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

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

Insect Respiration

I’ll admit that I am a bit A.D.D. when it comes to biological research.  I find everything interesting!  This makes it hard for me to focus on one specific thing all the time, so I have a lot of little side projects going at most times.  (And if any of you are in grad school out there, this is NOT the way to finish quickly!)  However, if I were to say I had one specialty, insect respiration (particularly respiratory behaviors) is it.  So, it’s time to dive into the science of insect respiration!  The next 3-4 major posts will focus on several different facets of insect respiration (finishing up with another From the Literature post), but today I’m going to go over the basic system of insect respiration, the one you would find in most typical terrestrial insects.

human respiratory system

The human respiratory system. Image taken from Wikipedia: http://en.wikipedia.org/wiki/File:Respiratory_ system_complete_en.svg.

 

Humans have a rather complicated respiratory system.  We use our lungs to draw oxygen into our bodies through the nose and mouth.  Our lungs branch and branch and branch some more, and air travels down these branches until it reaches the alveoli, small air sacs at the very end of the lungs.  These air sacs are very thin.  The capillaries (part of the circulatory system) that run along the outside of the lungs are also very thin.  This allows oxygen to travel from inside the lungs, through the lung tissue, through the capillary tissue, and into the red blood cells inside the capillaries.  The red blood cells then carry the oxygen from the lungs to other parts of the body and deliver it to needy cells.  The human respiratory system requires both the lungs and the circulatory system to function.

Insects are much less complicated than this.  For starters, they don’t have much of a circulatory system.  They have a blood-like substance called hemolymph filling their bodies, but it tends to slosh around inside without a lot of direction.  If you don’t have a fancy circulatory system that directs blood into specific areas, you can’t depend on your blood to transport oxygen from the outside of your body to your cells.  So, insects use a different system.

insect spiracle

A spiracle on a caterpillar. The blue arrow points to the opening.

Most people know that insects are divided into three major body sections: the head, thorax, and abdomen.  You may not know, however, that these main body segments are made up of several subsegments.  The thorax is divided into three sections called the prothorax (pro means forward), mesothorax (meso means middle), and metathorax (meta means after).  The abdomen of a typical insect is made up of 11 subsegments.  So why do we care about these subsegments?  Because most or all of these subsegments contain a pore in the insect exoskeleton that allows oxygen to enter the insect.  These pores are called spiracles.  Take a look at the caterpillar image and you’ll see several of these spiracles running down the length of its body.  Many insects have muscles associated with their spiracles that allow the insect to open and close them on command.  Other insects leave them open all the time and some don’t have spiracles at all!  But most insects do, so we’ll focus on the ones that do here.

insect respiratory system

Diagram of a simple insect tracheal system.

Spiracles connect the air outside the insect to the inside of the insect where it is needed by the cells.  The spiracles connect to the tracheae (plural of trachea), big, open tubes that travel from the spiracles some way into the body.  The tracheae then branch again and again into ever smaller tubes.  At some point, the diameter of the tubes gets so small the tubes become tracheoles.  Air passes to the end of the tracheoles and is delivered to a single cell or a small group of cells, where it is taken in and used for necessary cellular functions.  So, unlike the human system where air is collected in the lungs and is then transported to cells via the blood, the insect respiratory system delivers air directly to the cells!

You may be wondering how air gets into that system of tunnels in insects.  Well, many insects don’t “breathe” the way that humans do.  Humans have a muscle called the diaphragm that causes us to take breaths as it contracts and relaxes.  While some insects do use their muscles and simple air sacs to actively pull oxygen into their respiratory systems, many do not.  These insect depend entirely on a property of physics called a concentration gradient.  A gradient forms when oxygen (or another gas or a liquid or a chemical or molecules) is in a lower concentration in one location compared to another, i.e. there are fewer molecules of oxygen in one place than another.  When concentration gradients form, molecules tend to move from areas of high concentration to low concentration to restore an equilibrium between the two areas.  This happens in the insect respiratory system.  The insect cells use the oxygen that has traveled down the tracheal system, so there is less oxygen at the end of the tubes than there is outside the insect.  So, molecules of oxygen pass into the tracheal system to replace the oxygen that was used and end up at the tips of the tracheoles.  These molecules are also used by the insect’s cells, so more oxygen enters the tracheal system to compensate.  Thus, oxygen continues to flow into the respiratory system and the insect is continuously supplied with the oxygen it needs to survive.  This same system also takes the carbon dioxide the cells produce back out via the same process in reverse.

What I’ve described here is a sort of basic, generalized model of insect respiration, and a terrestrial one at that.  There are many, many variations on this theme.  Some insects use what is called ventilation and actively pull oxygen into their respiratory system, such as those insects I mentioned above that use muscles and air sacs.  Other insects use a system called discontinuous gas exchange where they hold their spiracles shut most of the time and then open them quickly every now and again, likely to conserve water.  These insects sort of hold their breath and then take in big gasps of air only when they really need to.  Aquatic insects exhibit many variations on the basic insect respiration plan that allow them to breathe more efficiently in water.  These adaptations will be the focus of my next post, so I hope you’ll stay tuned!

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Text, caterpillar image, and tracheal diagram copyright © 2010 DragonflyWoman.wordpress.com