Caterpillar Camouflage

Today I have a short post for you!  I was out collecting with my students during class last week and one of them took us to a nice spot on campus, a secluded little courtyard of one of the old buildings with a handful of citrus trees.  We looked around and found some stink bugs on a tree, some butterfly cocoons hanging off the buildings, and some spingtails.  One of my students found this:

Papilio cresphontes

Papilio cresphontes

If you think this looks like something that was ejected from the back end of a bird, you’re not alone!  This is the caterpillar of the giant swallowtail (Papilio cresphontes).  As you might imagine, looking like bird droppings has its advantages.  This caterpillar blends very well into the background and it is hard to recognize that it is an insect at all.  That’s the whole point of looking like bird droppings!  Any insectivore (an organism that eats insects) looking for a tasty caterpillar to eat is likely to pass right by this one because it looks so much like something else – and something most animals wouldn’t consider eating.  The appearance of this caterpillar is part of its defense against predators.  If it stays still, most predators won’t even notice it’s there.

But say something happens to bump into the caterpillar (such as an insect systematics student looking for insects for her collection) or otherwise detects the caterpillar’s presence.  Then the caterpillar brings out it’s backup defense!  It’s depicted in this video:

That little orange slimy looking thing that pops out of the caterpillar is called an osmeterium.  Normally, it’s hidden in a pouch inside swallowtail caterpillars, right behind the head.  When disturbed, the caterpillar can squeeze some of it’s hemolymph into the osmeterium, causing it to pop out of the pouch.  The everted osmeterium is then waved at the predator.  Now how might this little organ be useful in deterring predators that might want to eat the caterpillar?  It’s covered in potently stinky chemicals!  Any animal that gets a big whiff of a foul smelling substance from something it’s considering eating, especially from something that looks a whole lot like bird poop in the first place, is probably going to pause for a moment and consider whether it’s worth eating.  Most things will leave the caterpillar alone rather than eating it.  And when the predator wanders off and leaves the caterpillar to itself, it can pull the osmeterium back into the pouch behind its head until the next time it’s needed.

Pretty fun, eh?  If a caterpillar that looks like bird poop isn’t fun, I don’t know what is!  :)

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

 

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Click! Click! Click!

In the summer months in Arizona, it’s common for things to come crawling into your house to avoid the heat.  Leave a door open and your house will soon be full of mosquitoes, geckos, several types of beetles, webspinners (which are insects and NOT spiders), green lacewings, sun spiders (also not spiders), moths, maybe even the occasional bark scorpion or snake.  I happen to have a dog door that doesn’t quite close all the way due to the pressure built up by the evaporative cooler, so all kinds of insectoid creatures make their way into the house that way.  My personal favorites of the things that end up in my house are these:

click beetle

Click beetle, likely Melanotus sp.

Click beetles!  If you haven’t ever gotten to play with a click beetle (and let’s face it – most people don’t squeal with delight and immediately put their hands on random beetles they find the way my crowd does), you’re missing out.  You might not know why they’re called click beetles either.  Rather than explaining it, take a look at this short video I took a few nights ago of a click beetle that made it’s way into my bathroom (where all click beetles seem to end up in my house).  The quality isn’t fantastic as it was taken in a poorly lit inner room at night with a hand held camera, but you’ll get the idea.  And make sure you have the sound on!  Clicking the arrow near the lower right corner and selecting 1080p after you click play will improve the image quality.

Isn’t that wild?!  And did you hear the clicks?  These beetles are called click beetles for a reason: they make clicking sounds when they do their super awesome ninja-like jumps.  Those jumps are possible due to this structure on the underside of the beetles:

clicking mechanism

Click beetle underside. The arrow points to the click mechanism. Pryophorus sp.

click mechanism side view

Click mechanism side view. Pyrophorus sp.

Every entomologist knows this much about click beetles:

1) They click
2) They jump
4) They’re flat, elongate beetles with spines on the middle section of the thorax (you can see them in the pictures)
3) Both clicking and jumping are made possible by the spine indicated by the arrow on the second thoracic segment (the middle section of the thorax) fitting into the groove on the third thoracic segment.

