Wading For Bugs

Wading for Bugs coverLast week I was reading through the quarterly newsletter for the Society for Freshwater Science when I came across a book review for a book I hadn’t even heard of.  It was called Wading For Bugs and the review described the book as a series of stories told by aquatic biologists about their interactions with aquatic insects.  I of course had to have this book immediately (the book had been in print for three whole months by the time I discovered it after all!), so I clicked over to Amazon.  $13.16 and two days later I held a copy of the book in my hands.  And oh, it is marvelous!

The book has two main goals as I see it.  First, it introduces the reader to the benefits of aquatic insects and succinctly explains why everyone should appreciate them.  My only (minor) complaint is that the book focuses almost entirely on their usefulness as biological indicators of water quality to the near complete exclusion of other benefits they provide, but it’s understandable.  Aquatic insects do play a very important role in monitoring water quality around the world and that importance is rarely advertised to the public.  The book also provides basic information about aquatic insects.  Each section begins with information about an order (their structures, life histories, and role as bioindicators) to teach the reader a little about each group.  There’s a fair amount of knowledge contained in this 160 page book!

The second goal of the book is to help readers see aquatic insects through the eyes of the scientists who study them.  After a brief introduction to a group at the start of a chapter, you read through a series of stories (mostly non-fiction) that allow you to follow along with an aquatic entomologist as he/she works.  These stories are what attracted me to the book.  A lot of big name aquatic entomologists talk about their work and fascinations with aquatic insects while simultaneously teaching the reader a bit about a specific insect.

The stories are, I think, beautiful.  Many are love stories from scientists to the organisms that both enthrall them and provide their bread and butter, but there is a lot of variation in story styles and topics.  Ever been curious about how scientists discovered that the giant water bug Abedus herberti leaves streams before flash floods?  You’ll find out in the story by Dave Lytle.  Or maybe you’ve wondered if aquatic insects are useful in murder cases.  John Wallace can answer that.  The book contains stories about mayflies, stoneflies, caddisflies, true flies, dragonflies/damselflies, bugs, and beetles written by researchers studying a huge variety of topics.  In essence, it provides an overview of what aquatic entomologists actually do while giving you a unique insight into their psyches.

If you have an interest in aquatic entomology, this is a great little book to add to your collection.  The approach is rather unique and the book presents a viewpoint you’re unlikely to find anywhere else.  It’s a short book, but it’s full of inspiration and information.  I highly recommend it!

Wet Beaver Creek

Wet Beaver Creek

In the spirit of the book, I would like to share a very brief story about an encounter with an aquatic insect I’ve had.  About 5 years ago, I helped out a Park Service friend who was part of a team developing an aquatic monitoring plan for Arizona’s national monuments.  They wanted an outside opinion about the effectiveness of their plan and invited me to evaluate it.  We met up at the tragically named Wet Beaver Creek near Montezuma’s Well in central Arizona and got to work, spending the rest of that day and the following day sampling the insects in the stream.  It was great!  And the monitoring plan was sound too.  Fun, fun, fun!

Most of the team went back to Tucson at the end of the second day, but my friend and I stayed another night.  Lacking anything better to do, we wandered up to the Well in the dark, leaned against the railing overlooking the big water-filled crater, and talked about the monitoring plan and aquatic insects for about an hour.  I was really enjoying the whole experience!  Two days of collecting bugs in a beautiful river was making me very content with the world.

Right about as that feeling started to sink in, however, I felt something bite my calf just below my shorts.  Just a tiny pinch, so I swatted my hand at it and didn’t think more about it until I felt another one.  And another.  Then another.  The moon was very bright, so I eventually looked down to see what was nipping at my legs.  They were no see ums (aka, biting midges), tiny flies in the family Ceratopogonidae that are aquatic as larvae and terrestrial as adults!  Their common name stems from the fact that they’re so small they’re hard to see, but they are bloodsuckers.  I hadn’t ever encountered no see ums, so I thought, “What damage can such tiny flies possibly cause?” I started jiggling my legs a bit to discourage their landing on me and winced slightly whenever one bit me, but didn’t worry about it that much.  I fell asleep that night thinking, “That wasn’t so bad…”

Fast forward to the next morning.  Remember that photo I shared in my post about the downsides of entomology, this one showing all the bites on my legs?:

bug bites

No see um bites!

