Better Breathing Underwater in Aquatic Insects

If you’ve been keeping up with my blog in the last few weeks, you already know about the basic insect respiratory system and how aquatic insects breathe.  As you know, aquatic insects have a wide variety of respiratory adaptations that allow them to breathe in water, adaptations that have enabled them to live in water after insects developed on land.  However, aquatic insects can improve their ability to breathe underwater (i.e. improve their respiratory efficiency) by choosing to live in areas that have more oxygen (a higher dissolved oxygen content) or by using a variety of behaviors that allow them to take more oxygen from the water.  These are the topic of today’s post!

diagram of diffusion

Molecules move from areas of high concentration (on the left) to areas of low concentration so that they are evenly distributed throughout (on the right). Image taken from Wikipedia.

There is a lot of physics involved in understanding how insects are able to breathe in water and you’ll need to know some of the basics.  The most important thing to remember is that molecules, in general, want to be evenly distributed.  If there are more molecules of one type in one area versus another area, a concentration gradient forms (see my post on terrestrial insect respiration for more information about concentration gradients).  Molecules then tend to move from areas of higher concentration to areas of lower concentration (see image at right).  This happens with oxygen in both air and water and is the reason why the insect respiratory system works at all.

However, oxygen moves very differently in water than it does in air.  Most importantly, oxygen moves much more slowly in water than in air – up to hundreds of thousands of times more slowly!  If you allow an open container of water that has no dissolved oxygen in it to sit, oxygen will eventually become distributed throughout the water in the container, but it will take a very long time to do so.  And even when it does become evenly distributed in the water, there is still far less oxygen in the water than there is in air.  In air, oxygen has a concentration of about 210,000 parts per million.  This means that for every million molecules of the gas mixture we call air, 210,000 of them are oxygen.  In water, a concentration of 5 parts per million oxygen is considered pretty good and oxygen concentrations max out at only about 15 parts per million.  So, oxygen takes a very long time to move through water (it diffuses from areas of high concentration to areas of low concentration) and is present in only very small amounts.

The concentration of oxygen in water (the amount of oxygen in water, the parts per million) depends on a variety of factors.  Any movement of water will increase its dissolved oxygen concentration.  Turbulence of any sort, whether caused by wind, stream flow, objects in a stream, etc, stirs the water so that oxygen from the surface is pulled further into the water more quickly than if the water was still.  Water is also able to hold more oxygen when it is cold than when it is warm.  Very cold water, close to freezing, can hold nearly the maximum concentration of oxygen, close to that 15 parts per million.  Very warm water, like what you get in Arizona during the summer, might only hold 2 or 3 parts per million oxygen at best.  Finally, in general, water that has a lot of organic pollution, algae blooms, or is otherwise compromised often has less oxygen than waters that are very clean.  Pollutants and some biological organisms consume oxygen, driving the concentration of dissolved oxygen in the water down and making less available for the everything else to use.

fast flowing, cold water

An exmaple of a fast flowing, cold water stream where stoneflies like to live. I know it doesn't look like it in the picture, but the water was really ripping! This stream is in the White Mountains of AZ.

These facts have some important implications for aquatic insect respiration.  Let’s consider the types of habitats that aquatic insects live in.  Insects that need a lot of oxygen are going to live in places that have a lot of oxygen rather than places that do not.  Stoneflies, for example, live in very high oxygen environments – at least by aquatic standards.  They are usually found in fast flowing, very cold water streams.  Many of these streams have a lot of turbulence as well.  This type of water generally has about the maximum dissolved oxygen concentration possible.  And some stoneflies have large gills, which allows them to absorb even more oxygen from the water!  You probably won’t ever see a stonefly in a warm, slow flowing, polluted stream.  However, you might find a bloodworm there!  Because bloodworms have hemoglobin, they are able to live in very low oxygen environments where many other insects are unable to live.  Their bodies are designed to attract oxygen so that they are able to draw in oxygen even when there is only a tiny amount available in the water.  In fact, in some very polluted, slow flowing or still waters (like the water you find coming out of some poorly designed wastewater treatment plants), bloodworms might be the only insects in the water at all!

