Edge

The theme for Photography 101 today is “edge,” which immediately made me think of the under surface of the water and the aquatic insects that must visit the surface to breathe.  So, I give you a backswimmer getting air:

Notonectid at surface

Backswimmers swim upside down and carry an air bubble with them underwater that they use to breathe (think scuba tank).  Most of their body is coated with a thin film of air as well, which you can see as the shiny, silvery spots in the photo.  All that air they carry with them only lasts so long, however, so they have to go to the surface now and again to get more.  They break through the surface with their butts and allow air to fill their storage space.  Sometimes they’ll sit at the surface for a little while, but most of the time they’ll dart back underwater where birds and other predators have a harder time getting to them.

I love the look of the water’s surface when photographing things underwater.  My kind of edge for sure!

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth.

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From the Literature: The Cost of Breathing

I have a subscription to the science journal Science, but I rarely do more than shuffle through the contents every 6-8 issues.  While it’s considered one of the premier science journals, there are a lot of articles that are not applicable to what I do, so I usually choose to focus my attention on journals more closely aligned with my research.  However, I happened to look at the October 14, 2011 issue of Science and I’m happy I did!  I came across a great paper by Nick Lane that proposes a possible link between respiration and fitness, fertility, and life span within eukaryotes and explains how this might work.  I’ll talk about Lane’s hypothesis today, but I want to go a step further too.  The examples in the article are all vertebrates and the hypothesis fits well to these organisms.  But how well does it fit with insects?  I’ll discuss that at the end.

Note: if you don’t remember what the respiratory chain of cellular respiration is, you’re likely to get a little lost as you read through this.  I created a handy-dandy respiration refresher yesterday though, so you might want to take a look at that before you go any further.

Lane begins his paper by describing how respiration depends on two different sets of genes within cells, the mitochondrial genome (the genes found in the mitochondria) and the nuclear genome (the genes found in the cell nucleus).  The mitochondrial genome is passed directly from a mother to her offspring while the nuclear genome results from the combination of genes contributed by the mother and the father.  These two genomes must work together to create a functional respiratory chain, the important last step in cellular respiration.  If the mitochondrial and nuclear genomes are perfectly compatible, the respiratory chain will work nearly flawlessly and produce ATP and water as it should.  If there is a mismatch between genomes such that the respiratory chain develops incorrectly, electrons can escape the respiratory chain and react with oxygen to form free radicals, specifically reactive oxygen species (ROS).  Leaks in the respiratory chain can result in excessive ROS production, which can in turn cause damage and even programmed cell death.  It is thus essential that the mitonuclear compatibility be sufficient to form a functional respiratory chain capable of supporting an organism’s energy requirements.  In fact, offspring with severe mitonuclear mismatches will likely die during development or shortly after birth, leaving only those individuals with functional cellular respiration alive to reproduce.

hummingbird

A hummingbird has a very high metabolic rate.

But, Lane wondered, how good does this mitonuclear match really need to be for an organism to survive?  Consider an animal with a high metabolic rate such as a hummingbird or a bat.  These animals must have high mitonuclear compatibility to produce a respiratory chain capable of supporting the high energy demands of flight.  In these animals, any individual with even a minor mitochondrial mismatch should die because it cannot produce the necessary energy to survive.  However, more sedentary animals, such as rats and other rodents, can likely tolerate a higher ROS leak because their energy demands are lower.  It’s not as crucial that every electron make it to the end of the respiratory chain in these species.  If so, then offspring with mitonuclear mismatch (up to a point, the ROS threshold) may survive long enough to pass their genes on.