That was what I woke up with!  SO many bites, SO itchy, all over my legs and arms.  The 3.5 hour drive home was excruciating because I couldn’t stop scratching.  I essentially doused myself in hydrocortisone when I got home.  Then I counted my bites.  I had over 300!  THREE HUNDRED!  No wonder I was clawing my skin off.  No wonder I was miserable!  300 little bloodsucking flies had feasted on my legs!

That was my only bad encounter with no see ums though.  Now I wear long pants and long-sleeved shirts, even when it’s hot.  I would rather get my pant legs wet than live through that misery again.  That night I was almost taken down by a 1mm long fly!  Never again.  Never again…

So that’s one quick little story, but I’d love to hear your stories too!  Does anyone want to share an encounter you’ve had with an aquatic insect?  If so, leave a comment below!  Let’s make our own little Wading for Bugs!  But read the book too!  Maybe, just maybe, you’ll understand aquatic entomologists like me a little better.


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


Friday 5+1: The Brief Life of a Lethocerus Egg

In case anyone reading this doesn’t know already, a large part of the research I do deals with giant water bug eggs.  I spend a huge amount of my time staring at eggs with an electron microscope, rearing eggs, doing experiments on eggs, grinding eggs up to do chemical analyses, counting eggs…  Perhaps I spend a little too much time with eggs, though I’ll leave you all to decide that on your own.  Giant water bugs have a lot of interesting features to recommend them (including some really beautiful structures on the egg-shell), but I think one feature in particular is especially worth mentioning.  If you know much about eggs in general, such as bird eggs, reptile eggs, or other insect eggs, you probably know that most animals lay their eggs and the embryos develop within the confined space inside.  This isn’t what happens with giant water bug eggs!  Instead, they absorb water (a lot of water!) and puff up the eggshell from the inside so they get bigger over time.  In fact, giant water bug eggs, as big as they are to begin with, nearly double in size between the time they are laid to the time the nymphs hatch and swim away.  Their eggs GROW!  Simply spectacular.

For today’s special Friday 5+1, I’m going to share a series of photos I took of the eggs of the giant water bug Lethocerus medius a few years ago that show how they grow as they develop.  It may a little difficult to see if you don’t spend as much time around these things as I do, but compare the Day 1 eggs to Day 6 eggs and you should be able to see the change clearly.  I’m also going to give you a bit of commentary so you know what to look for.  Let’s start at the obvious place…

Day 1.  Lethocerus medius eggs start off just shy of 3 mm long and about 6 mg, a substantial insect egg.  This species is an emergent brooding giant water bug (see my post about giant water bug child care for more information), so it lays its eggs on vegetation out of water.  As you can see, the eggs are very tightly packed so that most of each egg is touching the others with only a small part of the top free:

day 1 eggs

Day 2. On the second day, things look rather similar from the outside, though the eggs get a little taller and a little heavier:

day 2 eggs

Day 3.  By day 3, the eggs have gained almost half a millimeter in height and 0.2 mm in width.  The weight has gone up too, nearly 2 mg.  You can start to see the eggs bulging at bit at the top:

day 3 eggs

Day 4. The eggs are growing more noticeably now, gaining another 0.5 mm and 2-3 mg overnight!  You can see how the eggs start to crowd each other a bit.  They’re fixed in place at the bottom, but they start to spread out at the top so that they can all fit:

day 4 eggs

Day 5. By day 5, the eggs have stopped growing up and begin to grow out a bit, adding 1/10th of a millimeter and another 2-3 mg in weight.  The eggs are now over 4 mm tall and 2 mm wide and weigh nearly 13 mg! The eggs continue to spread apart at the top end as they increase in size so that you begin to see gaps between the eggs and can start to see the sides of the eggs as well as the tops:

day 5 eggs

Day 6. During their last day in the egg stage, the eggs have topped an enormous 5 mm (that’s HALF A CENTIMETER!  Huge!) in height, nearly 2.3 mm in width near the top of the eggs, and reached 14+ mg!  These are truly big eggs now, and have nearly doubled in height in 6 days.  You can see nearly all the way down to the stick in some of the gaps between the eggs and the eggs themselves look like they’re ready to pop:

If they make it this far, you’ll usually see the following events the same night.  Hatching:

hatching eggs

… and then the newly hatched nymphs swim away, leaving behind only a stick and some empty shells:

hatched, empty eggs

And there you have it!  A wonderful set of growing insect eggs! Lethocerus medius isn’t the only water bug that exhibits this amazing growth either.  Other giant water bugs have shown similar patterns, including a mix of emergent brooders and back brooders.  Growing eggs seem to be quite common, if not universal, within the family to which the giant water bugs belong, the Belostomatidae.  Just one more way that giant water bugs are among the most amazing insects ever!


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

Sexing Giant Water Bugs

Female giant water bug

Female giant water bug

I am asked how to tell male and female giant water bugs apart more often than I would have ever imagined.  It would seem that there are a limited number of people in the world who can sex water bugs, so I have a rather steady stream of people asking me how to do it.  A few weeks ago I started looking around online to see if there was any information about sexing water bugs, but there’s really nothing available.  Well, there’s one article that is absolutely and completely wrong (I’m not even going to link to it, but don’t listen to anything the person who wrote the eHow article on the subject says!), so that’s no help at all.  It’s high time that this information be made available online!

Sexing belostomatids (aka, giant water bugs) isn’t always trivial.  The males and the females have the same coloration, so you can’t use color differences.  The females are often a little bigger than the males, but there’s a lot of variation in size in both sexes and it can be difficult to tell whether an individual is “big” or “small” unless you have many other individuals to compare with it.  You can sometimes use behaviors to distinguish sexes, especially during the mating season.  For example, if a bug is carrying eggs on its back or climbing up a stick to tend to eggs, it’s a male.  But… you also can’t be sure you have a female just because one doesn’t have eggs, even during the peak of the mating season.  Instead, you need to rely on a structure that varies reliably between the sexes.

Giant water bugs DO bite, but you’re going to have to hold them in place if you want to tell which sex you’ve got.  That usually involves picking them up, so if you do choose to sex water bugs yourself, be careful!  When I pick up water bugs, I use my middle finger and my thumb and press the bug down onto the bottom of whatever habitat/container they’re in.  I then grasp water bugs by the sides of the thorax, as it’s a very rigid part of the body where you can get a firm grip, and use my index finger to support the bug (I apologize that I only have a grainy black and white photo of this…):

Holding giant water bugs

Holding giant water bugs

You need to hold on tight, especially with the larger species, because they are surprisingly strong and will try to wiggle free.  Once you get a good hold on one, you’ll need to flip it over.  The part you need to look for is on the bottom of the abdomen:

Genital plate

Genital plate

This structure is called the genital plate, and it conceals the reproductive organs underneath.  Don’t worry!  You don’t need to go digging around to find internal parts and can use the shape of the genital plate itself.  The shape of the plate varies from genus to genus, but there are some general rules.  The genital plates of male water bugs are smoothly rounded at the tip (the part closest to the back of the bug) and complete (i.e., have no splits or gaps).  For example, the genital plate of males in the genus Abedus look like this:

Abedus male genital plate

Abedus male genital plate

The arrow points to the continuous and smoothly rounded tip of the genital plate.  Here’s a drawing in case the shape is difficult to see in the photo:

Abedus male genital plate

Abedus male genital plate

See?  Rounded at the tip, no splits or gaps.  The females are different.  In some water bug genera, there are splits, notches, or gaps at the tip of the genital plate, so that the line around the tip of the plate is broken.  Many of them have flattened areas at the tip of the genital plate so that they are not completely round.  Most have two distinct little tufts of hairs, either at the edges of a notch at the tip of the genital plate or alongside the midline of the genital plate near the tip.  For example, here’s a female Abedus:

Abedus female genital plate

Abedus female genital plate

In this image, the arrow pointing up from the bottom indicates the flattened, slightly notched part of the genital plate while the arrow coming from the side points to the tuft of hair on the right side of the midline.  The drawing:

Abedus female genital plate

Abedus female genital plate

The little tufts of hair can be quite small and the shape of the genital plate only subtly different from the male, but there are definitely two little tufts of hair on either side of the midline, a flattened area at the tip, and a small split at the tip of the genital plate in Abedus females.

