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?

stonefly

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!

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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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A long, long time ago, in a galaxy.. well, right here

I grew up loving minerals and geology.  My dad was a rather obsessive mineral collector when I was a kid and is still as passionate about the subject as I am about entomology.  He never really bothered with fossils though, so it wasn’t until grad school that I became interested in them.  My advisor adores insect fossils and even worked at an amazing fossil insect deposit in Germany, so I heard all sorts of fascinating stories.  When he offered a fossil insect class, I jumped at the chance to take it.  (What’s not to love about combining my interest in minerals with my interest in insects?!)  It quickly became my favorite grad-level class and I formed a deep and lasting appreciation for insect fossils during the class.

For the next two Monday posts, I’m going to share some of the insect fossil love! Today I’m going to cover the giant insects of the Carboniferous and Permian (358-289 and 290-248 million years ago respectively) and my favorite theory suggesting how these giant insects were able to develop.  Next week I’ll discuss a recent paper.  Let’s jump right into the giant insects, shall we?

Goliath beetle

Goliath beetle. Image from Wikipedia.

There are arguments over which living insect species is actually the largest insect on the planet.  Most of the debate centers around one problem: how do you define “largest?”  If you go by weight, the heaviest adult insect on record is the giant weta in New Zealand.  It’s thought that some of the African Goliath beetles might actually be heavier, but fewer of them have been weighed.  The heaviest immature insect on record is a Goliath beetle, so they’re definitely up there near the top.  If we go by length, the longest insect is (not surprisingly) a stick insect from Borneo.  There’s often a biggest species identified for each insect order too.  For example, the largest true bug is a giant water bug from South America that tops out at nearly 5 inches long.  All of these insects are big, the giants of the living insects.  However, they all pale in comparison to the largest insects ever discovered on Earth!

Griffenfly

Griffenfly. Image from Wikipedia.

The largest insect was a member of the Meganisoptera, an extinct order of insects called the griffenflies or “giant dragonflies.”  As you can see, griffenflies superficially resemble dragonflies and have similar wing and body shapes, so they are commonly confused with the Odonata.  If you’ve ever heard that the largest insect ever was a dragonfly, this is why, but it’s not quite correct.  They were not true dragonflies, rather the precursors to the modern dragonflies.  And they were BIG!  REALLY BIG!  The largest insect ever discovered was a griffenfly called Meganeuropsis permiana, a giant with a wingspan of nearly 28 inches (71 cm) and a body length of almost 17 inches (43 cm).  Can you imagine an insect with a two foot wingspan buzzing around your head?!  Still, as amazing as the griffenfly fossils are, there’s still very little known about them.  Most fossils contain only wings fragments with no body attached.  The immatures remain unknown.  No one has any idea what these things ate, but given their relationship and similar appearance to the dragonflies, it is assumed they hunted flying animals just like their modern odonate relatives do.  Ultimately, as cool as fossils are, they leave you longing for more information.  The griffenflies have been extinct for well over 200 million years, so we might never learn much about them.

Paleodictyoptera

Paleodictyoptera. Image from http://www.geocities.co.jp/NatureLand/5218/ pareodhikuthioputera.JPG

The griffenflies were the biggest insects ever, but they weren’t the only big insects around during their time.  Giant mayflies and an extinct group called the Paleodictyoptera (at right) were also roaming the planet at the time.  Some enormous  scorpions and myriapods (like centipedes and millipedes) were also present, as were giant amphibians.  (How cool would it be to see a giant proto-frog eating a giant proto-dragonfly?)  That’s not to say all arthropods were giant during the late Carboniferous.  Most were similar in size to the insects we see today, with a few amazing exceptions that absolutely dwarfed their relatives.   But why did they get so big?  And why are none of these truly giant insects alive today?

insect respiratory system

Diagram of a simple insect tracheal system.

