Insect Respiration

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

human respiratory system

The human respiratory system. Image taken from Wikipedia: system_complete_en.svg.


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

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

insect spiracle

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

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

insect respiratory system

Diagram of a simple insect tracheal system.

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

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

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


Text, caterpillar image, and tracheal diagram copyright © 2010

Field Stories: The Stuff of Nightmares

I believe that almost all entomologists have at least one arthropod or other animal that they really don’t like and find disturbing on some primal level.  Several of my best entomologist friends, including one who thinks ticks are the best animals ever, think roaches are the most vile beasts on Earth.  A herpetologist friend of mine gleefully handles rattlesnakes but completely loses her nerve when faced with a scorpion.  I know several biologists who are terrified of grasshoppers and other jumping insects, and even a few who really hate moths.  I personally don’t have any problem with roaches, or most other insects for that matter.  But there is one arthropod that I find incredibly disturbing, and that animal is the centipede.

Something about a centipede screams “This is an unnatural spawn of the devil!” to me.  I really, really hate them.  REALLY hate them.  They terrify me beyond almost any other animal.  I am not a scream at the top of my lungs kind of gal, so there’s rarely girlie shrieking involved when I come across one, but serious chills do run down my spine and I always involuntarily shudder.  Just thinking about them makes me anxious!

Because I dislike them so much, it figures that I live in a place that has some of the biggest centipedes in the world.  Meet Scolopendra heros:

Scolopendra heros

Scolopendra heros

This beast is also known as the giant redheaded centipede, which is yet another example of biologists giving organisms highly descriptive (aka, uncreative) names.  As you can see, this is an arthropod with many legs, but only one pair per segment, which makes it a centipede.  This thing is about 6-8 inches long, so it’s giant.  And it has a red head, hence redheaded.  I feel like I should think this centipede is beautiful and I certainly don’t begrudge anyone who does.  The colors really are fantastic!  And the ones in Arizona have even more red on them than the individual pictured here (there are several different color variations in this species).  Still, these are things of nightmares for me.  I think the problems I have with these animals are based on the fact that they are venomous (they’re predators and use their venom to subdue their prey) and they are fast.  Very fast.

Now that you know a bit more about these centipedes, allow me to tell you a story about an encounter I had with one.  If you’ve been keeping up with my blog, you’ve probably read about my field site already.  One day a few years ago, my advisor and I were made our daily summer trip to the pond to collect water bug eggs.  I strapped my waders on and climbed out into the pond as usual.  However, the water was remarkably clear that day and I could actually see all the way down to the bottom for the first time ever.  What I saw there, however, was horrifying: a Scolopendra heros sitting on the bottom of the pond, right by one of the sticks I needed to check for eggs.  The conversation with my advisor went something like this:

Me: (Yells to advisor) Whoa!  There’s a Scolopendra on the bottom of the pond here!
Advisor: That’s great!  Pick it up and bring it over here!
Me: Oh hell no!  I’m not picking it up!
Advisor: Chris, don’t be a wimp.  Pick it up!
Me: No!  I don’t think it’s dead.  (Pokes it with a stick to see if it moves.)
Advisor: It’s not moving?
Me: No.  (Poke, poke)
Advisor: If it’s not moving and it’s on the bottom of the pond, it’s probably dead.  Just pick it up!
Me: (A bit of hysteria creeps into my voice) No!!!  I’m not picking it up, even if it IS dead!  I hate these things!  But I really don’t think it’s dead…  (Poke, poke)
Advisor: (Shakes head sadly, conveying his utter disappointment at my squeamishness.  I have clearly failed his test of entomological robustness.)

In a bout of sheer wussiness, I eventually consented to pick the thing up with a stick.  I draped it’s limp body over the very far end of the three or four foot long stick and held it as far away from my body as I could, just in case it suddenly came back to life.  I was terrified I would get stuck in the mud and fall over and I could just see the demon spawn I was carrying flying through the air and landing on my head.  But, I made it to the shore unscathed and made my advisor hold a bag open for me (which he did only after making fun of me again) so I didn’t have to get the centipede close to my unprotected hands.  I was in the process of making an insect collection for a K-12 outdoor education center and knew it would make a good addition to my collection.  My advisor handed the bag to me and I quickly tied it shut.  I carried it back to the car holding it out from my body and grabbing only the tiniest part of the corner furthest from the centipede so I could keep it as far away from me as I could.  I kept looking at it and expecting it to wake up.  I was absolutely convinced that it was still alive.  I happily tossed it in the plastic box with my waders and slammed the lid on, thankful the centipede would be riding home in the back of the truck while I was safely in the cab.