These structures can vary in appearance from species to species, and especially between genera.  For example, this is the genital plate of a male Lethocerus:

Lethocerus male genital plate

Lethocerus male genital plate

Even though it’s much longer and narrower than Abedus, the arrow indicates the same sort of rounded, unbroken tip of the genital plate.  There is a fringe of hair along the tip of the plate in this species, but note that there are no distinct tufts of hairs anywhere.  There’s a groove that runs the length of the middle of the plate, but it doesn’t leave a gap at the tip.  In contrast, here’s the female:

Lethocerus female genital plate

Lethocerus female genital plate

The arrow here points to the tuft of hair to the right of the midline.  In this species, the tufts are at the very tip of the genital plate rather than on the upper surface and recessed from the edge as in Abedus.  There is also a small notch between the two tufts, though it’s a little difficult to see in the photo.

And that’s it!  Smooth, round genital plates in males and flatter, sometimes broken genital plates with two tufts of hairs in females.  In the US, the genital plates of the genus Belostoma are very similar to those shown here for Abedus and other American Lethocerus are similar to the Lethcerus pictured here, so the images here will help with sexing US water bugs.  Other genera in other countries follow similar patterns, but may have some slight differences.

Good luck!


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

Hissing Cockroaches as Pets

A few years ago, one of my Insect Behavior lab students did his independent project using Madagascar hissing cockroaches.  Then he left them with me because his mom was “definitely not going to be okay with having them in her house all summer.”  I hadn’t ever cared for hissers, and suddenly I had a dozen of them.  I was determined to keep them alive, however, so I bumbled my way through the first few weeks and eventually gave all but 2 away.  The last two have been my pets ever since.

I hadn’t ever expected hissers to be enjoyable pets.  I knew people who’d had hissers, but very few of them kept them as pets that they actually interacted with.  I interacted with mine though, and I’ve found them to be really fun!  Today, I’m going to share how I care for my roaches in case any of you out there want to get your own roachy pets.  (You know you want to!)

But first, let me introduce my roaches.  Meet Mr. Darcy:

hissing cockroaches

Mr. Darcy

Mr. Darcy is an active, aggressive, and enormous hisser!  He spends a good part of his day hissing, roaming about the cage, and pushing his roomie around.  I can hear him hissing all the way in the next room!  He entertains me to no end.  And this is his equally large roomie, Elizabeth (who else?):

hissing cockroach


Elizabeth doesn’t hiss and spends most of her day being pushed around by the big bully she lives with or clinging to the walls of the cage near the lid.  She’s much less exciting to watch and has a less vibrant personality, but she’s also a lot easier to handle as she doesn’t run around constantly like Mr. Darcy does.

Caring for my pair of roaches has been ridiculously easy!  They live in one of those cheap little plastic aquariums that you can get at any pet store:

plastic aquarium

Plastic aquarium

I’ve set mine up so that they have a 1/2 inch layer of coconut husk in the bottom of their cage.  It’s great stuff!  It is sold in dense bricks in the reptile section of the pet store, but it expands enormously when you add water.  The coconut holds water well, and considering that these roaches are decomposers in forests in the wild, I think it probably mimics their natural habitat fairly well.

