Friday, September 27, 2013

Darner Hovering: Bobbing With Bursts

Hovering is an energy-intensive activity.  It's not surprising, then, that dragonflies might employ strategies to reduce the effort – or at least rest the muscles a bit.  Here's an example.

I was observing a Happy-face Darner (Paddle-tailed Darner) hovering at Beaver Pond in Winthrop, WA.  As it hovered, I noticed it was bobbing up-and-down in a regular manner.  This might not seem surprising, considering that its wingbeats don't provide constant lift.  The frequency of the "bobbing" motion was much smaller than the frequency of the wingbeats, however.  The wings beat at about 35-40 beats per second, whereas the bobbing motion was clearly only a fraction of that frequency.  I decided to record a video to study the hovering in more detail.

Here's one of the videos I recorded.  It's recorded in "real time"; that is, normal speed.

video

A better view can be had at the following link to the Dragonfly Whisperer Channel on YouTube:

http://www.youtube.com/watch?v=r8oLKmfMLBc

As you can see, the wings are beating so fast they're just a blur.  In contrast, the body bobs up-and-down at a much slower rate.

I made frame-by-frame measurements of the flight level of the darner from the video and have plotted the results below.

Data from a video showing the up-and-down bobbing motion of a hovering darner.

The data show a clear periodicity to the flight level.  The darner is indeed bobbing up-and-down with quite a well-defined frequency.

The bobbing motion has a period of about 0.2 s, which corresponds to 5 bobs per second – or roughly one bobbing cycle for every 8 wingbeats.  The angular frequency of this motion, w, is about 31 radians per second, and the amplitude, A, is about 0.4 cm.  It follows that the maximum upward and downward acceleration is roughly Aw^2 = 3.8 m/s^2, which is just under 40% of the acceleration due to gravity.  Thus, the darner is never in free fall; it's always getting lift from the wings, but the lift varies with a period of 0.2 s.

It seems, then, that the darner alternates weaker and stronger wingbeats as it hovers, a little like the flight of finches and woodpeckers.  This is indicated in the plot below.  With this strategy, the darner can give a burst of 4 strong wingbeats, rest for 4 wingbeats, give another burst of 4 strong wingbeats, rest for 4 wingbeats, and so on.  Perhaps the intermittent rest helps it hover for long periods of time without getting fatigued.

Same plot as above, but with times of weak and strong wingbeats indicated.

Monday, September 23, 2013

Darner Mating Behavior: Attaching In Tandem

All dragonflies and damselflies mate in the so-called "wheel" or "heart-shaped" position.  In this position, the tip of the male's abdomen grabs the female behind the head in dragonflies, or on the front of her thorax in damselflies, and the tip of the female's abdomen attaches to the base of segment 2 of the male's abdomen.  A good example in the Common Green Darner is shown below:

Common Green Darners in the wheel position during mating.

The first step in attaining this position is for the male to grab the female and attach his abdomen to her, forming a tandem pair.  The following is a slow-motion video (1/4 speed) showing this attachment process in the Happy-face Darner (Paddle-tailed Darner):

video

A better quality video can be seen on the Dragonfly Whisperer YouTube Channel at the following link:

http://www.youtube.com/watch?v=p_ap7Aex9Y4

What's particularly interesting about this process is the way the male grips the female tightly as he makes the attachment.  Notice how the rear legs of the male slip down behind the female's forewings and pull them forward, folding them tightly against her thorax.  The male holds her like this for a moment as he shudders rapidly – apparently acquiring the desired connection – and then releases her wings and tries to take off.  In this case the female doesn't cooperate, and the male eventually gives up and departs alone.

I hadn't known before that the male folds the wings forward like that when it attaches in tandem.  It seems like quite an extreme maneuver.  You often see females with large tears in the forewings, and I wonder how often the tears are the result of this behavior.  This particular female had a large tear on the left forewing, as you can see in the photos below:


Notice the large tear just behind the nodus (bend) in the left forewing.

When the male pulls the forewings forward you can see that his leg fits nicely into that tear.  Before this video I might have considered the tear to be the result of a bird attack, but I think another possibility must be considered.