Because I work with insect respiration, my favorite theory of how insect gigantism came about has to do with how insects breathe.  If you recall from my post on insect respiration, insects depend on tubes called tracheae and tracheoles to exchange gasses with the environment.  The system works because there is less oxygen within the insect than outside the insect, so oxygen tends to flow down the tubes in an attempt to create an equilibrium.  It’s thought that insect size is limited by this system and that insects like the Goliath beetle and the giant weta are about as big as modern insects can be and still get all the oxygen they need.  So how was it possible for a 17 inch long griffenfly able to survive if the biggest insects today are as big as they can get?

Happily, there is evidence that oxygen levels on Earth have changed dramatically over time.  In fact, life on our planet began when there was very little or no oxygen on the planet.  By the late Carboniferous, 280 million years ago, there was so much oxygen on Earth that it made up about 35% of the gasses in the atmosphere.  This high level of oxygen could have in turn led to increased flow of oxygen into the insect respiratory system, at least compared to what we see at our current oxygen level of 21%.  Increased flow of oxygen into the tracheal system meant that the size limits the respiratory system imposed on insects also increased and insects were able to get bigger.

And it looks like they did!  The biggest of the giant insects happened to be flying around about the same time the planet’s oxygen levels were the highest they’ve ever been, suggesting that respiration played a role.  The giant insects then disappeared during the Permian, right about the time the oxygen levels dropped to a low 15%.  And when the oxygen levels rose again during the mid-Jurassic?  You guessed it!  Giant insects popped back up for a while, only to disappear when the oxygen levels dropped to the modern 21%.

This is only a theory of course and it’s unlikely we’ll ever know for sure whether this was really how it all worked, but the hypothesis certainly fits the fossil and climatological data well.  It has also been well received by entomologists, so the hypothesis is likely to hold its own for some time.  Several researchers have even pursued experiments in an attempt to support the validity of the high oxygen – giant insect correlation and gotten some interesting results.  Next week, I’ll discuss one such recent paper that deals with an oxygen study performed on stonefly nymphs.  It makes some interesting points regarding ancient insect gigantism, so I hope you’ll check back!

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References:

Grimaldi, D and Engel, MS.  2005.  Evolution of the Insects.  Cambridge University Press, 755 pp.

Graham, JB, Dudley, R, Aguilar, NM, and Gans, C.  1995.  Implications of the late Palaeozoic oxygen pulse for physiology and evolution.  Nature 375: 117-120.

Dudley, R.  1998.  Atmospheric oxygen, giant Paleozoic insects and the evolution o aerial locomotor performance.  Journal of Experimental Biology 201: 1043-1050.

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Friday 5: 5 Fantastic Insect Horror Movies

I am a huge wuss when it comes to watching horror movies.  I am jumpy in general (this might be the result of spending my early childhood in a place that has rattlesnakes in the gazillions, my current city!) so movies where things jump out really bother me.  Movies where things move in creepy ways are incredibly disturbing to me.  That scene from The Exorcist (I saw the director’s cut) where the girl crab-walks down the stairs?  Eeek!  And those horrible torture movies that are popular at the moment (Texas Chainsaw Massacre, the Saw series, etc) worm their way into my brain and stick there so I can’t think of anything else for weeks.  I really hate those movies…  However, I adore cheesy, low-budget, B-movie horror movies.  And guess which group of animals is frequently featured in these gems?  Insects!  Thus I can combine my passion for insects with my craving for bad horror movies on a reasonably regular basis.

Over the years I’ve seen dozens of insect horror movies, but there are a few that I absolutely adore.  Some are brilliant examples of classic horror films, but the others are so cheesy I laugh hysterically all the way through them no matter how many times I see them.  So, without further ado, I present my top 5 insect horror movies:

The Fly

IMDB Rating: 7.0. Image from IMDB.com.

5. The Fly. Now many of you will be familiar with the modern version of The Fly, the one with Jeff Goldblum and Gina Davis that was released in 1986, or maybe The Fly II with Eric Stoltz.  These are both fine insect horror films.  However, if I’m going to watch The Fly, I prefer the 1956 Vincent Price version.  Vincent Price was a brilliant horror actor and really makes this movie work.  You probably all know the story: a scientist develops a teleportation device and tests it on himself.  However, he didn’t know there was a fly in the machine with him when he turned it on.  The result: the scientist’s body comes out the other side with the fly head on top!  Vincent Price’s character is a the brother of the doomed scientist and attempts to help the scientist’s wife cope with her husband’s disfiguration.  The movie is actually good, with skilled actors and a a touching plot.  The end of this movie is fabulous, but I’m not going to ruin it!  You’ll just have to watch it yourself.