If you’ve ever had wild roaches in your home, you are aware that they like to have places to hide.  I got my roaches another item from the reptile section of the pet store, a half a hollowed log, to give them a hiding place.  My roaches are weird and spend most of their time on top of the log rather than under it…

hissing cockroaches

My hissers

…but it’s available for the rare occasions when they want to have an out-of-the-way place to go.  My setup looks like this:

hissing cockroach cage

My hisser cage

I feed my roaches a combination of foods.  I give them dry dog food because it has a lot of necessary protein and fat in it and they love it.  I toss in some of the alfalfa pellets that people feed to rats and other rodents, mostly because I happened to have a lot leftover when the last of my pet rodents died, but also to give them some carbs and nutrients.  My roaches also get a mixture of leafy greens (whatever salad greens I have in my fridge) and baby carrots.  According to what I’ve read, hissers REALLY love carrots and I’ve found that to be true:

hissing roaches eating carrot

Hissing roaches eating carrot

The combination of the dog food, alfalfa pellets, and fresh vegetables provides my roaches a well-balanced diet.  I leave alfalfa pellets and dog food in their cage all the time, but I only give my roaches the veggies every few weeks.

My hissers get water several ways.  Their main source is a piece of a sponge that I cut up, rinsed very well, and placed inside a yogurt cup lid.  Mr. Darcy and Elizabeth can suck water out of the sponge and I keep it wet all the time.  I also pour water into the coconut husk every few weeks to rehydrate it and spray their cage down with water to keep their log a little damp.  I’ve seen them suck water droplets off the side of the cage, the log, and the bedding, so they seem to take advantage of any water that I provide.

Then it’s just a matter of cleaning out their cage!  I only clean them every few months as they’re not very messy and I don’t have to worry about mold most of the year.  Cleaning them is a simple matter of removing the roaches (and listening to Mr. Darcy hiss – I imagine him screaming, “Put me down, you miserable wench!”), removing the dishes and log, dumping out the coconut husk, and then putting new coconut in before replacing everything else.  Easy!

If you happen to have a male and a female, you can tell you’re doing a good job caring for your roaches when you find a whole bunch of these in the cage:

immature roaches


Mr. Darcy and Elizabeth just had their first children!  The little roaches mean I’m going to need to ramp up my care a bit so I’m providing more food more often.  I also had to add a layer of fine mesh to the lid to keep them from escaping through the slats, but their care is otherwise exactly the same.

Hissers are great insect pets, especially for children, because they are completely harmless: they don’t bite, they don’t fly, they don’t sting, they’re not inclined to run very quickly, and they’re large.  They usually stop hissing once they get used to being handled, but Mr. Darcy isn’t your average roach.  He’s 33% bigger than any other hisser I’ve seen and he always hisses.  But that’s part of what makes him so fun too!  If you have more than one roach, you can also see how each has its own personality.  It’s really fun to see how different my two roaches are, and they’re both completely different from the roaches I’ve worked with for outreach events.  Their personality makes them feel a little more like a traditional pet rather than a giant insect too, which is always a plus when I have to explain why I have roaches as pets.

So, you all want pet roaches now, right?  (Hah!)  If you do, there are a few options.  Pet stores that specialize in reptile often have them for sale.  They should cost less than $3.  Or, you can buy them online and have them shipped to you in the mail.  Unless you live in Arizona.  Or California.  There are weird restrictions for mailing animals to some states, so check with the supplier before buying to be sure they can be shipped to your home.

My roaches are probably the lowest maintenance pets I’ve ever had and I enjoy watching them, so I’m happy that I got dumped with reject roaches.  I know it will probably seem a little strange to many of you reading this, but they really have been fun pets.  I highly recommend them!


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

The Long, Involved Process of Giant Water Bug Mating

For my dissertation, I am researching how giant water bug eggs benefit from the care provided by their fathers.  It’s a really interesting subject and I’ve learned many exciting things.  You’ll hear more about my work in the future.  You can’t just study the eggs of giant water bugs though because they are intricately linked with the adult males.  To do my work, I have to know a lot about brooding behaviors so that I understand how the eggs in different groups respond to the particular care they receive.  To get eggs to work with in the first place, I also have to know a lot about giant water bug mating practices and preferences.  For example, I have learned that it is nigh impossible to convince giant water bugs in the genus Lethocerus to lay eggs in the lab.  Sure, you can do it and some people have successfully, but my lab doesn’t provide the right conditions.  Thus, I have to collect mine in the field.  The genus Abedus is much more cooperative.  Put a gravid female (a female full of eggs, ready to be fertilized and laid on the back of her mate) and a male in a little bowl of water with a rock and the next morning you usually have 30-100 eggs that you simply scrape off the back of dad.  Easy!

