He’ll sniff the beetle, then jerk back in horror as the beetle launches itself into his nose.  He might then put a paw on it, at which point it will click again, so he’ll jerk his paw back.  He usually tries the paw thing twice before he leaves the beetle alone.  Granted, this only works so long as the other dog doesn’t see him playing with the beetle.  The smaller dog is so jealous the bigger one is getting something that she’s not that she’ll run over and eat the beetle, clicking or not, just so her brother can’t have it.  :)

But back to the beetles!  The other reason these beetles likely click is to be able to flip themselves over when they end up on their backs.  These beetles have rather wide bodies and stubby little legs, so they have a hard time getting themselves right side up again.  Clicking to the rescue!  The clicking mechanism works whether they’re right side up or upside down, so the beetle can simply click, launch itself in the air, and hope it lands on its feet.  If not, it will click again until it does.  You can see this in the video too.   The beetle easily rights itself every time I flip him over.  Pretty neat trick, don’t you think?

So now you know how and why these beetles jump.  I’ll end this post with a brief comment on variations in size and coloration in these beetles.  While most of the click beetles most people see are smallish, drab beetles, there are some amazingly beautiful click beetles in the world.  I leave you with pictures of what I think are the two most spectacular species found in Arizona.  Enjoy!

Chalcolepidius ostentus

Chalcolepidius ostentus. Not sure of a common name, but I call this one the really pretty click beetle. :)

Alaus zunianus

Alaus zunianus, the Zuni eyed elater

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

Giant Water Bugs Eating

I didn’t post anything last week because I was in the midst of chaos with family and friends, making final preparations for my wedding last weekend.  But now I’m back at the computer and getting things done!  Since I just took a short break, I thought I’d take this opportunity to express my thanks to everyone who’s been keeping up with my blog.  I appreciate your comments and your support!  It’s gratifying to know that something I enjoy doing so much is informative and helpful to others.

Lethocerus indicus eating a small fish

Lethocerus indicus eating a small fish

Today I thought I’d post a short video that illustrates how giant water bugs eat.  Giant water bugs are fierce predators and are known for being able to take down very large prey such as snakes, fish, turtles, and even birds!  Even more amazing is that they accomplish these feats while they barely move at all!  Giant water bugs are called sit and wait predators.  If you think about that phrase for a moment, the behavior it describes should become obvious: giant water bugs sit in one place and wait for prey to swim by.  As much as I love giant water bugs and try to get people excited about them, I’m the first to admit that they sit in one place for long periods of time without moving at all.  In fact, they can even be a bit boring to watch at times.

Abedus herberti

A giant water bug in the pose they normally adopt while waiting for food to swim by.

However, any boredom instantly disappears if the giant water bug you’re watching encounters food!  Part of what makes them exciting to watch is their structure.  I wrote about how to tell the American giant water bug genera apart several months ago and talked about the raptorial forelegs that giant water bugs possess.  Most of the time, you’ll see giant water bugs sitting underwater, holding onto a rock, the bottom of the pond or stream, or a piece of vegetation with only their hind two pairs of legs.  The front legs, those strong raptorial forelegs, are held in front of them, as in the photo above.  If food swims by, they thrust those muscular forelegs forward very rapidly, fold the legs over the food, and then retract their legs back toward their bodies, bringing the prey close to their heads.

The giant water bug then begins to eat its food.  First, it has to find a place it can insert its piercing-sucking mouthparts, that beak that you can see hanging off the front of the head of the bug in the image above.  It will probe the prey item with its beak until it finds a soft place into which it can insert its mouthparts.  The water bug then injects the prey with chemicals that break the tissues down, turning them into a sort of soup.  Finally, the bug sucks the liquid out of the prey and into its own body.  This part is rather like what you do when you take a drink or eat a smoothie with a straw!