Let’s consider what this hypothesis, if correct, means with regard to reproductive success (i.e. fitness), fertility, and life span.  Think about the hummingbird and the rat for a moment.  If the hummingbird requires low ROS leakage, many of its offspring are likely to die during development because the ROS leakage threshold is low.  In contrast, a rat has a higher ROS leakage threshold and more of its offspring will survive relative to the hummingbird.  This means that fertility should be low in hummingbirds and high in rats, which is indeed the case.  Fitness could also be affected by ROS leakage thresholds.  Hummingbirds have fewer offspring relative to rats, but their offspring are also less likely to survive to reproduce themselves if their ROS threshold is very low. But there’s a trade-off too (there’s almost always a trade-off in biology!): a higher ROS leakage threshold means that more ROS are being formed.  Excessive free radicals can lead to cellular damage and death.  Rats, with their higher ROS leakage threshold, will have a higher ROS production as well as higher cell damage.  High free radical production is known to decrease life span, so the rat, with its high ROS production relative to the hummingbird, will have a shorter life than the hummingbird.

gerbil

A typical rodent has a low metabolic rate.

So Lane’s hypothesis fits the data for terrestrial vertebrates very well and he has generated a testable idea that people will be able to experimentally support or refute.  That’s good!  I think this idea, that a cellular process helps determine fitness, fertility, and life span, is very interesting and I’m eager to see the research that results from this paper.  However, at the end of the paper Lane suggests that the ROS leak hypothesis should apply across species, presumably across eukaryotic species such as fungi, plants, and invertebrates.  He might be right, but let’s consider insects for a moment.  There’s nearly always an exception among insects, and I think this is a case where there might be a big exception.

Let’s think about flies for a moment.  Many flies have high metabolic rates, which, according to Lane’s hypothesis, should mean that their ROS leak threshold is low with a corresponding low fertility and long life.  This isn’t necessarily the case.  Some flies produce few offspring, but many of them produce a lot of offspring!  Many flies also have very short lifespans of only a few weeks from egg to dead adult.  Let’s then contrast this to the flightless giant water bug I work with (not an altogether fair comparison as they are larger than most flies and that needs to be taken into account, but this is just a thought experiment).  The water bug spends most of its life sitting in one place, a highly sedentary insect with a low metabolic rate.  If Lane’s hypothesis applies to all eukaryotes, this giant water bug should have a short life and high fitness.   Giant water bugs have huge eggs, so they don’t produce as many offspring as some species, but 1000 or more offspring in a mating season is nothing to sneeze at and they seem to fit Lane’s hypothesis.  However, these bugs tend to have comparatively long life spans, sometimes a year and a half as adults!  Last, think about social insect queens.  These are sedentary insects that sometimes have very long lifespans (sometimes decades long!).  According to Lane’s hypothesis, these insects should have short lifespans, which isn’t always the case.

I don’t want to sound like I think Lane’s hypothesis is worthless because I think it can potentially explain several patterns of metabolism, fitness, and life span that we see in nature and I honestly hope it pans out.  However, I have two predictions, one or both of which I suspect will hold true as researchers begin exploring the limits of the hypothesis:

Prediction 1.  The ROS hypothesis will not apply to all eukaryotes and instead apply mainly to warm-blooded, terrestrial vertebrates.

Prediction 2. If the hypothesis holds up to experimentation, I suspect researchers will find other forces at play in insects and other invertebrates that overshadow or alter the impact of ROS leaks on life span and fitness so that they do not follow the same patterns that we see in warm-blooded, terrestrial vertebrates.

Ultimately, I don’t think the hypothesis in its current state is going to be as widely applicable to eukaryotes as Lane suggests, but I hope it will lead to some interesting new research dealing with the link between cellular processes and the life histories of organisms.  If it does that, even if it proves to be incorrect, I think this hypothesis can contribute something very meaningful to science.  And even if it doesn’t pan out, at least it was an exciting idea that got people thinking.  I consider that a success!

Literature Cited:

Lane, N. (2011). The Costs of Breathing Science, 334 (6053), 184-185 DOI: 10.1126/science.1214012

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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?

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

From the Literature: On Diving Spiders and Physical Gills

I love Twitter!  Since I’ve started using it, I’ve learned more about a wider variety of scientific studies than I ever did before.  It’s a great source of science news!  This story absolutely exploded on Twitter a couple of weeks ago.  I can see why it was so popular though: it’s a super cool story!  Aquatic spiders + special webs for holding air underwater = SCUBA spiders!  This story has it all.  It also happens to be closely aligned to my own area of expertise, so it’s time for another From the Literature!

