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

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Aquatic Insect Tolerance Values

fast flowing, cold water

Sampling insects in a clean, high elevation stream

About 6 months ago, I wrote a post about aquatic insects and water quality that highlighted the differences in the diversity of species in a polluted river compared to a clean mountain stream in Arizona.  Considering how much I enjoy this subject, it’s been far too long since I wrote about it!  It’s time to do something about this sorry state of affairs.  My next few Monday posts are thus going to be about how insects are used as indicators of water quality in streams and lakes.

Aquatic insects are very useful for making environmental policy decisions and in deciding when managers need to step in and actively manage a body of water.  I think it’s useful to know how this works!  But first I should introduce the concept of macroinvertebrates.  If you ever delve into the insects-as-indicators-of-water-quality literature, you’ll see this term over and over again.  Although I tend to talk about insects more than the other invertebrates in streams and lakes, not all invertebrates that live in freshwater habitats are insects.   There are lots of other inverts, including crustaceans (crayfish, shrimp, and their relatives), aquatic worms (earthworms, tube worms, leeches, etc), flatworms, mites, snails, and clams.  The inverts are divided into the microinvertebrates and the macroinvertebrates.  Essentially, anything that you can see with the naked eye is a macroinvertebrate.  Everything else is a microinvertebrate.  I personally don’t find this definition very satisfying because one person’s macroinvertebrate might be another person’s microinvertebrate (e.g. my macroinvertebrate is very small).  Any one person’s cutoff for what makes a macroinvertebrate can change over time and as they gain experience too.  Still, water resource managers love the term macroinvertebrate and everyone uses it, including me.

An example of water full of organic pollution. This is a constructed wetland intended to clean up water coming out of a wastewater treatment plant before being released into the river.

In my first post on using aquatic insects as indicators of water quality, I focused on the changes in diversity that you see along the clean to polluted water continuum and I’ll talk about it again in a future post.  The number of species of macroinvertebrates in a stream is a quick and dirty way to compare bodies of water and determine the relative amount of pollution or impairment because clean streams and lakes tends to have more species in them than highly polluted bodies of water.  It isn’t precise though.  A fairly dirty stream can have almost as many macroinvertebrate species in it as a clean stream under the right conditions.  In this situation, it becomes important to consider the specific species that are found in a body of water. This is where tolerance values come in handy.

Tolerance values tell you how tolerant any given species is to pollution in its habitat (go figure).  The scale most commonly used goes from 0 to 10.  Things with low numbers are very sensitive to pollution.  Things with tolerance value numbers closer to 10 tolerate a lot more pollution in their habitats and can live in some pretty nasty water.  And, just to make everything confusing, sometimes you find super tolerant species with scores that go right off the top of the regular scale.  (They’re like Nigel’s amplifier in Spinal Tap – they go to 11!)

Arivaipa Creek looking toward the canyon

A nearly pristine, low elevation stream

Considering how often tolerance values are used in aquatic research and how valuable they are to water resource agencies and managers, I think it is worthwhile to know where tolerance values come from.  It takes a lot of time and effort, and often a lot of money, to calculate tolerance values for macroinvertebrates, but the concept is very simple.  First, someone (often a water manager for a state’s environmental protection department or a scientist) will take measurements of pollution or other impairments in many different bodies of water.  These could be simple physico-chemical measurements (such as pH, dissolved oxygen, temperature), measurements of embeddedness (how far down into the silt/sand the rocks and pebbles are buried) or periphyton (the algae growing on the surfaces of rocks, soil, and plants in the water), or full water chemistry analysis.  Which measurements are taken will depend on the region, the group doing the work, the funding, and the time available to put toward the project.  After measurements are made, bodies of water are grouped according to the level of pollution/impairment they exhibit, such as pristine, impaired, and polluted.

Next, the researchers send out a hoard of samplers to pull out every invert they can find from as many bodies of water as possible.  Some poor group of technicians then “picks” the samples (separates the inverts from the massive amount of junk that you get in aquatic samples such as leaves, sand, silt, twigs, trash, etc) and passes the inverts off to the identification guru to identify.  After all the water measurements and invert ID work is done, then the researchers compare the species present in each water body to its pollution classification and use statistics and other mathematical tools to look for overall trends.

Rio de Flag

A highly impaired, effluent dominated stream downstream of a wastewater treatment plant. Photo by Dave Walker.

Inverts that are found only in pristine lakes and streams and never in impaired or polluted waters have narrow pollution tolerances and are assigned low pollution tolerance values, usually 3 or less.  Inverts found in impaired and pristine waters but not highly polluted waters have a wider tolerance for pollution.  These inverts prefer clean water, but they can tolerate some pollution in their habitats and are usually assigned mid-range values around 5 or 6.  Things commonly found in highly polluted waters get high scores, between 8 and 10, though they are sometimes found in clean water systems too.  And those things with scores of 11?  Well, they can live in some of the filthiest water you can imagine!  I don’t know about you, but I can imagine (and have worked in) some pretty nasty water, and there are insects living in nearly all of them.

It is important to note that macroinvertebrate pollution tolerance values vary from region to region.  Here in Arizona, we can’t use the pollution tolerance values calculated for inverts on the east coast, even when the species are the same, because our waters and the inverts living in them behave differently than those on the coast.  Thus, every region develops their own pollution tolerance values.  When I’ve done water quality studies using insects as indicators of pollution/impairment in the past, I’ve used a list of tolerance values developed within Arizona that was given to me by the Arizona Department of Environmental Quality.  The tolerance values therefore accurately reflect how inverts in Arizona react to pollution/impairment that occur in Arizona.  The list doesn’t have every species, but you can often use what you know about which waters you find them in and published records of their presence to fill in the gaps.

