Why Dragonflies Are the Best Insects (The Dragonfly Trilogy, Part One)

Hello everyone!  I took a week off for Christmas, but I’m now back at my computer and looking forward to blogging during the new year.  Hope you all enjoyed your holidays!

For the next few posts, I’m going to talk about odonates.  Dragonflies have been my favorite insects for ages and I find them completely fascinating.  They are also special insects – they do things no other insects do and they have body structures that no other insects have.  So, for this post, I’m going to talk about these special traits, the reasons why I think dragonflies are the most amazing insects in the world.  Next time, I’m going to talk about dragonfly territorial behaviors, and then I’ll finish up my dragonfly trilogy with a From the Literature entry that relates to dragonfly territoriality.  I hope you enjoy The Dragonfly Trilogy!

Last summer, I wrote about the difference between dragonfly and damselfly nymphs.  If you haven’t read that post and/or don’t know how to tell dragonfly and damselfly nymphs apart, I recommend that you do so before reading further.  If you have read the post or know a bit about dragonflies already, you know that one of the very special things that dragonflies nymphs have is their fantastic mouthpart (see photo).

Dragonfly mouthparts side view

Dragonfly mouthparts, side view

This bizarre mouthpart is definitely one of the most amazing things about odonates – and they are the only insects in the world that have them.  I personally think it gives the odonates a sort of alien-like appearance.  Can’t you just see this sort of structure popping up in an alien invasion movie?!  More practically, this mouthpart allows the odonates to be fierce predators and places them firmly near the top of most aquatic food chains.

All odonate nymphs have this mouthpart, but the dragonflies have another unique structure that makes them even more bizarre and interesting: rectal gills.  Recall that dragonflies and damselflies have different appendages on the ends of their bodies.  The damselflies have three leaf-like gills attached at the tips of their abdomens (see photo).

Damselfly gills

Damselfly gills

Though the dragonflies don’t have these structures, they still have gills.  Rather than keeping them exposed to the water like the damselflies do, however, they keep their gills inside their bodies!  Dragonflies have a chamber inside their abdomens called the rectal chamber that is lined with gills.  Due to the physics of respiration in water and because the gills are inside the body, dragonfly nymphs have to pump water in and out of the rectal chamber to allow the gills to absorb oxygen from the water.  (If you ever have a chance to see a dragonfly nymph, you’ll likely see the abdomen pulsing – it is pumping water through its rectal chamber.)  The opening to the rectal chamber is at the back end of the bugs, so the dragonfly nymphs are effectively breathing through their butts!  If that doesn’t make dragonflies one of the coolest animals in the world, I don’t know what does.

The rectal chamber also allows the dragonfly nymphs to do a great behavior: jet propulsion.  Due to some properties of physics, they can forcefully squeeze the water out of their gill chamber to propel themselves forward very rapidly.  If you’ve ever seen a fan boat (they’re usually used in swamps) on TV or in person, it works about the same way.  By pumping water into the rectal chamber and shooting it back out at a high speed, a dragonfly can move very fast and entirely without the use of its legs!  To see this behavior in action, take a look at this video, paying special attention to their legs:

Did you see the little dimples in the surface of the water behind the dragonfly?  That is the water jet being expelled from the rectal chamber and hitting the surface of the water.  The jet propulsion behavior helps the dragonfly nymphs avoid being eaten by predators by allowing them to escape very quickly when they are startled.  And, it is one more reason why dragonflies are amazing insects.

So dragonfly nymphs have the alien-looking extendable mouthpart, internal gills that make them breathe through their butts, and jet propulsion.  What about the adults?  The adults are just as bizarre!

green darner in flight

Anax junius in flight

First, let’s consider the wings.  Dragonflies are among the strongest and most agile fliers of the insects.  They are able to do this because of another strange bodily structure.  In almost all insects, the wing muscles don’t attach directly to the base of the wings.  Instead, they usually attach to the inner walls of the thorax so that when the insect contracts the wing muscles, the whole thorax deforms.  This indirectly makes the wings move.  These sort of flight muscles are called indirect flight muscles because the wing muscles do not attach directly to the wings.  Dragonflies, however, have direct flight muscles.  Moreover, each wing has its own muscle.  This means that dragonflies can move all four wings independently of each other.  It is this trait that gives them their awesome flight capabilities, such as their ability to come to a dead stop while making a 180 degree turn and their ability to fly backwards.  Dragonflies are such great fliers because they have different wing muscles than most other insects.  This is also what allows them to grab a flying insect (or the occasional hummingbird!) in flight so that they can eat it.

Adult dragonflies have one other unique body structure that makes them fabulous.  The males and females both have their genitalia at the ends of their bodies, the tips of their abdomens.  However, due the way dragonflies mate, dragonfly males can’t use these structures to transfer sperm to the females.  A dragonfly male grasps the female behind the head as they mate, which means that his genitalia are far from the female’s.  So, dragonfly males have a second set of genitalia on the abdomen near the thorax!  Before he mates with a female, a male will transfer sperm from the genitalia at the end of his abdomen (the primary genitalia) to the genitalia at the base of his abdomen (the secondary genitalia):

male dragonfly genitalia

Male dragonfly genitalia (Pantala hymenaea)

Once a male dragonfly grasps the female with his abdomen, he curls her body around so that her genitalia come into contact with his secondary genitalia, thus transferring sperm from his body to her’s:

mating dragonflies

Dragonflies mating (Pachydiplax longipennis). The male is the blue dragonfly on top and the female is the brown dragonfly on the bottom.

Some dragonflies fly around while they mate and others will find somewhere to land, but you’ll see this formation in virtually all mating dragonflies.  Pretty neat, eh?  I’ll talk more about dragonfly mating next time when I discuss dragonfly territorial behaviors.

Because dragonflies have this complicated mating system (which is amazing even if there wasn’t anything else going on), they also have two sets of genitalia, one of many reasons why I think dragonflies are the best insects.  This also makes it very easy to tell male and female dragonflies apart.  If you have a dragonfly in your net or your hand or a really good photo, look for a bump on the underside of the abdomen near its joint with the thorax.  If there’s a bump there, it’s a male.  If the underside of the dragonfly is smooth, it’s a female.  It’s easy!

So, did I convince you that dragonflies are the best insects?  If not, I hope you at least have a greater appreciation for the things that make dragonflies unique and fascinating.  Between their anatomical oddities and their amazing behaviors, dragonflies are some of the most interesting animals to study and one of the most widely appreciated insects.  If you have an opportunity, I highly encourage you to watch any dragonflies near your home.  You are sure to discover many new and interesting things about dragonflies that you can add to the list of things that make dragonflies so cool.  And who knows?  You might even find yourself agreeing with me!

Tune in next time for part two of the Dragonfly Trilogy: Territorial Behaviors in Dragonflies.


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