This adorable little guy is the diving bell spider, Argyroneta aquatica:

diving bell spider

Diving bell spider. Image from http://www.nytimes.com/2011/06/14/science /14obspider.html?_r=1&smid=tw-nytimesscience&seid=auto

This spider is one of the only spiders in the world that spends its entire life underwater, from egg to adult.  Like its aquatic insect relatives, the diving bell spider has a number of adaptations that help it survive in water.  Its respiratory adaptations are especially interesting.  See that sheen along the abdomen of the spider in the photo?  That is an air bubble held in place by specialized hairs that trap air against the body when the spider surfaces.  Underwater, the spider can use that air store to breathe.  In addition to carrying the air film, this species does something truly spectacular: they build special silk webs underwater, fill them with air that they carry from the surface, and then use the air stored in the silk balloon (the diving bell) to breathe!  However, neither the air film carried by the spiders on their bodies nor the air contained within the diving bell are enough to completely satisfy the oxygen requirements of the spiders, so they depend on occasional surface trips to survive.

Scientists have known that these spiders use the diving bell as a sort of SCUBA tank for a while, but many questions remained.  No one knew what the oxygen conditions inside the bubble are, how much gas exchange occurs between the air inside the diving bell and the water outside, or how effectively the diving bell acts as a physical gill for the spiders.  (Please see my post Better Breathing Underwater in Aquatic Insects for a complete description of how physical gills work, why oxygen flows from water into an underwater air bubble, and why these air bubbles do not last forever.)  These unanswered questions drove researchers Roger Seymour and Stefan Hetz to look into the oxygen dynamics of the diving bell more thoroughly.  In the process, they discovered some exciting things about both the spiders and the unique air containers they build!

The researchers did several different things for their study.  First, they measured the volume of empty diving bells and used specialized oxygen detecting equipment to measure the oxygen levels both inside the bells and in the water surrounding them.  This information allowed the researchers to use mathematical equations to determine the potential rate of flow of oxygen from the water into the air bubble.  They also measured the oxygen inside the bubble while the spider was inside to calculate the rate of flow of oxygen from the diving bell to the spider, the spider’s oxygen consumption.  Using these two pieces of information, they could then show how effectively the diving bell acted as a physical gill, that is how long a spider could stay submerged when using a diving bell.  The researchers also measured the oxygen levels within the diving bells when spiders voluntarily returned to the surface  to reveal the oxygen level at which the respiratory requirements of the spiders were no longer met by the diving bell and they were compelled to replenish it with fresh air from the surface.  Finally, the pair measured the metabolic rates of the spiders directly using a respirometer, a machine that measures the amount of carbon dioxide released and/or oxygen absorbed by a biological organism.

Based on the result of all these measurements, Seymour and Hetz revealed several interesting things about the spiders and their diving bells.  First, they showed that the larger a diving bell, the more effectively it acted as a physical gill and the longer the spider could remain underwater.  Not only do larger bells contain more oxygen to begin with, but the flow from the water into the bell as the spider consumes oxygen is greater in larger bells than smaller.  Also, the larger spiders, those with greater oxygen requirements as measured with the respirometer, produced larger bells.  The authors further showed that diving bells easily provided the entire oxygen requirement of inactive spiders in warm, stagnant water (i.e., water with low dissolved oxygen) for over a day.  If the spiders moved around, built or cared for cocoons containing eggs  (cocoons are stored inside the diving bells!), ate prey that they captured, etc, then they had to return to the surface more often.  If they remained mostly still, the diving bell more than adequately provided their respiratory oxygen requirements for long periods of time.  Clearly the bells are acting as highly effective physical gills for the spiders!

Seymour and Hetz also demonstrated that the spiders stay within the diving bells until the oxygen drops d to 5-20% of the original level.  At this point, the bell apparently provides insufficient oxygen for the spiders and they return to the surface to collect air to replenish the diving bell.  The spiders also appeared to enlarge their bells if their oxygen demands increased or if the dissolved oxygen levels in the water dropped.