Mansquito

IMDB Rating: 3.1. Image from IMDB.com.

4.  Mansquito / Mosquito Man. This is a Sci-Fi Channel exclusive, and if you know anything about made-for-Sci-Fi Channel movies, you know how terrible this movie really is!  The plot is decent enough.  Once again, we have a scientist, this time a woman who is testing some new compound she’s developed on prisoners.  However, an explosion in the lab as the prisoner is about to be injected has disastrous effects!  Both the convict and the scientist begin to transform into giant mosquitoes.  And if that isn’t enough to make you want to run out and watch this tonight, let me just say that the “love” scene between mostly transformed convict and partly transformed scientist is about the most hilariously bad scene ever created for a movie.  This movie definitely falls into the so bad it’s good category, but it made me laugh.  A lot.  Hence its appearance on my list.

Skeeter

IMDB Rating: 2.8. Image from IMDB.com.

3.  Skeeter. This movie follows a standard plot in insect horror movies: pollution caused by man irradiates or otherwise mutates the insects in an area (usually a remote area) and turns them into giants.  Apart from the fact that an insect this big would collapse under its own weight, I really love this particular plot.  I believe Skeeter takes place in a small town in Nevada, a town that has an illegal toxic waste dump conveniently located in a damp cave or mine shaft with a lot of water.  The mosquitoes become gigantic, about the size of a basketball, and go on a killing rampage through the area around the town.  Add to this a love story between a lawman and a woman from the town and you’ve got yourself one fabulous so bad it’s good insect horror movie!

Them

IMDB Rating: 7.4. Image from IMDB.com.

2.  Them! Okay, okay.  I know I should put Them! first for several reasons.  First, this movie is surprisingly detailed and correct when it comes to the science. You can actually learn something about ants watching this movie!  Second, the story is fabulous – atomic testing in New Mexico creates a hoard of ant giants that terrorize the humans who created them.  Third, this is probably THE most classic insect horror movie.  Don’t get me wrong.  Them! is a truly brilliant movie and makes for great commentary on the consequences of the nuclear age.  If I were going to recommend a good insect horror movie to someone, this would be it.  However, I just can’t put this movie above my all-time favorite insect horror movie…

Empire of the Ants

IMDB Rating: 3.2. Image from IMDB.com.

1.  Empire of the Ants.  This movie is pure B-movie greatness!  There’s a ton of bad, overly dramatic acting.  The story is completely ludicrous.  The beginning of the movie is supposed to put the plot into context – we humans are destroying the planet, consequences be damned – but the narration is so over the top it’s impossible to take the movie seriously.  Once again, we have giant ants created due to toxic waste.  Once again, the giant ants are terrorizing people, this time a group of potential investors who are visiting a bogus land-development project headed by the scamming Joan Collins.  They have to fight off the ants to save their lives, only to get into worse and worse situations with fewer and fewer people as they go along.  The movie oozes more cheese than a big pile of nachos!  But I think it’s absolutely hilarious to watch.  Plus, the ants in this movie are actually pretty cool for the most part.  I can’t be sure, but I believe they filmed some scenes through an ant farm-like enclosure (or superimposed film of ants in such an enclosure) so that the ants crawling on the buildings and the docks look much more realistic than they do in most insect horror movies.  Still, the big showdown between the ants and the survivors at the end is so shockingly bad it can make you forget about the more redeeming qualities of this movie.  I was recently very excited (perhaps too excited!) to discover that it had been released on DVD, so you might actually be able to get your hands on a copy of this gem of an insect horror movie.  I highly recommend it.

So that’s my list.  Anyone want to share their favorite insect horror movie?  If so, leave a comment below!  I’d love to discover a new movie or two to watch!

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