A predacious diving beetle in water.  The blue thing at the back end is the part of the air bubble it carries that is exposed to the water.  This allows the beetle to use the bubble as a physical gill.

A predacious diving beetle in water. The blue part at the back is the portion of the air bubble the beetle carries with it underwater that it exposes to the water. This allow the beetle to use the bubble as a physical gill.

Concentration gradients allow some other nifty things to happen for insects that carry air with them underwater, those insects that use the scuba tank style of respiration like the beetle at the right.  In my last post, I said I would explain why a giant water bug might want to expose the air bubble it carries under its wings to the water, and here’s the reason!  When an insect carries a bubble of air with it underwater, the concentration of gasses in the bubble are those found at the surface, about 21% oxygen, 78% nitrogen, and the rest a mixture of other gases including carbon dioxide.  When the insect dives underwater, it begins to consume the oxygen in the bubble.  However, the bubble wants to remain at equilibrium, it wants to keep its gas composition the same as the atmosphere at the surface.  If the insect is using oxygen from the bubble, the equilibrium is thrown off – the oxygen level is no longer at 21%.  But if the insect happens to expose its bubble to the water (see the blue air bubble at the back of the beetle in the image), oxygen will actually flow from the water into the bubble, replenishing the oxygen lost!  This works because, even though there is much less oxygen in water than in air, oxygen makes up a greater % of the gases dissolved in the water (about 35%) than in air (21%).  This sets up a concentration gradient so that oxygen flows from the area of higher concentration, the water, to the area of lower concentration, the bubble.  This type of respiration is called physical gill respiration because the insect is using the air bubble like a gill.

Now, it may seem like a physical gill would allow an insect to remain underwater indefinitely, but this is unfortunately not the case.  As the oxygen is consumed by the insect, the concentration of oxygen in the bubble decreases and the gas mixture is thrown out of equilibrium – the ratio of oxygen to nitrogen is no longer the same.  Remember how the bubble wants to remain at equilibrium?  Well, it will do whatever it needs to so that equilibrium is restored.  There are two ways to do it: increase the oxygen in the bubble or decrease the nitrogen in the bubble.  As oxygen is consumed, nitrogen starts to flow out of the bubble to restore the equilibrium.  Oxygen flowing into the bubble from the water slows the loss of nitrogen, but there is so much less oxygen available in water compared to air that consumption of oxygen often outstrips the flow of oxygen into the bubble.  So, nitrogen slowly seeps out of the bubble, making the bubble smaller and smaller until the bug must go to the surface to replace the bubble altogether.

Physical gill respiration is a really excellent adaptation for aquatic insects that rely on atmospheric air.  It considerably increases the length of time an insect may remain underwater and decreases the trips to the surface the insect must make.  However, it only works when the bubble is exposed to the water.  Insects that don’t expose their air bubbles don’t gain the benefits of physical gill respiration.   And any insect that carries lots of air with it, regardless of whether it exposes its bubble to take advantage of physical gill respiration or not, has some negative side effects.  One of these is the insect becomes very buoyant and tends to float to the surface any time it isn’t holding on to something or actively swimming.  This requires energy to do.

water boatman

A water boatman, one of many aquatic insects that use ventilation to improve their respiratory efficiency.

Another downside to physical gill respiration is related to the speed at which oxygen moves through water.  Imagine an insect that carries an air bubble in still water.  As the insect consumes oxygen from the bubble and it is replaced by oxygen from the water, the oxygen close to the bubble is quickly depleted.  Because oxygen moves so slowly through water, this is problematic: if the insect doesn’t move, it will take a very long time for oxygen to move into contact with the bubble once more, likely longer than the insect can stay underwater.  This is where our friend turbulence comes into play again!  Several insects exhibit behaviors that ventilate their respiratory system, that is behaviors that bring freshly oxygenated water into contact with the bubble or other respiratory structures while pushing de-oxygenated water away.  The beetle in the image above is an excellent example of this.  It swims around through the water a lot, exposing its air bubble to the water the whole time.  The water boatman at the left uses his legs to stir the water around his air bubble.  And some insects that use physical gill respiration live in flowing water so oxygenated water is almost always flowing by them.