A few important implications are suggested by the results of this study.  The observations and measurements reported in the study were made in rather unfavorable conditions: warm, still water with low dissolved oxygen.  If diving bells are enough to meet the respiratory needs of the spiders for over a day under poor conditions like these, then spiders may be able to stay underwater nearly indefinitely in cooler and/or flowing waters.  This is important for several reasons.  If you happen to be an aquatic spider, you represent a nutritious meal to other aquatic animals such as fish, amphibians, and large insects.  Minimizing your trips to the surface, and thereby minimizing the attention you draw to yourself, is desirable.  It likely requires a lot of energy to make trips to the surface and back as well, so staying underwater as long as possible may help the spiders conserve energy.

diving bell spider in diving bell with cocoon

Diving bell spider in diving bell with cocoon. Image source: http://www.arkive.org/water-spider/argyroneta-aquatica/image-A12753.html

The spiders are also apparently able to respond to their environmental conditions and adjust the properties of their diving bells to match!  Seymour and Hetz observed their spiders enlarging bells under several conditions.  Spiders that captured prey enlarged their bells and added air to them before they started eating.  Spiders with cocoons also enlarged the bells as the broods inside developed, so the parents may be able to compensate for the increasing oxygen demands of their offspring as they develop by modifying the bell.  Apart from demonstrating how effectively the diving bell acts as a physical gill, I think the most exciting result from this study is that it reveals how these spiders intentionally modify their environment in response to their changing needs.

In summary, the diving bell does act as a physical gill for diving bell spiders as scientists have long proposed.  These bells allow the spiders to stay underwater for a very long time, and the spiders can adjust the bells to match their oxygen requirements and the dissolved oxygen levels of the water.  However, even under the most favorable conditions, the air contained within the diving bell will eventually need to be replenished, so the spiders will always depend on air from the surface and must have access to the surface to survive.

Super cool, right?  Aquatic spiders are amazing enough on their own, but spiders that build little air balloons to breathe underwater are infinitely more interesting!  Because I think it’s helpful to see it, I’ll end this post with a YouTube video (not my own) of a diving bell spider building a diving bell.  (I recommend turning the sound off – it’s got obnoxious music).  Enjoy!

Literature Cited:

Seymour RS, & Hetz SK (2011). The diving bell and the spider: the physical gill of Argyroneta aquatica. The Journal of experimental biology, 214 (Pt 13), 2175-81 PMID: 21653811

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Unless otherwise stated, all text, images, and video are copyright © 2011 DragonflyWoman.wordpress.com

Hellgrammites

We had an insect trading session in the class I’m TAing this semester, so everyone brought in extra insects they had in their collections to trade for things they didn’t have.  I’m going to discuss some of my observations about the trading session in an upcoming post (I was fascinated!) but today I’m g0ing to focus on the specimen I was most excited about: a live hellgrammite.

Hellgrammites are the larvae of the insect known as the dobsonfly and they are fabulous (or at least I think so).  In their adult form, dobsonflies are pretty gnarly looking.  Males tend to have long, intimidating mouthparts:

Dobsonfly male

Dobsonfly male. Awesome photo by Jessica Lawrence, available at http://bugguide.net/node/view/ 419853/bgimage

Though the mouthparts look scary, they’re really pretty wimpy.  The males of most species can only inflict a minor pinch because the mouthparts are so large they can’t get any leverage on them.  But these giant mouthparts do have a purpose – and, as in most cases where insects have supersized body parts, it all comes down to sex.  Female dobsonflies size up potential mates according to the size of his mouthparts, and in the world of the dobsonfly, bigger is definitely better!  The males with the biggest mouthparts are the sexiest, most desirable males, so some dobsonflies have evolved truly massive ones.

So a male with giant mouthparts mates with a female with more reasonably sized mouthparts to produce eggs.  Those eggs then hatch and these crawl out:

Hellgrammite (Corydalus cornutus)

Hellgrammite!