When I decided to write about click beetles, it occurred to me that I didn’t know exactly how this mechanism worked.  The online search was worthless as nearly every site simply copy and pasted the Wikipedia entry on click beetles, and that told me what I already knew – spine fits into groove, makes beetles jump.  Looking through my entomology books wasn’t much more helpful: spine fits into groove, makes beetles jump.  Before I dug into the literature on the subject, I decided to try one final textbook, The Insects: Structure and Function by the late, great Reg Chapman (a very lovely and brilliant British man who taught one of my first entomology classes in grad school).  The book is one of the most dense books I’ve ever read – and I read it cover to cover to prepare for my comprehensive exams for my Ph.D. – but it’s an amazing treasure trove of information about insects.  Reg’s book told me how the mechanism works.

First, the beetle arches the center of its body upward off the ground so that only part of the thorax and the tip of the abdomen are still in contact with the ground.  It then contracts the muscles around the spine.  Normally this would result in the movement of the thorax, but the spine catches on the groove so the thorax doesn’t move.  Instead, energy is stored up as the beetle continues to contract the muscle and the spine remains trapped in the groove.  Eventually, the spine slips off the groove and all of that energy is released.  The front and back ends of the beetle, the parts that were still in contact with the ground as it arched, snap upward off the ground at a high velocity.  The velocity is so great that the rest of the body is pulled up after them, launching the entire beetle into the air.  If this is hard to picture, imagine shooting a rubber band off your finger toward the ceiling.  You pull back on the rubber band with one hand while catching it on a finger of the other hand.  The rubber band represents the spine and the muscles attached to it while the finger is the groove.  Energy is built up as you stretch the rubber band further and further back, similarly to how the beetle stores up energy as it contacts the muscle around the trapped spine.  When you release the rubber band, the part you pulled back launches forward, releasing the energy stored in the rubber band and pulling the entire band off your finger in the process.  (Well, assuming you’re holding it right, otherwise it releases all that energy into your soon-to-be-painful finger instead.)  Now imagine the rubber band and finger setup inside the beetle and it should give you a pretty good picture of how this works.

So why do these beetles do this?  There are two main reasons.  This clicking-jumping behavior is likely primarily a strategy to avoid being eaten by predators.  Most things that try to eat a click beetle will think twice if the beetle launches up into their face as they try to eat it.  It’s a startle tactic: if they can distract the predator for even a moment or two they just might be able to run away.   You can see this in the video.  The beetle clicks when I (a possible predator) touch it, then it runs.  I think this anti-predator mechanism is likely very effective.  Watching one of my dogs messing with them certainly suggests that it is!  He’ll sniff the beetle, then jerk back in horror as the beetle launches itself into his nose.  He might then put a paw on it, at which point it will click again, so he’ll jerk his paw back.  He usually tries the paw thing twice before he leaves the beetle alone.  Granted, this only works so long as the other dog doesn’t see him playing with the beetle.  The smaller dog is so jealous the bigger one is getting something that she’s not that she’ll run over and eat the beetle, clicking or not, just so her brother can’t have it.  :)

But back to the beetles!  The other reason these beetles likely click is to be able to flip themselves over when they end up on their backs.  These beetles have rather wide bodies and stubby little legs, so they have a hard time getting themselves right side up again.  Clicking to the rescue!  The clicking mechanism works whether they’re right side up or upside down, so the beetle can simply click, launch itself in the air, and hope it lands on its feet.  If not, it will click again until it does.  You can see this in the video too.   The beetle easily rights itself every time I flip him over.  Pretty neat trick, don’t you think?

So now you know how and why these beetles jump.  I’ll end this post with a brief comment on variations in size and coloration in these beetles.  While most of the click beetles most people see are smallish, drab beetles, there are some amazingly beautiful click beetles in the world.  I leave you with pictures of what I think are the two most spectacular species found in Arizona.  Enjoy!

Chalcolepidius ostentus

Chalcolepidius ostentus. Not sure of a common name, but I call this one the really pretty click beetle. :)

Alaus zunianus

Alaus zunianus, the Zuni eyed elater

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