The process by which those eggs are fertilized and deposited by the female onto her mate’s back is anything but easy though.  Water bug mating is a very long, involved process in the back brooders, but there’s a good reason for it.  Ponder this for a moment: you’re a giant water bug male and you’ve just found the water bug woman of your dreams!  You want to be her baby daddy, but children tie you down.  They’re expensive too: while you carry her eggs around with you, you won’t be able to fly, are more vulnerable to predators, will have a harder time swimming, etc – huge costs if you’re a studly male giant water bug!  Plus, you suspect that your new lady love may have been a bit promiscuous in the past…  If you’re going to be a water bug daddy and raise your gal’s expensive kids, you want to be darned sure that those kids are yours before you commit.  Luckily, giant water bug mating practices ensure just that.  Let’s go over how it works, shall we?

The most important step of all comes first: finding a mate.  If you can’t find a member of the opposite sex, having kids is pretty much out of the question.  The back brooders I work with the most, Abedus herberti, exhibit a very interesting behavior to attract mates: they do frantic push ups:

As you can see, the push ups create little waves in the water.  And, if you know anything about how ear drums work, you have a pretty good idea of what these little waves do: the male water bugs are calling out to potential mates by creating waves in the water that are perceived as sound by the females.  It’s the water bug equivalent of a pickup line!  If the female wants to mate, she’ll track the male down and indicate that she’s willing to mate.

Once the pair gets together, they stay in near constant contact until they’re finished mating.  The bugs will climb all over each other and rub each other with their legs.  But first things first!  They mate:

giant water bugs mating

Giant water bugs mating (Abedus herberti)

This usually takes several minutes and involves the usual parts coming into contact with one another and the transfer of sperm from the male (red dot) to the female (white dot).  When they’re done mating, the female will try to climb onto the male’s back to lay eggs.  However, I’ve never seen a male that will allow his mate to lay eggs after mating only once.  No, they have to mate again!  And probably again.  And maybe once (or thrice!) more, for good measure.  If the male isn’t ready for his lady to lay her eggs, he’ll move around so she can’t lay and do more push ups to indicate that he wants to mate again:

As you can see in the video, there’s a bit of a struggle: the female wants to lay her eggs, so she’s persistent.  But the male’s also determined to mate again, so he keeps thwarting her attempts.  Eventually, however, the male will have mated enough and will allow the female to climb on his back to lay eggs:

Abedus herberti mating

Abedus herberti laying eggs. Female at top, male at bottom.

So, the bugs mate several to many times and the female lays her eggs.  They’re done, right?  Wrong!  The males generally only allow the females to lay 1-4 eggs at a time.  After that, he shakes her off his back and insists that they mate again.  Then she lays a few more eggs.  Then they mate again.  Then they rinse and repeat, oh, 10-50 more times.  The male eventually looks something like this:

water bug dad with eggs

Water bug dad with eggs

The whole process can take several hours!  As I said, it’s a very involved behavior.  Then the female swims away and leaves dad to take care of the kids until they hatch.

Let’s think back to those expensive kids for a moment.  Females are essentially big bags of eggs and do not care for their young at all.  It is in their best interest to mate once, lay as many eggs as possible as quickly as possible, and be on their way.  But the males will drag their needy children around for 1-3 weeks.  If they’re going to bear the costs of parenthood, they want to be sure that the kids they’re lugging on their backs are actually theirs.  The mating system that I described helps them do just that!  Suppose a female has mated before.  Many insects can store sperm so that sperm from multiple males mix together as the eggs are fertilized and laid (or oviposited if you want to use the technical entomological term).  For male water bugs, this isn’t the ideal situation.  To help ensure that all the eggs on their backs are their own offspring, they insist on mating 10, 15, 20, 30, 50 times.  In doing so, a male flood the female’s reproductive tract with his sperm, making it highly unlikely that any of the eggs on his back belong to other males.  It’s a complicated mating system, but it is an important one, at least for the male.  This way, he’s sure that he’s not wasting energy and resources caring for another man’s kids and is instead devoting himself entirely to his own children.