Now I love hellgrammites and find them completely fascinating.  I am always thrilled to find these in the streams I work in and I can spend hours watching them.  Even so, I’ll be the first to admit that these are some truly vile looking larvae.  They’ve got big, strong mandibles they use to rip apart their prey and they are formidable predators.  They’ve got a pair of hooks on each of two fleshy prolegs on the back end (more about these in a moment) that stick to your fingers or clothes like burrs.  They’re big larvae too.  The hellgrammite in the photo is nearly 3 inches long!  And then there are the long, spindly gills sticking off the sides of the abdomen that give them an alien look.  These do nothing to diminish their threatening appearance and I think it makes them look like big, aquatic centipedes.

But those hooks and gills are also part of why I love hellgrammites.  If you’ve kept up with my blog, you know that my research broadly involves respiratory behaviors of aquatic insects.  Judging from the adaptations hellgrammites display and the habitats they live in, they need a lot of oxygen to survive.  That’s where the hooks and the gills come in: they both help the hellgrammite get as much oxygen from the water as possible.

Let’s consider the hooks for a moment.  If you’re an aquatic animal that requires a lot of oxygen, there is a specific type of water that is best suited to your needs: cold, turbulent, fast flowing streams or rivers.  That’s exactly where you’ll find hellgrammites, clinging to rocks right out in the areas of the strongest flow in cool or cold streams.  However, a giant three-inch long larva, even a flat one like a hellgrammite, is going to have a hard time holding onto the rocks when there’s water slamming into it constantly.  So, they’ve got these:

hellgrammite hooks

Prolegs and paired hooks at the posterior end of a hellgrammite.

Those little hooks grab a hold of the rock so that they aren’t ripped off the substrate and washed downstream.  Hellgrammites are also usually found under big rocks in these fast flowing streams, so the currents they experience are weaker than those on the upper surface of the rock.  Those little hooks aren’t always enough to keep a large hellgrammite in place if they venture out onto the top of the rock.

Hellgrammites are highly adapted for collecting oxygen from the water as well.  If you recall from my post on aquatic insect respiration, insects living in turbulent, cold water maximize their opportunities to collect oxygen from the water.  If they expand their exoskeleton into gills, their surface area increases and they can absorb as much of that relatively abundant oxygen as possible.  Hellgrammites have a lot of extra surface area in their gills.  The feathery looking gills sticking off the sides are rather immobile and simply increase the surface area.  The other set of gills, the puffy dandelion fluff looking ones, have muscles attached to them.  When a hellgrammite become oxygen stressed, it can wave those gills around through the water:

Waving the gills around is a form of ventilation and allows the hellgrammite to extract as much oxygen from the water as possible, especially under less than ideal situations.  The gill movements stir the water around the hellgrammite, pushing deoxygenated water away from the body and bringing oxygen-rich water into contact with the gills so that it may be absorbed.  Behavioral ventilation of this sort is common in aquatic insects and gill movements like this have been recorded in several species, especially within the mayflies.  Still, I can’t help but marvel at just how beautiful the hellgrammite gill movements are!  I hadn’t ever seen this behavior before I noticed it in the insect trading session and I was amazed.  I found it shocking that something that ugly could also have such a stunning characteristic.  It was almost hypnotic watching the hellgrammite pulsing its gills and I could have watched it for hours.

But then I was snapped right out of my gill-inspired reverie when the hellgrammite started to swim around the jar:

I don’t know about anyone else, but I find this sort of abdomen flicking, backwards swimming kinda creepy.  Crayfish do it too and it’s just bizarre.  Doesn’t that look like rather inefficient way to maneuver around your environment?  I can’t easily come up with a reason why this sort of swimming would have developed, though I’m sure there’s a good explanation.

Yep.  Hellgrammites are appalling to look at, but they are amazing in so many ways that I have to love them anyway!  I hope I’ve given you at least a little taste of my appreciation for these monsters of streams and rivers.  I’ll probably describe my plan for making a horror movie called “Hellgrammite!” at some point in the future.  I am sure you are all eagerly looking forward to hearing all about it.  It’s going to be fantastic!  :)

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

From the Literature: Beetles that breathe through their shells!