When I got home, I carefully carried my wader box inside and pulled the lid off slowly, carefully peering in and expecting to see a lifeless centipede inside.  What I saw instead was exactly what I had feared!  The centipede was indeed still alive and was now running frantically around the bag.  I imagined that it was now a very angry venomous creature trapped in a very thin film of plastic that I was sure it could find a way out of.  I had to do something and fast!  My worst nightmare was about to come true: a livid centipede bearing down upon me across my kitchen counter while I was paralyzed in fear and helpless to prevent its leaping onto my face.  (Okay, so I have a vivid imagination!)  I grabbed the corner of the bag, the one now holding a squirming, probably unhappy centipede, and tossed it into the freezer, slamming the door shut before slumping against the fridge door and sighing in utter relief.  I conquered the menace that was the evil centipede!  And I was preserving it for a good cause, thereby killing two birds with one stone.

Or I would have been killing two birds with one stone except I’ve never taken it back out of my freezer.  I’m too creeped out by it to retrieve it from it’s frigid habitat.  Knowing it’s in there still is bad enough (and I carefully avoid it when rooting around in there for food), but actually getting it out, facing it’s horribleness once again?  Well, that’s just not going to happen willingly.  The real question is, when I eventually move, will I have the courage to take the centipede out and finally add it to a collection, or will someone from the rental company be in for a very nasty surprise?  I can’t say for sure until that day comes…


Text copyright © 2010

From the Literature: Jocks and nerds in the damselfly world (The Dragonfly Trilogy, Part Three)

Welcome to the third and final segment of the Dragonfly Trilogy – and another installment of From the Literature!  If you don’t know anything about dragonfly territoriality, I recommend reading part two of my trilogy for more information on how dragonflies and damselflies set up and defend territories.  You’ll get more out of this post if you know something about territoriality before diving in.

Last time, I discussed how odonates benefit from being territorial, why they set up territories and defend them from other males.  Like many things in biology, it boils down to sex: males that defend high quality territories generally mate with more females than males with low quality territories.  Likewise, males that defend territories generally mate with more females than males that do not defend territories.  It is usually better to be a male with a territory than a male with no territory, but there are often many more males than there are available territories and some males are inevitably left out.  Presumably the stronger, better males (the most fit males) end up successfully claiming and holding territories while the weaker, wimpier males are left without territories and become wanderers.

Calopteryx virgo male

Calopteryx virgo male. Photo from Wikipedia, by Michael Apel.

This is the situation that a group of researchers  in Finland recently investigated.  They chose to study the damselfly Calopteryx virgo, a European damselfly also known as the beautiful demoiselle.  This gorgeous creature is pictured at left and is one of the damselflies known to be territorial.

The researchers asked a simple question: are C. virgo males that defend territories larger than males that do not defend territories?  They wanted to know if the damselflies that were able to protect a territory from other males were somehow better suited to being territorial than the damselflies doomed to be wanderers and unable to gain their own territory.  They also wanted to know if this changed over time, whether the non-territorial males eventually became territorial.

To answer these questions, the researchers captured mature C. virgo males arriving at their study stream in Finland, then marked them (so they could tell them apart), measured their right hindwings, weighed them, and released them back into the study area.  They observed the damselflies on two different days ten days apart and determined whether the males were territorial (they stayed within a small, 2 meter area for at least three hours) or non-territorial (they did not remain within a 2 meter area).  Males were considered wanderers if they moved more than 100 meters during the observational periods.

The team discovered that there was a significant difference in size between territorial and non-territorial males.  Territorial males had longer wings and were heavier than wandering males, so the bigger males were the ones claiming and holding territories.   The researchers also discovered that time didn’t have much to do with whether a damselfly male was territorial or not.  The wing length and weight of the wandering males was about the same and wandering males were consistently smaller than the territorial males on both days.  Wandering males made up about the same percentage of the population both days too.  So, the smaller males weren’t ever getting territories and were consistently being excluded.