This is just one of the many interesting behaviors that giant water bugs exhibit!  I think my bugs are absolutely fascinating, so look for most posts about them in the future.  :)


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

Well-Nigh Wordless Wednesday: Puddler

I love it when I walk out my front door and see this sort of thing going on in the parking lot:

puddling butterfly

Giant swallowtail puddling

The irrigation system in my housing complex breaks all the time, so we end up with little puddles all over the parking lot that attract butterflies.  Butterflies, such as this giant swallowtail, have a hard time getting enough salt and minerals in their diets, so they “puddle” – they suck up moisture from damp areas rich in the nutrients they need.  Apparently we have a salty parking lot because butterflies LOVE it!  And I love photographing them, so I consider it a win-win situation.


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

From the Literature: Oxygen, Temperature, and Giant Insects

I hope everyone liked the giant insects post last week!  It was one of my favorites to write.  The topic is just so fun!  I continue with the subject this week by describing a scientific paper that was released in July.  It combines several things I love (giant insects, aquatic insects, and respiration)  into one manuscript of pure science fabulousness!  Let’s get to it, shall we?

You probably learned as a kid that insects are ectothermic (aka, cold-blooded).  Ectothermic organisms are largely unable to regulate their body temperatures, so their bodies remain close to the temperature of their environments.  As the temperature increases, processes like metabolic rates speed up.  The opposite happens at cold temperatures and everything slows way down.  Ectotherms survive best under a range of temperatures where their body processes work efficiently, but the animal is still able to get everything it needs (food, water, oxygen, etc) from the environment.  They’re like Goldilocks: they don’t like things too hot or too cold and prefer for things to be just right.

Oxygen plays a big role in the interaction of ectotherms with their environments, especially at extreme temperatures.  Let’s consider a hypothetical insect, say a grasshopper.  As the grasshopper gets warmer, its metabolic rate increases and its body processes become more efficient.  However, as the grasshopper’s metabolic rate increases, so does its oxygen consumption.  At some point, the oxygen demand of the grasshopper may become greater than its oxygen availability and all sorts of bad things start to happen as its body processes start to break down.  Oxygen plays a role at very cold temperatures as well, leading scientists to propose that oxygen can set thermal limits (the maximum and minimum temperatures our grasshopper can survive) in ectotherms.

One problem though: terrestrial insects don’t fit the pattern observed in many other ectothermic animals.  This may be because their respiratory systems do not depend on lungs and blood to deliver oxygen to their cells and instead deliver oxygen directly to their cells via a series of tubes that connect to the outer environment.  This creates a terribly efficient system that provides enough oxygen even at high temperatures for many terrestrial insects.  Quite simply, their respiratory system provides enough oxygen even under the worst conditions.  But what if the insects live in oxygen-limited environments, such as water?  Might oxygen play a role in setting those upper thermal limits then?


Image from http://www.glommaguiden.com/foto_2003/ bilder/030416_dinocras_cephalotes.htm.

Researchers Wilco Verberk and David Bilton considered this question and determined that if the thermal limits of any insects were to be limited by oxygen levels, aquatic insects were the most likely suspects.  So, they chose an insect that requires a low temperature and flowing water as their subject, the stonefly Dinocras cephalotes.  If the maximum temperature the stonefly could tolerate was limited by oxygen consumption, the maximum tolerable temperature would decrease in low oxygen water while it would increase in high oxygen water.  They then developed a simple experiment to determine whether this was the case.

The team placed stoneflies in flow-through chambers in a water tank and ran 10°C water containing various mixtures of oxygen and nitrogen (20% O2/80% N2 = normal, 5% O2 /95% N2 = low oxygen treatment, and 60% O2/40% N2 = high oxygen treatment) through them.  After letting the stoneflies acclimate for an hour, they ramped the temperature of the water up 0.25 degrees per minute until the critical temperature was reached, i.e., the stoneflies started showing signs of thermal stress such as lack of movement and leg twitching.  Then they compared the critical temperatures for each treatment to determine if their hypothesis was correct.

