Monday, June 28, 2010

Trying to make sense of Teneral Spreadwings

These immature spreadwings, photographed at the marsh bordering the Eastern Ontario Trail about one kilometer west of town, lack the colors and patterns of adults. Virtually tenerals, they are about the length of an Emerald Spreadwing, about 40 mm, but much less robust. Distinguishing the Northern (or Common) Spreadwing (Lestes disjunctus) and the Sweetflag Spreadwing (Lestes forcipatus) can confound even experts and lacking experience I'm finding it difficult to determine the species. To add to the chaos, although they can usually be differentiated in the field there are teneral and immature Emerald and Slender Spreadwings competing in this same habitat, all within the same few square yards. Pairs of adults will be studied in future to determine if both species are extant in this area.

Although the dorsal colors of the thorax and shoulder stripes don't agree with the illustrations for an adult Northern Spreadwing (Lestes disjunctus), the sides of this immature male are pale yellow. The teeth on the cerci, shape of the paraprocts and narrow profile of the apical hood also look correct for this species.

Lateral view of another male; note the pale yellow sides of the thorax and claspers similar to those of the other male above.

Dorsal view of a third immature male's claspers.

Lateral and ventral views of a fourth male's claspers.

Oblique and lateral view of an immature female, possibly Lestes disjunctus. The sides of this individual are pale yellow. The backs of the heads of both males and females depicted thus far are dark.

The ovipositor looks correct for Lestes disjunctus.

Now things get confusing. Unfortunately I was unable to acquire of photo of the entire damselfly as a view of the postocular area would be helpful. The claspers are different than the three featured above. While the paraprocts appear to be the same the profile of the apical hood looks wider and the teeth on the cerci are subtly different. Is this a normal variation of Lestes disjunctus? An effect of the camera angle and lighting? Or are we looking at the terminal abdominal segments of an immature male Sweetflag Spreadwing (Lestes forcipatus)?

Views of an immature female; the postocular area appears pale. The pattern of the darker areas on the dorsal surface of the abdomen agree with those for Lestes forcipatus (but is this significant in the case of a teneral?).

Detail of the terminal abdominal segments. In the images above it's obvious that the large ovipositor extends well past the end of the abdomen. Although the Emerald Spreadwing has a long ovipositor, it is nowhere near this size in any individuals that I have seen, and the Emerald Spreadwing is much chunkier in appearance in any event. A darker lower rim would be desirable to help nail the identity down, however, the other criteria strongly suggest this is an immature female Sweetflag Spreadwing (Lestes forcipatus).

A teneral male Slender Spreadwing (Lestes rectangularis). In addition to the length of the abdomen this spreadwing can be identified in the field by the pale veins on the outer margins of the wings.