All this boils down to a simple concept: many aquatic insects exhibit adaptations, either structural or behavioral, that allow them to remain underwater as long as possible.  They actually have some control over how much oxygen they are able to take in while underwater!  Next time, it’s another From the Literature post.  I’ll be going over a recent paper on a structural adaptation in some beetles that allows them to breathe in water.  I think it’s amazing, so I hope you’ll check back soon!

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Text copyright © 2010 DragonflyWoman.wordpress.com

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Presentation at the Biosphere II this weekend!

backswimmer

A backswimmer, one of Arizona's many aquatic insects.

I haven’t had time to finish my respiration post for this week (it’s just been one of those weeks), but I wanted to give everyone a head’s up about an event I’ll be doing at Biosphere II this weekend.  One of the requirements of being a Biosphere II Science and Society fellow is that we give two presentations at the Biosphere during our year as fellows.  Last semester I gave a talk on giant water bugs on Halloween.  This semester, I’ll be running a booth inside the main visitor’s center.  Anyone who happens to be in Tucson is welcome to stop by between 11am and 2pm this Saturday, February 20, to learn about Arizona’s fabulous aquatic insects!  I’ll have several live insects to observe and interact with, including the giant water bugs that are my specialty.  I’ll also have a mini behavioral project available for visitors to complete while they’re at my tables.  And I’ll of course be there to answer any questions that Biosphere visitors have about aquatic insects.

If you’ve ever had any interest in aquatic insects in general, the species we have in our desert, want to get up close and personal with an aquatic insect, or simply want to check “talk to an entomologist” off your life’s to-do list, I encourage you to stop by.  There should be lots to see, do, and learn.  Hope to see you there!

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Text and image copyright © 2010 DragonflyWoman.wordpress.com

Aquatic Insect Respiration

Creeping water bug

Creeping water bug (Order: Hemiptera, Family: Naucoridae)

Welcome to part 2 of my insect respiration series!  Last time I focused on the basics of insect respiration, the sort you would find in the standard terrestrial insect.  The basic type of insect respiratory system has been around for millions of years and works quite well for most terrestrial insects.  There are several variations on the basic theme of insect respiration in terrestrial insects, but the aquatic insects exhibit a very wide range of respiratory adaptations.  These adaptations, the modifications aquatic insects have made to their respiratory systems, are the focus of today’s post.

First things first.  Insects first evolved on land about 350 million years ago.  Because they originated on land, they have terrestrial respiratory systems that work best when breathing in air.  When insects first began invading freshwater systems, they had this terrestrial respiratory system to deal with.  As you might imagine, breathing in water is a very different thing than breathing in air, so insects had to adapt their terrestrial respiratory systems to a freshwater habitat if they were going to live in water.  And this is exactly what they did!  Aquatic insects have undergone a huge variety of structural and behavioral modifications that have helped change their respiratory systems from those that worked best on land to those that work well underwater.

Let’s imagine a scenario where a terrestrial insect crawls into the water for the first time.  Luckily, the pores through which insects breathe (their spiracles) were already largely waterproof, so it didn’t drown instantly.   This insect breathed air, though, and it needed to keep doing so to survive.  How did it do this?  Before I answer this question, first consider how we humans survive when we’re submerged in water.  There are three main ways you can prevent yourself from drowning underwater: hold your breath, use a snorkel or other device that maintains your contact with the air, or take air with you in the form of an oxygen tank (scuba diving!) or a submersible watercraft.  And why are we thinking of how humans survive in water?  Because insects can do these same things!

Lethocerus medius

Lethocerus medius (Order: Hemiptera, Family: Belostomatidae). Notice the long respiratory tube that extends off the back end of this bug!

Snorkels are actually pretty common in aquatic insects and are one of the simplest adaptations for breathing underwater.  The image at the right is a picture of the giant water bug Lethocerus medius.  This insect uses it’s long respiratory siphon to allow it to remain underwater while still maintaining contact with the surface to breathe.  If the bug needs to dive into the water for some reason, such as to avoid a predator or capture prey, it is able to hold its breath for several minutes (close to a half hour!) until it is able to return to the surface, stick its respiratory siphon back out, and continue breathing air.  This sort of respiration is also very common in several fly species, including the mosquitoes and the rat tailed maggots.

