Tuesday, November 28, 2017

Species Spotlight: California Spreadwing

The next species in the "spotlight" is the California Spreadwing, the largest damselfly in the Puget Sound area. It is so large, in fact, that on the wing it can look like a female meadowhawk.

This species was first seen in our area at Magnuson Park in Seattle, but since then it has been found at a number of other Puget Sound locations as well. It is more common in western Oregon and much of California. Here's the range map for this species:

Definitely a western species.

In addition to being conspicuously large, the California Spreadwing is easily identifiable by its overall brown color, with pale markings on the sides of the thorax. In particular, there is one prominent light-colored side stripe with contrasting dark borders that is an excellent field mark for this species. It also has pale rings along the length of the abdomen. You can see these features clearly below:

The female looks much the same as the male, with the same side stripes on the thorax. Her abdomen is rather stocky, however, and it has a prominent ovipositor at the tip. In the following photo a female is being held by a male, who grabs her by the front of the thorax. Male dragonflies hold the females by the back of the head, but the head is so small in damselflies that it is better to grab the female by her thorax.

This is an interesting damselfly, and one that is easy to spot due to its size.

Read more about this and other west coast odonates in my field guide:


Monday, November 20, 2017

Splash-Dunk Analysis, 2011-2017

As in past years, Betsy and I recorded all of our observations of dragonflies splash-dunking and spin-drying. This was a particularly slow year, as far as dragonfly activity goes, and for only the second time since we started in 2011 we had fewer than 100 Events—86 in fact.

Before continuing, let's make it clear what is meant by an "Event." For our purposes, an Event begins when a dragonfly hits the water in a splash-dunk. If the dragonfly rises after the splash-dunk and performs a spin-dry, then that was a one-splash-dunk Event. If the dragonfly does two splash-dunks, then rises for a spin-dry, it's a two-splash-dunk Event, and so on. Each time we see an Event beginning, we keep count of the number of splash-dunks before the spin-dry.

Secondly, let's give a reminder why the dragonflies are splash-dunking in the first place: They are bathing. When dragonflies perch they hold their wings straight out, and they cannot clean them. The wings collect various types of debris, and the way to clean them is to plunge into the water one or more times.

Here are the results for splash-dunk events in 2017:

Splash-Dunk Events for 2017. Total number of Events is 86; average number of splash-dunks per Event is 2.41.

This is a typical distribution, with the number of splash-dunks per event ranging from 1 to 7. Compare this with the cumulative results for 2011-2017:

Splash-Dunk Events for 2011-2017. Total number of Events is 688; average number of splash-dunks per Event is 2.32.

These are the results from observations of 688 splash-dunk Events. The average number of splash-dunks per Event is 2.32. Notice that more Events have just a single splash-dunk than any other number. In addition, the record number of splash-dunks in an Event is still 8—the number of splash-dunks in the famous event associated with the constipated darner. You can read details about the constipated darner at the following link: http://thedragonflywhisperer.blogspot.com/search/label/constipated.

Each year we see that there seems to be a bit of an excess in the number of Events around 3 and 4 splash-dunks. Let's look at this a bit more carefully. We begin with an exponential fit to the data:

The red dots are the data points, and the blue line is an exponential fit of the form a Exp[–bx], with the parameters a and b taking on the values a = 425 Events and b = 0.488 Events/splash-dunk. This shows clearly that the data is generally exponential in its fall-off, but with an excess of Events at 3 and 4. This has been a significant feature of the data each year.

The exponential fall off implies that each splash-dunk is an independent occurrence; that is, after each splash-dunk there is a certain probability that the dragonfly will do another splash-dunk independent of how many splash-dunks it has already performed. For the most part, this seems to be a valid description of the splash-dunk behavior. For some reason, however, there is a greater probability that a dragonfly will perform 3 or 4 splash-dunks. Perhaps fewer than 3 splash-dunks is usually not enough to clean the dragonfly, whereas more than 4 splash-dunks starts to get tiring, making the 3 to 4 range sort of a "sweet spot" for the dragonflies.

Tuesday, November 14, 2017

Swarming Meadowhawks

The Autumn Meadowhawks are the last dragonflies flying in our area. I thought I would re-post a description of a swarm of Autumn Meadowhawks that I encountered at Cranberry Lake 6 years ago. Here it is:

Recent reports of mass gatherings of dragonflies make it seem there is “something in the air” when it comes to events like these.  Large numbers of baskettails have been observed in Canada, and a large gathering of Striped Meadowhawks was recently seen in Oregon.

In my case, the dragonflies involved were Autumn Meadowhawks.  These friendly dragonflies, which like to land on people, are a common sight at Cranberry Lake in Anacortes, Washington late in the season.

Betsy experiences a red dragonfly on the shoulder, and a second one on her hat.  Both dragonflies are male Autumn Meadowhawks, the friendliest dragonfly we know.

Autumn Meadowhawks are well described by a famous haiku:

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

I’ve often had them “on my shoulder,” but last autumn I had them covering my entire body – literally from head to toe.  Here’s what happened.

I went to Cranberry Lake on November 9, 2011 to observe the dragonfly activity.  The weather was sunny and calm, with an air temperature of 57 ˚F.  On other similar days I would observe about a dozen Autumn Meadowhawks and half a dozen Shadow Darners.  On this day, however, I immediately realized something was different – there were so many Autumn Meadowhawks on the gravel walking path that I had to choose my steps carefully to keep from stepping on them.