Sabino Canyon

A normally clean, but impaired, stream a few weeks after the end of a major fire. Photo by Dave Walker.

Next Monday I will go through an example of how scientists and water managers use tolerance values by discussing a project I was involved in a few years ago looking at the insects in Arizona’s effluent dominated streams.  Tolerance values played a huge role in the analysis of our results, and it was an interesting (but disgusting) project.  Until then, have a great week – and don’t forget to enter my latest contest!

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

Friday 5: My Favorites Places to Collect Aquatic Insects in Arizona

I shall begin today’s Friday 5 with a quick true story.  Imagine a girl of 21 who loves insects and is applying to grad school.  She knows she wants to be an entomologist, but she hasn’t narrowed down her area of focus.  All she knows is she loves dragonflies, those gorgeous aquatic insects that flit happily around streams, wetlands, and ponds.  She applies to schools and then has to choose which one to go to.  She eventually chooses Arizona, where she will work with an aquatic entomologist.  She tells her family members the good news: she’s moving to Arizona to work on dragonflies!  Hooray!  Now imagine the look of dismay on the face of each relative when she tells them.  That look is followed by what quickly becomes the dreaded question: “You’re going to Arizona to study AQUATIC insects?!”

So, yeah.  My family generally thought I’d lost it when I told them I was packing up and moving to Arizona for grad school.  Never mind that a good number of them had been to Arizona several times themselves and know that there’s a decent amount of water here.  I myself remember trips to the local spring-fed oasis and several streams in the mountains when I lived here as a young child and came back to visit my grandparents.  I knew there was water here and I wasn’t going to let any of those naysayers get me down.  I was going to study aquatic insects in the desert, gosh darn it!

Since I started grad school, I’ve had the opportunity to visit many, many aquatic habitats in Arizona.  Some of them, like the area where I do my summer field work, are appallingly disgusting.  Others are gorgeous and pristine.  Today I’m going to share my top 5 places to collect aquatic insects in Arizona.  Some are favorite locations due to the insects they contain and others because the area itself is so amazing, but they’re all special to me.

Arivaipa Creek

Arivaipa Creek

Arivaipa Creek. I just wrote about this creek, so I won’t say much more here.  This creek is one of my favorites because getting to go there is something special in and of itself.  The area is also incredibly beautiful and is home to some fantastic insects.  Really love this creek!  Check out the post linked above if you would like more information about the area or my recent trip there.

Madera Canyon

Madera Canyon

Madera Canyon.  I’ve been going to this canyon stream all my life.  In fact, some of the very first photos I ever took were at Madera!  Madera Canyon is in the Santa Rita Mountains south of Tucson and east of Green Valley, AZ.  The creek flows mostly over the big rocks you see in the photo, and for the most part it flows year round.  (Yes, I count that little 4 inch wide trickle you sometimes get in the summer as “flow!”).  Madera is very pretty, but I also love the insects I find there: lots of caddisflies, sunburst beetles, two types of whirligig beetles, water scorpions, fly larvae, lots of other beetles and bugs.  The creek is even home to a unique beetle (an riffle beetle)  that is thought to be found only in this one creek!  The downside is the canyon is VERY popular for birding (there are some rarely spotted birds there), so there are usually a lot of people there.

Reynolds Creek

Reynolds Creek

Reynolds Creek. I recently wrote about aquatic insects with suction cups and described my joy at discovering net-winged midge larvae for the first time.  I found them in this creek.  Reynolds Creek is in the mountains south of Young, AZ and north of Globe.  It’s way out in the middle of nowhere, so it’s usually visited only by campers and hikers.  The pine forest surrounding the creek is stunning and the water is cold and clear, so it is an entirely pleasant place to spend a few hours or the night.  There are all kinds of interesting things in this creek too.  However, the sheer elation I experience every time I find the blepharacerid fly larvae here would be enough to keep me coming back, even if there was nothing else to find.

Salt River

Salt River

Salt River (a few miles upstream of Roosevelt Lake).  The Salt River is one of the few big, perennial rivers in Arizona.  As such, it is heavily utilized by people who enjoy water sports (tubing and rafting are both very popular – the location in the photo is a raft pullout point) and is therefore far from pristine.  However, this is still one of my favorite places to collect.  The water flows swiftly and powerfully, and it gets quite deep in places.  This means that there are some excellent flow-adapted insects in the river.  My favorite: the gigantic hellgrammites this river produces!  They’re close to 3 inches long and they’re fierce.  In fact, I tell my aquatic entomology students to put them into their own bags when they collect them from this river.  The hellgrammites will eat everything else in the bag before they expire, leaving you with a single bloated hellgrammite floating amongst an assortment of insect legs.  This river is also one of the only places I’ve found sisyrid larvae, but I’ll discuss them further in a future post.

Three Forks

Three Forks

Three Forks. Three Forks is located in the White Mountains east of Alpine, AZ at the confluence of the East Fork of the Black River, Coyote Creek, and Boneyard Creek.  The photo doesn’t do this location justice at all as the bright sun at the high elevation consistently causes me problems when photographing this area.  Three Forks is a high elevation, cold, fast flowing stream, so it’s got some great insects in it.  My favorites are the water pennies, the flat mayflies (heptageniids), and the aquatic moth larvae.  You can only collect in specific areas of Three Forks though.  It has become a conservation site for an endangered snail that is being decimated by invasive crayfish, so you now need special permission to access the protected area.

So those are my top 5 areas in Arizona for collecting aquatic insects!  If you ever visit Arizona, any of these places are well worth visiting even if you have no interest in collecting.  I think they are some of the most beautiful areas of Arizona.

I wish everyone a happy New Year!

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Unless otherwise stated, all text, images, and video are copyright © 2010 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