The other interesting feature of this video is the male's rapid oscillation of his abdomen (about 60 oscillations per second) just before he tries to take off.  Is he making sure he's got a good attachment?  I found one reference to something like this in Corbet, Dragonflies: Behavior and Ecology of Odonata.  On page 276 he describes a male Common Green Darner that is in tandem when it's attacked by another male.  According to Corbet, "the tandem male shakes its abdomen in a convulsive movement detectable to the human observer only when portrayed in slow motion."  I wonder if this is the same basic behavior.

Wednesday, September 18, 2013

Autumn Has Arrived!

Well, not according to the calendar – for a few days yet – but as far as the dragonflies are concerned it has.   Recall the following haiku:

Red dragonfly on my shoulder,
calls me his friend.
Autumn has arrived.

The Autumn Meadowhawks are out in numbers at Cranberry Lake, and they are landing on your shoulders, arms, legs, shoes, etc.  In fact, you have to be careful where you step to make sure you don't step on one or more of them.  Here are a few pictures from Cranberry Lake last year to give you an idea of what it's like there now:

A pair of Autumn Meadowhawks in the wheel position on my finger.

The Autumn Meadowhawks are fairly numerous, and they seek out perching sites with good exposure to the sun.

Dragonfly on the shoulder – and the hat, too!

The Autumn Meadowhawk has been named the world's friendliest dragonfly by DASA, the Dragonfly Appreciation Society of America (membership consists so far of Jim and Betsy Walker).

The darners are also present in numbers right now.  The sunlit bushes at the lake were hosts to 6 to 8 perched darners at any given time, mostly Paddle-tailed Darners, but also a couple Shadow Darners and a Variable Darner.

Update:

The next day we made our first observation of a Dark-eyed Junco in our backyard.  This is another species whose occurrence is linked with the seasons – we see them on or close to the first day of Fall, they stay with us all winter, then disappear again around the first day of Spring.  They are extremely accurate, usually to within a day or two, as they were again this year.

Here's a little haiku to commemorate their appearance:

Dark-eyed Junco in my yard,
hopping to and fro.
Autumn has arrived.

Friday, September 6, 2013

Band-winged Meadowhawk

On a recent field trip to Burke Lake near Quincy, Washington, we saw a number of Band-winged Meadowhawks.  Here's a young female that posed for its picture, happy to be a part of the Dragonfly Whisperer blog:

Band-winged Meadowhawk (female).

This meadowhawk is named for the band of amber color near the base of the wings.  When I first started dragonflying, it was called the Western Meadowhawk, and there was a similar species in the eastern U. S. called the Band-winged Meadowhawk.  These species have been merged due to the fact that intermediates were found.  They all go by the name Band-winged Meadowhawk now.

Band-winged Meadowhawks seem very finicky about where to land.  They often hover above a perch, and just when it looks like they're going to land they lift off again, only to come back for another try.  This can happen numerous times before they finally "take the plunge" and settle down on the perch.  Here's another view of this species from Burke Lake, showing the bands in the wings a little more clearly:


This is dragonfly is widespread across the northern part of North America, as can be seen in the following dot map from Odonata Central:


Here's a closer look at the dot map for the Pacific Northwest area:

Wednesday, September 4, 2013

Cherry Springs Interpretive Sign

Not long ago, I was contacted by naturalist Sheri Covert at the Cherry Springs nature area near Pocatello, Idaho.  She was putting together an interpretive sign to inform visitors about some of the interesting insect life to be found there, and asked if she could use a couple of my dragonfly pictures.  I was happy to help with a project like that, and supplied her with pictures of the species she was looking for.  Here's a rough draft of the sign, which looks very nice.


Here's an enlargement of the dragonfly section:


I look forward to visiting Cherry Springs one of these days.  If any of my intrepid readers gets there first, please take a picture of the sign in place and we'll include it in the blog.

Monday, September 2, 2013

"Monster Soup" with a Dragonfly on the Side

Recently, Jean Enersen on King5 TV did a cooking segment where she shared the recipe for her "Monster Soup".  Here she is in the kitchen preparing the soup:



What caught my attention, however, was the dragonfly towel hook on the wall behind her.  It appears to be one that is something like this:


As usual, the dragonfly stole the show – at least for me.