Depending on the size of the prey item, the eating part of the process can take a very long time, up to 10-12 hours.  It takes a long time to inject all those chemicals and suck up the resulting soup.  But the prey grabbing happens VERY fast!  So fast that most prey items probably don’t even see the bug before it’s too late.  And so fast that the very first time I fed a bug as a graduate student, I dropped the forceps in which I held the prey (a mealworm) and jerked my hand out of the way as hard as I could.  There may or may not have also been a loud girlie shriek involved, one that I may or may not have been very happy that no one else was in the lab at the time to witness.  :)

So, to get an idea of just how fast water bugs are when they grab food, I recorded my lab bugs the last time I fed them.  Without further ado, I give you a giant water bug (species: Abedus herberti) eating a mealworm!  Pay special attention to how fast the bug grabs the mealworm.  If you look closely, you can also see it probing the mealworm with its beak!  Look for the probing in the space between its eyes and its left foreleg:

Pretty cool eh?  That speed and power in their forelegs allow giant water bugs to catch and eat some very large things.  How amazing is it that an insect, and an aquatic insect at that, can capture and consume a bird?!  And I’m not talking about little birds either.  There is a published report of one taking down a woodpecker.  Now that’s just impressive.

I should be back to my usual posting schedule now, so look for a new post next week!  I’ll be installing an educational aquatic insect pond at the Biosphere II soon, so I’ll be posting about that for sure.  But first, another quick video, this time of a non-insect aquatic invertebrate: the flatworm!

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

Better Breathing Underwater in Aquatic Insects

If you’ve been keeping up with my blog in the last few weeks, you already know about the basic insect respiratory system and how aquatic insects breathe.  As you know, aquatic insects have a wide variety of respiratory adaptations that allow them to breathe in water, adaptations that have enabled them to live in water after insects developed on land.  However, aquatic insects can improve their ability to breathe underwater (i.e. improve their respiratory efficiency) by choosing to live in areas that have more oxygen (a higher dissolved oxygen content) or by using a variety of behaviors that allow them to take more oxygen from the water.  These are the topic of today’s post!

diagram of diffusion

Molecules move from areas of high concentration (on the left) to areas of low concentration so that they are evenly distributed throughout (on the right). Image taken from Wikipedia.

There is a lot of physics involved in understanding how insects are able to breathe in water and you’ll need to know some of the basics.  The most important thing to remember is that molecules, in general, want to be evenly distributed.  If there are more molecules of one type in one area versus another area, a concentration gradient forms (see my post on terrestrial insect respiration for more information about concentration gradients).  Molecules then tend to move from areas of higher concentration to areas of lower concentration (see image at right).  This happens with oxygen in both air and water and is the reason why the insect respiratory system works at all.

However, oxygen moves very differently in water than it does in air.  Most importantly, oxygen moves much more slowly in water than in air – up to hundreds of thousands of times more slowly!  If you allow an open container of water that has no dissolved oxygen in it to sit, oxygen will eventually become distributed throughout the water in the container, but it will take a very long time to do so.  And even when it does become evenly distributed in the water, there is still far less oxygen in the water than there is in air.  In air, oxygen has a concentration of about 210,000 parts per million.  This means that for every million molecules of the gas mixture we call air, 210,000 of them are oxygen.  In water, a concentration of 5 parts per million oxygen is considered pretty good and oxygen concentrations max out at only about 15 parts per million.  So, oxygen takes a very long time to move through water (it diffuses from areas of high concentration to areas of low concentration) and is present in only very small amounts.

The concentration of oxygen in water (the amount of oxygen in water, the parts per million) depends on a variety of factors.  Any movement of water will increase its dissolved oxygen concentration.  Turbulence of any sort, whether caused by wind, stream flow, objects in a stream, etc, stirs the water so that oxygen from the surface is pulled further into the water more quickly than if the water was still.  Water is also able to hold more oxygen when it is cold than when it is warm.  Very cold water, close to freezing, can hold nearly the maximum concentration of oxygen, close to that 15 parts per million.  Very warm water, like what you get in Arizona during the summer, might only hold 2 or 3 parts per million oxygen at best.  Finally, in general, water that has a lot of organic pollution, algae blooms, or is otherwise compromised often has less oxygen than waters that are very clean.  Pollutants and some biological organisms consume oxygen, driving the concentration of dissolved oxygen in the water down and making less available for the everything else to use.

fast flowing, cold water

An exmaple of a fast flowing, cold water stream where stoneflies like to live. I know it doesn't look like it in the picture, but the water was really ripping! This stream is in the White Mountains of AZ.