I wrote my last three posts on insect respiration specifically because I wanted to talk about a paper that was published in the Journal of Morphology in November 2009 that deals with aquatic insect respiration.  Since this is my personal area of expertise, I find this paper fascinating!  I hope you find it as interesting as I did.

One of the most common insects you’ll see in many freshwater systems is the predaceous diving beetle (Order: Coleoptera, Family: Dytiscidae).  As the name implies, these are beetles that live in water (hence “diving”) and they are fierce predators (hence “predaceous”).  They’re pretty amazing insects in many different ways and many of them are absolutely gorgeous to boot.  However, today I wanted to focus on one really interesting respiratory structure that a pair of researchers in Germany discovered.

Predacious diving beetles are generally atmospheric breathers and must go to the surface periodically to collect oxygen (see my posts on aquatic insect respiration and breathing more efficiently in water for more information).  They then carry air under their elytra while underwater.  Several species expose part of their air bubble to the water to allow them to take advantage of physical gill respiration as they swim around their environment.  However, some predaceous diving beetles are known to stay underwater a much longer time than would be expected if they relied solely on a physical gill.  Some of these beetles have been observed to have little air bubbles attached to their elytra, prompting some researchers to suggest that they might actually be breathing through their shells.

German researchers Siegfried Kehl and Konrad Dettner of the University of Beyreuth were interested in this observation.  They wondered whether certain species of predaceous diving beetles were able to use their elytra as a respiratory organ and set up a simple study to test the idea.  The team was interested in two things.  First, they wanted to know whether the elytra of the predaceous diving beetle Deronectes aubei were able to take up oxygen, that is whether they contributed to the respiration of these insects.  If so, they wanted to examine the structures on the surface of the elytra to determine which structures were contributing to respiration and how oxygen would be transferred from these structures to the rest of the body.

Meet Deronectes aubei:

Deronectes aubei

Deronectes aubei. Photo copyright Christoph Benisch and taken from http://www.kerbtier.de/cgi-bin/deFSearch.cgi?Fam=Dytiscidae.

This is one of the smaller predaceous diving beetles and is just over 1/8″ in length.  This particular species is European and found in Germany, so Kehl and Dettner were able to observe them in their lab.  They discovered that this species is capable of remaining submerged without any access to atmospheric oxygen for over six weeks!  Because these beetles live at the bottom of very fast flowing water, anything they can do to increase their submersion time will help prevent their bring washed downstream and/or damage to their bodies.  Being able to breathe through their elytra and absorb more oxygen from the water would be an excellent adaptation for living in this environment.

First Kehl and Dettner determined whether D. aubei was able to take up oxygen via their elytra using a technique called respirometry.  Respirometry is rather complicated to do and there is a ton of math involved, but the concept is simple.  First, you measure the oxygen content of air or water entering a chamber where an organism is housed, in this case a beetle or group of beetles.  Then you measure the oxygen content of the air or water after it has been exposed to the organism for some length of time.  The difference between the two measurements is the amount of oxygen that the organism used.

Kehl and Dettner measured the oxygen uptake of the beetles from the water using two different types of respirometry.  They also measured the oxygen uptake of beetles in two different treatments.  First they painted a resin onto their elytra, effectively blocking them from taking in oxygen.  Then they removed the resin and measured them again.  To determine whether the beetles were taking up oxygen via their elytra, they compared the oxygen uptake of beetles with the resin covering their elytra (the treated beetles) to the same individuals after the resin was removed (the control beetles).

Both types of respirometry provided the same results: beetles exhibited a significantly larger oxygen uptake when their elytra were exposed to the water than when the elytra were coated with resin and not allowed contact with the water.  These results mean that the beetles are taking up oxygen via the elytra and are using them as a respiratory structure.  The team also found that blocking the physical gill (that air bubble they expose to the water) did not significantly decrease the oxygen consumption of the beetles.  This told them that the beetles are getting most of their air through their elytra and that the air bubble they carry isn’t that important.

scanning electron microscope

The scanning electron microscope I use for my work. The sample goes into the chamber on the left and the image pops up on the computer screen on the right. The electron gun is the cylindrical part sticking up above the sample chamber.

