Wednesday, September 19, 2012

River Jewelwings

A few weeks ago Betsy and I drove to a location just south of Olympia to look for a striking damselfly known as the River Jewelwing.  We've only seen this odonate twice before, both times a single male at Stossel Creek near Duvall, WA.  According to reports, the River Jewelwing can be seen in numbers where State Route 12 crosses the Black River, so we headed south to check it out.  We had stopped at the same location on our way home from Arizona in May, but that was too early in the season and none were seen.  This trip was during the peak of the season, and several – both males and females – were seen flitting along the shore of the river.

The Black River is beautiful at the location populated by the jewelwings.  Here are a couple shots that show the setting.

Betsy on the shore of the Black River.

The Black River.  Notice the submergent vegetation just below the surface of the river.

Just visiting this location is pleasant enough, but if you look closely in the vegetation along the shore you'll see some flashes of iridescent blue and green.  You'll also see wings flapping with prominent black patches.  These are the River Jewelwings going about their daily activity.

Here are a couple shots of the male River Jewelwing.

A male River Jewelwing with its wings slightly parted.

Side view of a male River Jewelwing on the shore of the Black River.

The beautiful colors of this damselfly are due to iridescence, which is a very different mechanism from colors produced by pigments.  Here's a comparison between the two mechanisms:

Pigment Colors  Pigments absorb certain colors and reflect others, resulting in a specific color when viewed in white light.  The observed color is the same regardless of the viewing angle, since it depends only on which colors are reflected.

Iridescent Colors  Iridescent materials produce colors by interference.  What this means is that all colors of light are reflected, but some colors cancel themselves out due to phase shifts caused by the structure of the material.

One of the key characteristics that distinguish pigment colors from iridescent colors is that iridescent colors change with the viewing angle, because changing the viewing angle changes the phase shift.  This effect can be seen in cases like the head and neck of a male Mallard, which can appear either blue or green.  The same effect is seen in the River Jewelwing, as we can see below:

A male River Jewelwing viewed from one side.  The body is dark blue.

The same individual as above, this time viewed from the other side.  Its body is now brilliant green.

Notice that even the leading edge of the wings (the costa) changes color from blue to green along with the thorax and abdomen.

The males were constantly patrolling along the shore, defending their territories.  Here are a couple males flashing their wings as one tries to displace the other.

Male River Jewelwings competing for space along the shore.

Female River Jewelwings were present as well – our first opportunity to view them.  They share the iridescent body colors of the male, and also have white spots (the stigmas) at the tip of their wings.

A female River Jewelwing.  Notice the iridescent body and the white spots on the wings.

Many female River Jewelwings were seen in the male's territories, but no mating activity was observed.

All in all, it was a beautiful day at a wonderful location full of special damselflies.

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