These facts have some important implications for aquatic insect respiration.  Let’s consider the types of habitats that aquatic insects live in.  Insects that need a lot of oxygen are going to live in places that have a lot of oxygen rather than places that do not.  Stoneflies, for example, live in very high oxygen environments – at least by aquatic standards.  They are usually found in fast flowing, very cold water streams.  Many of these streams have a lot of turbulence as well.  This type of water generally has about the maximum dissolved oxygen concentration possible.  And some stoneflies have large gills, which allows them to absorb even more oxygen from the water!  You probably won’t ever see a stonefly in a warm, slow flowing, polluted stream.  However, you might find a bloodworm there!  Because bloodworms have hemoglobin, they are able to live in very low oxygen environments where many other insects are unable to live.  Their bodies are designed to attract oxygen so that they are able to draw in oxygen even when there is only a tiny amount available in the water.  In fact, in some very polluted, slow flowing or still waters (like the water you find coming out of some poorly designed wastewater treatment plants), bloodworms might be the only insects in the water at all!

A predacious diving beetle in water.  The blue thing at the back end is the part of the air bubble it carries that is exposed to the water.  This allows the beetle to use the bubble as a physical gill.

A predacious diving beetle in water. The blue part at the back is the portion of the air bubble the beetle carries with it underwater that it exposes to the water. This allow the beetle to use the bubble as a physical gill.

Concentration gradients allow some other nifty things to happen for insects that carry air with them underwater, those insects that use the scuba tank style of respiration like the beetle at the right.  In my last post, I said I would explain why a giant water bug might want to expose the air bubble it carries under its wings to the water, and here’s the reason!  When an insect carries a bubble of air with it underwater, the concentration of gasses in the bubble are those found at the surface, about 21% oxygen, 78% nitrogen, and the rest a mixture of other gases including carbon dioxide.  When the insect dives underwater, it begins to consume the oxygen in the bubble.  However, the bubble wants to remain at equilibrium, it wants to keep its gas composition the same as the atmosphere at the surface.  If the insect is using oxygen from the bubble, the equilibrium is thrown off – the oxygen level is no longer at 21%.  But if the insect happens to expose its bubble to the water (see the blue air bubble at the back of the beetle in the image), oxygen will actually flow from the water into the bubble, replenishing the oxygen lost!  This works because, even though there is much less oxygen in water than in air, oxygen makes up a greater % of the gases dissolved in the water (about 35%) than in air (21%).  This sets up a concentration gradient so that oxygen flows from the area of higher concentration, the water, to the area of lower concentration, the bubble.  This type of respiration is called physical gill respiration because the insect is using the air bubble like a gill.

Now, it may seem like a physical gill would allow an insect to remain underwater indefinitely, but this is unfortunately not the case.  As the oxygen is consumed by the insect, the concentration of oxygen in the bubble decreases and the gas mixture is thrown out of equilibrium – the ratio of oxygen to nitrogen is no longer the same.  Remember how the bubble wants to remain at equilibrium?  Well, it will do whatever it needs to so that equilibrium is restored.  There are two ways to do it: increase the oxygen in the bubble or decrease the nitrogen in the bubble.  As oxygen is consumed, nitrogen starts to flow out of the bubble to restore the equilibrium.  Oxygen flowing into the bubble from the water slows the loss of nitrogen, but there is so much less oxygen available in water compared to air that consumption of oxygen often outstrips the flow of oxygen into the bubble.  So, nitrogen slowly seeps out of the bubble, making the bubble smaller and smaller until the bug must go to the surface to replace the bubble altogether.

Physical gill respiration is a really excellent adaptation for aquatic insects that rely on atmospheric air.  It considerably increases the length of time an insect may remain underwater and decreases the trips to the surface the insect must make.  However, it only works when the bubble is exposed to the water.  Insects that don’t expose their air bubbles don’t gain the benefits of physical gill respiration.   And any insect that carries lots of air with it, regardless of whether it exposes its bubble to take advantage of physical gill respiration or not, has some negative side effects.  One of these is the insect becomes very buoyant and tends to float to the surface any time it isn’t holding on to something or actively swimming.  This requires energy to do.

water boatman

A water boatman, one of many aquatic insects that use ventilation to improve their respiratory efficiency.