Abedus herberti

The giant water bug Abedus herberti exposing its air store to the water. The silvery parts of the image is the air.

Other insects use the scuba tank style of respiration and carry oxygen with them underwater.  This is very common in many aquatic insect species, including several species of giant water bugs (the members of the Belostomatinae) and the predacious diving beetles.  In the image to the left you’ll see the giant water bug Abedus herberti exposing its air bubble, which it carries under its wings, to the water.  (I’ll explain why they might want to expose their air bubbles in my next post!)  While the bug is underwater, oxygen is drawn through the spiracles and into the respiratory system from this reservoir of air.  When the bubble shrinks and the oxygen supply runs out, the bug goes back to the surface to replenish it.  Insects using this form of respiration (called bubble gill respiration) are still using the same atmospheric oxygen insects have always used, but they can stay underwater for a long time.  Some insects even take this a step further and use what is called plastron respiration.  Plastrons are rather complicated to explain, especially before you’ve read my next post, so I’m not going to go into detail.  Just know that insects that use plastrons still carry an air bubble and still rely on atmospheric air, but they have special modifications that prevent the bubble’s shrinking so that they almost never have to go to the surface.  It’s like having a scuba tank that never runs out of oxygen!

The snorkels and scuba tanks are very simple modifications for the most part.  Insects that use these sorts of respiration still rely on atmospheric air and other than rearranging, modifying, or closing some of their spiracles, their respiratory systems are very similar to those of terrestrial insects.  These insects also need to be able to get to the surface periodically to breathe.  However, this is very difficult in some habitats, such as at the bottom of lakes or in very fast flowing water.  In these types of situations, being able to stay underwater all the time, breathing more like a fish than a terrestrial insect, becomes valuable.  And, of course, insects have figured out several ways to do this.

Hellgrammite (Corydalus cornutus)

A hellgrammite (Order: Megaloptera, Family: Corydalidae)

Insects that breathe in water rely on the oxygen that is available in water, dissolved oxygen.  Dissolved oxygen levels vary widely across aquatic habitats, so different insects have different modifications that allow them to live in their particular habitats.  Take a look at the hellgrammite that is pictured at right.  These are big insects and need a relatively large amount of oxygen.  However, there isn’t a lot of oxygen in water, even under the best conditions.  So, these insects live in the best conditions.  They are typically found in very fast flowing water in areas with a lot of turbulence (turbulence increases dissolved oxygen levels in water) and in cool to very cold water (cold water holds more dissolved oxygen than warm water).  From this fast flowing, cool water, they absorb oxygen directly through their exoskeleton.  But they have also modified their exoskeleton to increase their surface area so that they can absorb even more oxygen.  Those little pointy bits coming off the sides in the dark brown section in the back half of the hellgrammite aren’t legs – they’re gills!  Gills dramatically increase the surface area of insects that absorb oxygen through their exoskeletons, allowing them to breathe more efficiently.  Lots of aquatic insects have gills, including the mayflies, the stoneflies, the dragonflies and damselflies (remember that dragonflies have rectal gills!), and the hellgrammites, among others.

blood worms

Bloodworms (Order: Diptera, Family: Chironomidae), fly larvae also known as non-biting midges.

The last adaptation I wanted to go over is very rare in insects and is only known in two groups.  Take a look at the bloodworms at the left.  The color has faded in these specimens because they were preserved in alcohol, but if they were alive they would be bright, vivid red.  This is because these insects contain a sort of hemoglobin to help them breathe!  For those of you who don’t know, hemoglobin is the substance in human blood that a) allows our blood cells to absorb oxygen from our lungs and b) makes our blood red.  Bloodworms are red because their hemoglobin makes them red.  They use their hemoglobin to absorb more oxygen into their bodies than they could without it.  This is important because a lot of bloodworms live in very low oxygen environments such as the bottom of deep lakes, in polluted waters (where they are sometimes one of the only insects that can survive!), and lakes with a lot of bacteria.  These are habitats in which these insects would not be able to live without their hemoglobin helping them bring oxygen into their bodies.