And it was!  They discovered that the upper thermal limit increased almost 3°C in the high oxygen water compared to water containing normal levels of oxygen.  Conversely, the upper thermal limit decreased in low oxygen water by about 1.5°C compared to that at normal oxygen levels.  The conclusion: oxygen levels can set upper thermal limits in larval aquatic insects!

Now you might be wondering why this is exciting or what any of this has to do with giant insects.  The results are interesting for several reasons, but largely because they show that some insects do experience oxygen-induced changes in their upper thermal limits.  This means that, while terrestrial insects might be able to obtain enough oxygen at any temperature to meet their needs, aquatic insects and other insects that live in oxygen limited environments can reach a temperature at which their oxygen demand outstrips the oxygen available to them.  Consider how an insect such as a stonefly gets the oxygen it uses.  They don’t have any spiracles (the pores through which terrestrial insects “breathe”), so oxygen is simply absorbed through the exoskeleton.  Many stoneflies have gills to make this process more efficient (the bigger your body surface, the more oxygen you can absorb from the water), but it’s still a very slow process.  The size of these insects may be limited as a result.  Aquatic insects that rely on absorbing oxygen from the water rather than going to the surface to breathe are also unable to regulate their oxygen uptake very well.  They can do various behaviors to increase the flow of oxygen into their bodies when they become oxygen stressed, but oxygen becomes toxic at very high concentrations.  Aquatic insects can’t do much to prevent oxygen from flowing into their bodies, so this can be a problem.

And this brings us to the giant insect part of the paper.  Verberk and Bilton propose that oxygen limitation at temperature extremes may have contributed to the rise of insect gigantism in the late Carboniferous and early Permian.  This makes sense considering how many of the giant insects were insects that probably had aquatic nymphs (proto-dragonflies, mayflies, and stoneflies, among other aquatic organisms).  The high levels of oxygen at the end of the Palaeozoic meant that oxygen could be absorbed more efficiently by aquatic insects and allowed them to become larger.  I covered this hypothesis last week, so check that post for more details.

Alternatively, Verberk and Bilton suggest that oxygen toxicity may have played a significant role in promoting insect gigantism.  How can an aquatic insect cope with increasing levels of oxygen in water and prevent oxygen poisoning?  They can get bigger!  If insects increased in size as oxygen levels in water rose, then they could counteract the negative effects of high oxygen levels on their bodies.  Oxygen levels at the end of the Palaeozoic were so high that aquatic insects likely had to become very large to prevent oxygen poisoning.  Giant immatures then led to giant adults.  Hence, giant insects that resulted from the limits of their respiratory systems in very high oxygen environments!  It’s a very interesting, new idea.  I suspect many people will do further tests in the future to determine whether this might really have been possible, so we’ll see if it holds up to further study.

I love this hypothesis!  Still, I have to point out that there is one major assumption that the entire hypothesis is built upon, that the giant proto-dragonflies, mayflies, stoneflies, etc had aquatic nymphs.  Modern dragonflies, mayflies, and stoneflies have aquatic immatures, so it’s likely that their predecessors did too.  However, there is no fossil evidence of aquatic nymphs for these groups at the time of the giant insects.  For all we know, the griffenflies and giant mayflies may have had terrestrial nymphs, which would make Verberk and Bilton’s hypothesis fall apart completely.  While the authors did acknowledge this assumption, I think their position would be strengthened if a fossil of even one aquatic immature could be found from that time period.  Without that piece of evidence, I fear this hypothesis is built upon a shaky foundation, one that might not hold up to scrutiny.

But wow!  A new explanation for how giant insects may have evolved!  And focused on the aquatic stages of insects!  You can see why I’m excited by it.  I can’t wait to see the research generated by this paper in the future.  It’s going to make for some very interesting reading!


Literature Cited:

Verberk WC, & Bilton DT (2011). Can oxygen set thermal limits in an insect and drive gigantism? PloS one, 6 (7) PMID: 21818347

This paper is open access!  Full text available online here:  http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022610


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