Kehl and Dettner conclusively showed that the beetles were capable of taking oxygen in via their elytra, so the next step was to determine how exactly the elytra are used as a respiratory organ.  To do this, the team examined the structure of the elytra using electron microscopy.  Electron microscopy, like respirometry,  is rather complicated, but for now just know that there are two different types.  Scanning electron microscopy (SEM) is used to look at the surface features of an object and the samples can be pretty big, as large as the sample chamber will allow.  An electron gun is aimed at the object, they bounce off the sample, and sensors collect them to create an image of the object on a screen.  In contrast, transmission electron microscopy (TEM) is usually used to look at the internal structures of an organism, but you can only look at very small portions of the object of interest at a time.  An electron gun is aimed at a very, very thin slice of the thing to be examined and a sensor below the sample collects the electrons that pass through the sample to create an image on a screen.

Kehl and Dettner used both types of electron microscopy in their study.  First they used SEM to look at the surface structure of the elytra of D. aubei.  They discovered that there are three different types of hairs present on the beetles.  One is a simple hair that is found in deep pits.  Another is a hair that is found in shallow pits.  The third is a spoon-shaped hair with an enlarged base that is also found in pits.  Hairs are used for a lot of different things in insects including sensation (smelling and/or feeling the environment around them) and proprioception (determining which body parts are positioned where), so the team used TEM to see which specific hairs were involved in respiration.  They found that the spoon-shaped hairs contained tracheoles that traveled down cylindrical channels in the elytra.  These tracheoles then connected to one of four big tracheae inside the elytra that carried oxygen to other parts of the insect.  This meant that the spoon-shaped hairs were the respiratory structures of the elytra, the areas where oxygen was being taken into the elytra to be used by the insect.

What all of this means is that these beetles have tiny gills littering their elytra that they use to absorb oxygen from the water!  Kehl and Dettner call them setal tracheal gills.  These gills likely significantly increase the surface area through which the beetles can absorb oxygen, allowing them to remain underwater much longer than they could without them.  Because the beetles live in fast flowing, cold water, they live in an ideal aquatic oxygen environment.  These gills allow the beetles to make the most of the abundant oxygen.  The beetles also live in a high risk environment for moving around, so the longer they can stay underwater the better.  Kehl and Dettner suggest that other beetles that either live in very fast flowing waters or that live far under the substrate in streams likely have similar structures as well.  It is common for organisms that live in similar habitats to exhibit similar structural and/or behavioral features that suit the environment, even if they are widely disparate species.  The team predicts that many beetles living under gravel in fast flowing streams might have the same respiratory structures as D. aubei.

I believe that this study is a particularly good one.  Aside from my personal interest in the subject, Kehl and Dettner simply did a great job in creating an elegant study.  First they determined conclusively that the beetles were able to use their elytra to take up oxygen by comparing the oxygen consumption of beetles that were allowed to use their elytral structures to the same beetles when they could not use their elytra for respiration.  Then they used SEM to identify potential respiratory structurs on the elytra.  Finally, they used TEM to determine how the structures on the surface were connected with the internal respiratory system of the beetles, identifying the specific structures involved in respiration and the pathway oxygen takes to get into the beetle’s respiratory tract.  They showed both that the beetles were using their elytra as a respiratory structure and how they were able to do so.  Then they finished off their paper by suggesting the conditions in which this sort of respiration might be particularly well suited and made predictions that could extend to several other species.  It’s an example of a very neat, tidy experiment that uses simple experiments to discover new things with broad implications, which is what ever researcher hopes to achieve with his or her writing.

This concludes my series on insect respiration and aquatic respiration in insects!  If you have any questions or would like more information, please feel free to leave a comment.  Until next time!

Literature Cited:

Kehl S, & Dettner K (2009). Surviving submerged–Setal tracheal gills for gas exchange in adult rheophilic diving beetles. Journal of morphology, 270 (11), 1348-55 PMID: 19480011

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Unless otherwise noted, text and images copyright © 2010 DragonflyWoman.wordpress.com