The data that Koskimaki and colleagues presented in their paper suggest that the bigger, more physically impressive males get more mates.  To better understand the significance of these results, let’s consider a similar situation in humans that many people will recognize.  Think about what you know about stereotypical high schoolers.  Who gets more dates: the jocks (usually the bigger, more physically impressive males) or the nerds (usually the smaller, less physically impressive males)?  When I was in high school (I myself was firmly rooted in the nerd category!), the jocks got most of the girls while the nerds usually only admired the girls from afar.  The jocks outcompeted the nerds physically, and because they were generally more attractive, they excluded the nerds from finding dates by hogging all of the available girls.  If you ignore the possible confounding affects of wealth, intelligence, and overall personality that come into play in human mating behaviors, almost the same thing is happening in the high school students that we see in the damselflies that Koskimaki and colleagues studied!  In effect, the jocks among the damselflies were getting all the girls because they were better suited to protecting territories, and thereby attracted more mates, than the nerds who were unable to gain a territory.

I’ll end with two important questions: if it is so much better for males to be bigger so that they can more successfully hold valuable territories, 1) why are any males small and 2) why aren’t the damselflies getting bigger and bigger over time?  Koskimaki and colleagues suggest that that territorial and non-territorial males might form two distinct subgroups within the damselflies, each with their own strategies and goals for mating.  Even males without territories are able to mate with some females.  They could end up with the same number of offspring, thus ensuring the continued existence of smaller males in the population, if they have means for compensating for their relative lack of mating opportunities.  The team cites several other studies that suggest that this is happening in several territorial damselfly species, that non-territorial males are equally successful in producing offspring compared to territorial males.   It is likely that there are some benefits to being smaller or some costs to being larger that have not yet been accounted for.  Further studies in this area would be a great avenue for future research!

I hope you’ve enjoyed the dragonfly trilogy!  It’s been a lot of fun delving into the dragonfly literature for a few weeks and sharing information about my favorite group of insects.  I’m sure to post more odonate research in the future, but next time I’ll be telling a story of a centipede and a woman who is very, very scared of them – me!

Literature Cited:

Koskimaki, J., Rantala, M.J., & Suhonen, J. (2009). Wandering males are smaller than territorial males in the damselfly Calopteryx virgo (L.) (Zygoptera: Calopterygidae). Odonatoligica, 38 (2), 159-165.


Text copyright © 2010

Dragonfly Territoriality (The Dragonfly Trilogy, Part Two)

Welcome to part two of my odonate trilogy!  Last time I discussed the reasons why I think dragonflies are the best insects.  However, I didn’t talk about the final reason I think dragonflies are amazing and that is the subject of today’s post: dragonflies are territorial!

Even if you aren’t a biologist, you probably know a bit about territoriality already.  Ever see a dog lift his leg on a fence or a tree?  That is a territorial behavior, a way for the dog to say, “This is MY space, so I’m going to mark it as mine!”  In canines, males mark territories with urine and other odorous compounds so that they can chemically signal to outside dogs that the space they are marking is part of their territory (the space in which they hunt and find mates) and that outside males should stay away if they want to avoid a confrontation.  They’re trying to convince other dogs that they are bigger and badder and that they can take on anyone that wants to challenge them for their space.  Naturally, your dog peeing on a tree doesn’t have quite the same meaning as it might if it were a coyote or a wolf doing the peeing.  After all, it’s hard to defend a territory on a leash!  Still, the behavior hasn’t entirely disappeared from our domestic pets and so our male dogs continue to pee on trees.  (And yes, I am resisting the urge to post a picture of my dog peeing on my fence!)

Male dragonflies are also territorial, but their system is very different from canines and is more behaviorally complex in many ways.  Let’s go over the process!

dragonfly habitat

A typical dragonfly habitat

First of all, males are territorial because females choose mates based on who provides the best real estate for her eggs.  For a female dragonfly, this might be a nice mat of algae, open water, or a stand of cattails – different species will look for different things.  A female dragonfly will go to an appropriate body of water (flowing or still, depending on the species), find the best place to lay her eggs, and mate with whatever male happens to be in the area.  The females of some species require courting rituals, but others will just mate with the male in the area.  The pair mates as described in my last post and the female will lay eggs in the area she has chosen.  Then she’ll fly off, perhaps waiting a few more days to return to the water to lay some more eggs or simply moving to another spot.