Another downside to physical gill respiration is related to the speed at which oxygen moves through water.  Imagine an insect that carries an air bubble in still water.  As the insect consumes oxygen from the bubble and it is replaced by oxygen from the water, the oxygen close to the bubble is quickly depleted.  Because oxygen moves so slowly through water, this is problematic: if the insect doesn’t move, it will take a very long time for oxygen to move into contact with the bubble once more, likely longer than the insect can stay underwater.  This is where our friend turbulence comes into play again!  Several insects exhibit behaviors that ventilate their respiratory system, that is behaviors that bring freshly oxygenated water into contact with the bubble or other respiratory structures while pushing de-oxygenated water away.  The beetle in the image above is an excellent example of this.  It swims around through the water a lot, exposing its air bubble to the water the whole time.  The water boatman at the left uses his legs to stir the water around his air bubble.  And some insects that use physical gill respiration live in flowing water so oxygenated water is almost always flowing by them.

All this boils down to a simple concept: many aquatic insects exhibit adaptations, either structural or behavioral, that allow them to remain underwater as long as possible.  They actually have some control over how much oxygen they are able to take in while underwater!  Next time, it’s another From the Literature post.  I’ll be going over a recent paper on a structural adaptation in some beetles that allows them to breathe in water.  I think it’s amazing, so I hope you’ll check back soon!

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Text copyright © 2010 DragonflyWoman.wordpress.com

Dragonfly Territoriality (The Dragonfly Trilogy, Part Two)

Welcome to part two of my odonate trilogy!  Last time I discussed the reasons why I think dragonflies are the best insects.  However, I didn’t talk about the final reason I think dragonflies are amazing and that is the subject of today’s post: dragonflies are territorial!

Even if you aren’t a biologist, you probably know a bit about territoriality already.  Ever see a dog lift his leg on a fence or a tree?  That is a territorial behavior, a way for the dog to say, “This is MY space, so I’m going to mark it as mine!”  In canines, males mark territories with urine and other odorous compounds so that they can chemically signal to outside dogs that the space they are marking is part of their territory (the space in which they hunt and find mates) and that outside males should stay away if they want to avoid a confrontation.  They’re trying to convince other dogs that they are bigger and badder and that they can take on anyone that wants to challenge them for their space.  Naturally, your dog peeing on a tree doesn’t have quite the same meaning as it might if it were a coyote or a wolf doing the peeing.  After all, it’s hard to defend a territory on a leash!  Still, the behavior hasn’t entirely disappeared from our domestic pets and so our male dogs continue to pee on trees.  (And yes, I am resisting the urge to post a picture of my dog peeing on my fence!)

Male dragonflies are also territorial, but their system is very different from canines and is more behaviorally complex in many ways.  Let’s go over the process!

dragonfly habitat

A typical dragonfly habitat

First of all, males are territorial because females choose mates based on who provides the best real estate for her eggs.  For a female dragonfly, this might be a nice mat of algae, open water, or a stand of cattails – different species will look for different things.  A female dragonfly will go to an appropriate body of water (flowing or still, depending on the species), find the best place to lay her eggs, and mate with whatever male happens to be in the area.  The females of some species require courting rituals, but others will just mate with the male in the area.  The pair mates as described in my last post and the female will lay eggs in the area she has chosen.  Then she’ll fly off, perhaps waiting a few more days to return to the water to lay some more eggs or simply moving to another spot.

Because females choose mates based on the quality of oviposition sites (the word oviposition means to lay eggs, so an oviposition site is the location where eggs are laid), males who remain in the best areas will be able to mate with more females than those who are in lesser quality areas.  It’s even better if you’re THE ONLY male in that really great spot so you get all of the females.  Hence, territoriality began!  Male dragonflies will protect an area from other male dragonflies of their species so that they will have the best chance at mating with as many females as possible.  The best male (generally considered the most “fit” male by evolutionary biologists) is able to protect the best spots from his competitors.  The less fit males, the ones who are unable to chase the best males out of their areas, will take the less fabulous oviposition sites.  The males who are not able to hold any spot will sometimes hang out around the body of water and wait for a space to open up or try to sneak matings while the residents are preoccupied.  These comparatively unfit males will also sometimes leave the area entirely in search of another body of water.

