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

_______________

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

Advertisements

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 DragonflyWoman.wordpress.com