Because females choose mates based on the quality of oviposition sites (the word oviposition means to lay eggs, so an oviposition site is the location where eggs are laid), males who remain in the best areas will be able to mate with more females than those who are in lesser quality areas.  It’s even better if you’re THE ONLY male in that really great spot so you get all of the females.  Hence, territoriality began!  Male dragonflies will protect an area from other male dragonflies of their species so that they will have the best chance at mating with as many females as possible.  The best male (generally considered the most “fit” male by evolutionary biologists) is able to protect the best spots from his competitors.  The less fit males, the ones who are unable to chase the best males out of their areas, will take the less fabulous oviposition sites.  The males who are not able to hold any spot will sometimes hang out around the body of water and wait for a space to open up or try to sneak matings while the residents are preoccupied.  These comparatively unfit males will also sometimes leave the area entirely in search of another body of water.

Anax junius patrolling

A green darner (Anax junius) male patrolling his territory

So how does this territorial behavior work?  Let’s envision a hypothetical pond where no dragonflies have ever set up territories.  The first dragonfly arrives and takes the best oviposition site.  Depending on the species he belongs to, he will find a good place to rest while he watches his area or he will fly around his area continuously in a behavior called patrolling.  When another dragonfly comes along, unless there is another spot of nearly the same quality he can claim, he probably wants the same spot as the first male to arrive.  In this case, the new male will challenge the resident male for the position.  To do so, he will fly into the territory and the resident male will fly out to greet him.  The two will engage in a ritualistic fight where they chase each other, flying around one another in circles very rapidly and zipping across the pond, to demonstrate their strength (effectively their fitness) to one another.  Minimal physical contact occurs between the combatants (this is important when you have rather fragile wings that you depend on for everything you do!), but the male who would lose if they actually came to blows will likely give up his claim on the spot and take the next best spot.  More males arrive and fight for the spots that they want, shifting the territories between individuals.  Eventually, a sort of equilibrium is reached where the best males have the best spots, the lesser males have the lesser spots, and the weakest males have no spots.

Protecting a good territory is hard work and even the best dragonflies can’t protect them forever.  A male protecting the most popular oviposition site will be constantly challenged by neighbors and the males who are unable to claim territories, not to mention he’s getting more matings than any other dragonfly at the pond!  Male dragonflies also expend a lot of energy guarding their mates while they lay their eggs.  An unprotected female is likely to be grabbed by another male and taken to another location at the pond to mate again before she finishes laying the eggs the resident male just fertilized, and males put a lot of effort into guarding their mates.  So, territories often shift during the day as more energetic males overthrow tired resident males.  Younger males can usurp territories from older males as well.  And there are always those males who weren’t quite strong enough to claim a territory waiting for resident dragonflies to weaken to the point that they can finally overcome them and take over their positions.

Anax junius males in combat

Green darner (Anax junius) males in combat

Competition for territories can be fierce.  There are usually far fewer territories at a body of water than there are male dragonflies who want them, so they are constantly trying to claim better territories and mate with as many females as they can.  It’s effectively a war zone!  The competition doesn’t ease once all of the territories are taken either as more dragonflies may arrive at the pond and many of the less fit males stay nearby in hopes of eventually gaining a territory.

In general, males who hold territories mate many more times than males that do not have territories, but even the homeless males will secure some mates.  While a male is chasing another male from his territory, a weaker male might be able to slip in and grab a quick mating.  Males who are guarding females are less likely to chase intruders from their territories until the female is finished laying her eggs, so weaker males can sometimes take advantage of their lapse in attention to sneak in a mating.  It is also typical for males with lower quality territories to mate more often than males without territories.  You might thinks that males holding lower quality territories would never get mates and waste their energy protecting their sites because females choose mates based on who holds the best oviposition site.  However, females are in such short supply and such high demand that they are sometimes mobbed when they arrive at the best spots because so many males are competing for the same space.  A female who is harassed enough or has her egg laying interrupted enough times will seek a mate in a quieter area where she may lay her eggs in peace.

So, male dragonflies form territories so that they can mate with as many females as they can.  The more females they mate with, the more offspring they will produce and the more their genes are passed on.  Pretty simple really!  And it’s one major reason many other animal species set up territories too.

Next time, I’ll finish up my dragonfly trilogy by cheating a bit and using the British definition of dragonfly (they use dragonfly for both dragonflies and damselflies) so I can talk about a recent paper about territoriality in a damselfly species.  Damselflies are much less likely to protect territories than dragonflies, but the system works the same way.  I hope you’ll stay tuned!


Text and images copyright © 2010