Anax junius patrolling

A green darner (Anax junius) male patrolling his territory

So how does this territorial behavior work?  Let’s envision a hypothetical pond where no dragonflies have ever set up territories.  The first dragonfly arrives and takes the best oviposition site.  Depending on the species he belongs to, he will find a good place to rest while he watches his area or he will fly around his area continuously in a behavior called patrolling.  When another dragonfly comes along, unless there is another spot of nearly the same quality he can claim, he probably wants the same spot as the first male to arrive.  In this case, the new male will challenge the resident male for the position.  To do so, he will fly into the territory and the resident male will fly out to greet him.  The two will engage in a ritualistic fight where they chase each other, flying around one another in circles very rapidly and zipping across the pond, to demonstrate their strength (effectively their fitness) to one another.  Minimal physical contact occurs between the combatants (this is important when you have rather fragile wings that you depend on for everything you do!), but the male who would lose if they actually came to blows will likely give up his claim on the spot and take the next best spot.  More males arrive and fight for the spots that they want, shifting the territories between individuals.  Eventually, a sort of equilibrium is reached where the best males have the best spots, the lesser males have the lesser spots, and the weakest males have no spots.

Protecting a good territory is hard work and even the best dragonflies can’t protect them forever.  A male protecting the most popular oviposition site will be constantly challenged by neighbors and the males who are unable to claim territories, not to mention he’s getting more matings than any other dragonfly at the pond!  Male dragonflies also expend a lot of energy guarding their mates while they lay their eggs.  An unprotected female is likely to be grabbed by another male and taken to another location at the pond to mate again before she finishes laying the eggs the resident male just fertilized, and males put a lot of effort into guarding their mates.  So, territories often shift during the day as more energetic males overthrow tired resident males.  Younger males can usurp territories from older males as well.  And there are always those males who weren’t quite strong enough to claim a territory waiting for resident dragonflies to weaken to the point that they can finally overcome them and take over their positions.

Anax junius males in combat

Green darner (Anax junius) males in combat

Competition for territories can be fierce.  There are usually far fewer territories at a body of water than there are male dragonflies who want them, so they are constantly trying to claim better territories and mate with as many females as they can.  It’s effectively a war zone!  The competition doesn’t ease once all of the territories are taken either as more dragonflies may arrive at the pond and many of the less fit males stay nearby in hopes of eventually gaining a territory.

In general, males who hold territories mate many more times than males that do not have territories, but even the homeless males will secure some mates.  While a male is chasing another male from his territory, a weaker male might be able to slip in and grab a quick mating.  Males who are guarding females are less likely to chase intruders from their territories until the female is finished laying her eggs, so weaker males can sometimes take advantage of their lapse in attention to sneak in a mating.  It is also typical for males with lower quality territories to mate more often than males without territories.  You might thinks that males holding lower quality territories would never get mates and waste their energy protecting their sites because females choose mates based on who holds the best oviposition site.  However, females are in such short supply and such high demand that they are sometimes mobbed when they arrive at the best spots because so many males are competing for the same space.  A female who is harassed enough or has her egg laying interrupted enough times will seek a mate in a quieter area where she may lay her eggs in peace.

So, male dragonflies form territories so that they can mate with as many females as they can.  The more females they mate with, the more offspring they will produce and the more their genes are passed on.  Pretty simple really!  And it’s one major reason many other animal species set up territories too.

Next time, I’ll finish up my dragonfly trilogy by cheating a bit and using the British definition of dragonfly (they use dragonfly for both dragonflies and damselflies) so I can talk about a recent paper about territoriality in a damselfly species.  Damselflies are much less likely to protect territories than dragonflies, but the system works the same way.  I hope you’ll stay tuned!

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Text and images copyright © 2010 DragonflyWoman.wordpress.com