I walked to some bushes near the shore to see if any darners were perched there, but as soon as I stood still for a moment the meadowhawks began to gather on me.  It felt like a scene from Hitchcock’s movie, The Birds.  They were landing all over me in a frenzy.  I took some pictures showing the ones perched from my waist down, but as I took those pictures I could feel them perched on my arms, my upper body, my head, even on my face.  The pictures show over 30 on my lower body, and I would estimate there were 50 or more on my body as a whole.  I’ve had several Autumn Meadowhawks land on me before, but never anything like this.  

A gathering of Autumn Meadowhawks at Cranberry Lake in Anacortes, Washington on November 9, 2011.

The ones pictured on my lower body are only half the story – they covered me from head to toe.  It felt like a scene out of The Birds.

After taking a few pictures I looked up and saw that the air was just “full” of meadowhawks flying in all directions, hooking up in tandem or attempting to hook up.  It was similar to a mass flight of winged ants or termites.  A few darners were flying too, picking off individual meadowhawks, and also pairs in tandem, and heading for the bushes or trees to enjoy their catch.  It was quite a scene.  It’s hard to estimate the number of meadowhawks, but it must have been in the several hundreds.

I decided to go home and bring Betsy to see this phenomenon.  As I walked back to the car the meadowhawks went along for the ride on my body.  The car was a considerable distance away, and in the shade, but there were still a dozen or more dragonflies on me when I got there.  I had to “shoo” them away to keep them from getting in the car with me – though one managed to do so anyway.

When Betsy and I returned a few minutes later, the activity level was a bit lower, though still intense.  We marveled at the meadowhawks that seemed to be everywhere we looked, including all over us.  Along the shore we observed an egg-laying frenzy, with intense competition for prime sites.  As a result of the competition, many meadowhawks were being knocked into the water where they became stuck.  We ended up rescuing a dozen or more.

As we watched the egg-laying activity, the shadows of the afternoon (it was about 2:00 pm at this point) began to lengthen.  We expected to see the meadowhawks moving along the shore to stay in the sunlit areas, but at one point – quite suddenly – we noticed that the egg laying had ceased, and the air was now clear of meadowhawks.  It was almost as if someone had flipped a switch.  We’re not sure what the signal for stopping was – it wasn’t evident to us – but the meadowhawks seemed to respond en masse.

We returned the next several days, but each time the activity was completely normal again, with just a dozen or so meadowhawks along the shore.  The mass behavior seen on November 9 was a short-lived phenomenon, but one we’re happy to have experienced.

Saturday, November 11, 2017

Late Season Observations

Yesterday, Betsy and I went to Cranberry Lake. It was a beautiful, sunny, calm day, with temperatures in the low 50s. It's kind of late to be looking for dragonflies, but we're nothing if not optimistic.

The most common dragonfly we saw was the Autumn Meadowhawk—as one might expect. There were several perched in the sun on the concrete dam. Here's an example:

Notice the hamules under segment 2 of the abdomen, showing that this is a male—which is also quite evident from its bright red color. Here's another view:

We saw one of these guys do a splash-dunk, as well. It will probably be the last splash-dunk of the season.

We also saw a few Shadow Darners, flying along the shore and perched in the bushes. Here's an example of this species:

Notice the broad blue stripe on the top of segment 2 of the abdomen, plus the broad front stripes on the thorax and the lack of blue on segment 10.

You can find more information about both of these species in Common Dragonflies and Damselflies of the Pacific Coast.

Monday, November 6, 2017

Species Spotlight: California Darner

The next "C"species for our spotlight series is the California Darner.

This is the smallest darner in our range—barely bigger than a typical skimmer. It is also the earliest darner to appear each year. Here's the Flight Season for the California Darner:

Notice that they begin to appear in March, though only about 1% of observations occur in that month. They peak in June and July, with over 60% of all observations. Finally, the last few percent of appearances occur in September.

Their range is basically west of the Rockies, with a heavy density of observations in Northern California, Oregon, and Washington.

The California Darner is a lovely dragonfly, with lots of blue along the length of the abdomen, and striking blue eyes. It has a tendency to land on the ground, or very close to it, which is a bit unusual for darners.

The key field mark for this species is as follows: no front stripes on the thorax. It's our only darner that lacks the front stripes. In addition, notice that the tenth segment of the abdomen is cream colored, and the appendages are simple in shape.

You can read much more about this dragonfly, its behavior and field marks, in my new field guide Common Dragonflies and Damselflies of the Pacific Coast, available on Amazon at the following link:


Thursday, November 2, 2017

Species Spotlight: Calico Pennant

Well, we've reached the letter C in our cavalcade of dragonfly and damselfly species.

First up is the Calico Pennant. This is a small, delicate dragonfly with wings that have beautiful colors and patterns—as is common in many pennant species. Here's a look at this striking dragonfly:

Notice the heart-shaped red spots on the abdomen, and the lovely colored patches in the wings.

We saw this dragonfly in a small field with tall grass, near a business park in Virginia. It would fly from one patch of grass to another, always very light on the wing.

Here's another look at the Calico Pennant:

We plan to visit Virginia again this coming summer. We will surely be on the lookout for more Calico Pennants!