Wednesday, June 2, 2010

Which one is it? An Emerald Spreadwing (Lestes dryas), or an Elegant Spreadwing (Lestes inaequalis)?

My first sighting of an Emerald Spreadwing (Lestes dryas). I've likely stumbled across this damselfly many times in the past, but (like the Sedge Sprite, the Taiga Bluet and a few others) didn't know what it was until I acquired the two odonate guides referenced in earlier posts.

The metallic green color and small stocky build ... about 35mm to 40 mm long ... are correct for the species. This male is still relatively young as it isn't showing any pruinosity. The photos were taken on the Eastern Ontario Trail, near the marsh about one kilometer west of town.

Spreadwings can be challenging to identify, as colors often become obscured by pruinosity in older individuals. The claspers are blocked by vegetation in the lateral view above but I managed to get a satisfactory dorsal closeup, and they conform to the drawings in both of my field guides. Regardless of the damselfly's visible aspect, the claspers are the last word as to species; this is an Emerald Spreadwing (Lestes dryas).

Some views of a female Emerald Spreadwing. This damselfly is quite common along the trail, perching in the vegetation bordering the marsh. I've sighted several males and females, but no mating pairs as yet.

The ovipositor is visible in the lateral view but here is a bit more detail of the terminal abdominal segments.

As luck would have it I came across another metallic green spreadwing a couple of days later, perched on the emergent vegetation along the north shore of Stoco Lake near the boat ramp. At a cursory glance this damselfly looks the same as the Emerald Spreadwing in the photos above. It's a bit longer (50 to 55 mm), with a relatively slender build, and it's showing the pruinosity on the last abdominal segments and blue eyes typical of adult males. The luster of the abdomen is brassy rather than a true green. But how to be certain as to species?

I was unable to approach closely with the camera, and the following two overviews and the dorsal view of the claspers are scarcely adequate for the purpose of identification.

Fortunately I was able to capture the insect. (As a rule I dislike doing this for fear of mishandling or injuring it; even the oils from human skin can coat the wings and have a deleterious effect on the insect's ability to fly). Upon closer examination (and as the following ventral and lateral closeups show) it's clear that the paraprocts (lower claspers) are longer than the cerci (upper claspers). Again, what I'm seeing agrees with the diagrams in both field guides ... this is an Elegant Spreadwing (Lestes inaequalis), and my first encounter with this species of damselfly as well.

Whilst I had the insect in hand I took a couple more shots ...

This is a different male observed at the same location a few days later. Another good field mark for this species is the pale postocular area, visible in this picture. (I failed to take advantage of the opportunity to take a photo of this attribute while I had the other damselfly in hand.) It's obvious that the paraprocts are longer than the cerci despite the fact that the terminal segments of the abdomen are out of focus.

The Elegant Spreadwing is aptly named, as manifested by the brilliant green and lemon yellow of this mature female. At about 50 mm in length the Elegant Spreadwing is visibly larger than the Emerald Spreadwing. This female damselfly was photographed in the vegetation under the dam at the walking bridge ... interesting, since the fast waters and rock bottom of the river are quite a different habitat than the still waters and muddy bottom of the lake where the males were encountered.

The pale tibiae are more easily seen in the field than in most of the photos, but they are visible in the lateral view. Also note the pale postocular area in the closeup below.

The rim of the ovipositor is definitely dark, as illustrated in the field guides, but I cannot see the obtuse angle of the basal plate (compare this to the image of the teneral spreadwing's ovipositor below).

Now things get a little tougher. Female spreadwings can be hard to sort out and tenerals raise the level of difficulty. This teneral female, photographed at the marsh bordering the Eastern Ontario Trail, isn't showing much in the way of color. But the short body, about 40 mm long, robust build, ovipositor extending past the tip of the abdomen and habitat suggest an Emerald Spreadwing. Nevertheless, based on the foregoing information I cannot rule out a female Sweetflag Spreadwing (Lestes forcipatus).

The following photos of teneral female spreadwings were taken east of the boat ramp at Stoco Lake. Note how the damselfly often holds its wings closed in the manner of pond damselfly, and the luster of the wings. These damselflies were about 50 mm long.

Another teneral female, a bit older but her body still hasn't developed its adult colors. A female Elegant Spreadwing would show pale green at the back of the head and pale yellowish, not black, legs.

Furthermore, the ovipositor of an Elegant Spreadwing should have a dark rim. However, the basal plate is obtusely angled, which is what would be expected for this species. Due to a lack of experience I don't know what to make of this conflicting data, but pending further information I'm going to say this is a teneral female Swamp Spreadwing (Lestes vigilax), a damselfly that is common at this location.