In my next post, I’ll discuss the many things that aquatic insects do to make their respiratory systems work more efficiently in water.  Whether they rely on atmospheric or dissolved oxygen, insects exhibit countless behaviors that help them use the tiny amount of oxygen in the water as effectively as possible.  Check back soon!

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Text, images, and video copyright © 2010 DragonflyWoman.wordpress.com

Teaching Insect Behavior Using Blogs

There’s been a bit of a delay in getting a post out this week thanks to my coming down with a cold.  Before I continue my series on insect respiration (which requires more energy to finish than I have available at the moment), I wanted to take a quick detour to describe a new project I am using in the class I teach this semester.  Next time, assuming I’m cold-free by then, I’ll jump right back into the respiration series with a post on aquatic insect respiration.

As an entomologist who blogs, I have learned that there are a lot of people in the world who want information about insects.   I have also learned that these people often look up very obscure things.  I try to remember this when I decide what I want to write about and sometimes blog on obscure topics even though I don’t think many people will read them.  I’m constantly surprised by the things that people will read!  When I write new posts, I try to remember that someone, somewhere will eventually want to know what I have to share, so I should always go ahead and post it.  And even if no one reads it right away, at the very least I’m making information available so that people can easily access it later.  I find my blog very fulfilling and I’ve come to believe that blogging is a powerful tool in disseminating scientific information to the world.

Abedus herberti mating

The giant water bug Abedus herberti mating. Photo taken in class last week.

This semester, I am once again teaching the insect behavior lab at the University of Arizona, but I’m trying something new.  Here’s how the insect behavior lab generally works when I teach it.  The students come to lab once a week for close to two hours.   For the first four labs, they do highly directed work that teaches them the basics of collecting behavioral data from insects and how to present their data in scientific paper format.  For the majority of the remaining labs, they are simply given a topic for the day (locomotion, predation, etc) and a goal.  The students then figure out what questions they want to ask to achieve that goal, form hypotheses, and develop a quick experiment to test their hypotheses using the live insects I make available to the class.  The students submit several assignments, including a lab notebook of their observations, lab write ups written in proper scientific paper format, and a longer scientific paper reporting on an individual project they do outside of class.

lab notebooks

Some of my own lab notebooks.

One of the major problems I have with the class is that the information we collect almost never makes it outside of the classroom.  This is an issue for me because some of the students come up with some very interesting information and do really excellent, publishable work.  However, the knowledge my class produces belongs to only a handful of people, often only the group that worked together in class and me.  Who cares if one of my students discovers something amazing?  No one outside the class is ever going to learn of it, so it does absolutely nothing to further science or make others aware of some of the fascinating things that insects do.

In an effort to remedy this deplorable state of affairs, my insect behavior students are being given the opportunity to submit some of their classwork in blog format for the first time.  Students who choose this option may submit their lab reports and their lab notebooks online via science blogs they create.  It is my hope that some of the interesting and/or valuable things that my students learn will finally be made available outside of class for people who are interested in the behaviors we study.  Aside from the benefits to science-loving people outside the class, I also believe that making their classwork available publicly will benefit my students.  Writing something that everyone can read makes you think about things more thoroughly, convinces you to look up that fact you are not completely sure is true, makes you more concerned about embarrassing yourself in public.  Or at least, this is what I choose to believe.  :)

I don’t know how many students will choose this option.  So far, it doesn’t seem popular for submitting lab reports, but I believe at least a few of them are submitting their lab notebooks as a blog.  The lab notebooks are probably more interesting for others to read anyway.  Really, though, I’ll be excited if even a few students take the blogging option.  If they do, I will post links to thier blogs here when I get them.  Some of my current students are likely to produce very high quality work that will be well worth a read.  It’s a really excellent group so far, probably the best I’ve had!

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Text and images copyright © 2010 DragonflyWoman.wordpress.com

Abedus herberti mating

The giant water bug Abedus herberti mating

I find my blog very fulfilling and I’ve come to believe that blogging is a powerful tool in disseminating scientific information to the world.