Tuesday, June 9, 2015

The Rewards of Looking for Exuviae

Recently I was looking for odonates at a place called Grand Island on the Willamette River in Oregon. This is in Yamhill County which is one of the more under-surveyed counties in the state (only 24 species recorded at the time). I found a couple of new species for the county with little effort—Tule Bluet (Enallagma carunculatum) and Western Pondhawk (Erythemis collocata), both very common and widespread species in the region.

Male Tule Bluet (Enallagma carunculatum) on left, and immature male Western Pondhawk (Erythemis collocata) at Grand Island, Oregon.

I also saw a couple of snaketails (Ophiogomphus) flying around, but I could never get close enough for a look, a photo, or a capture. Then I decided to check the river bank for exuviae—and exuviae I found. They were clearly gomphids with their flat labium, relatively prominent antennae, and short legs. Based on their overall shape I knew they were Ophiogomphus (also partly because I know there are few gomphids to expect in this area). Several of these exuviae were found on a short section of bank about a foot above the current water level, and I collected a few that were intact and in pretty good shape.

There are two species of Ophiogomphus expected in this area: Pale Snaketail (O. severus) and Sinuous Snaketail (O. occidentis), and I couldn’t remember off the top of my head how the nymphs/exuviae were differentiated. So, I took my exuviae home and looked up the Ophiogomphus nymph key in the current edition of Dragonflies of North America (2014, Needham, Westfall, and May). The limited number of candidate species in this region made the task pretty simple, and it boils down to whether or not there is a lateral spine on abdominal segment 6.

One of the Ophiogomphus exuviae I collected. The arrow points to the lateral spine on S6 which identifies it as Sinuous Snaketail (O. severus).

The exuviae were certainly Sinuous Snaketail (O. occidentis), and this was another new species for the county—Pale Snaketail having already been recorded. The habitat made sense too: Pale Snaketail tends to be on smaller streams than Sinuous Snaketail, and the Willamette isn’t what I would call a small stream. Of course, this does not mean that the adults I saw that day were the same species as the exuviae, and that’s okay—records don’t depend on identified adults. I know what to call the exuviae which is certainly adequate to add the species to the county list.


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