|Click image for a larger version.|
It looks pretty bulky, the “face” looks relatively flat rather than “snouted”, and the head does not seem very elongated from side-to-side (hammer-headed or dumbbell-shaped with the eyes capping each end). These are all indicators of dragonflies, but another more concrete character is the width of the gap between the eyes at the top of the head. On damselflies that gap is greater than the diameter of either eye; on dragonflies, if a gap is present, it is less than the diameter of either eye. Even at this angle we can see that the gap is smaller. Definitely a dragonfly. You can also just make out that the margin of each eye is angular at the narrow point of the gap—something else you won’t see on a damselfly. Here’s a post I did a while back on differentiating dragonflies and damselflies which illustrates the head/eye shape.
Okay, so it’s a dragonfly. But which family? That obvious gap between the eyes that I’ve been talking about narrows it down to two families: the petaltails (Petaluridae) and the clubtails (Gomphidae). Suffice it to say that you won’t see any petaltail anywhere with a mostly yellow-green thorax and face, so we can eliminate them on that basis. Can we determine the genus that this clubtail belongs to? Because of the limited view, we can’t see any of the little technical, egg-head characters like wing venation, hamules, and abdominal appendages, or even the color pattern of the abdomen that could be used to to figure that out. But look at that yellow-green color on the thorax; that screams Ophiogomphus—the snaketails. The females of some snaketails have little spikes or hooks on the head between the eyes, so that’s a good fit too.
There are only four or five species of Ophiogomphus recorded in the Pacific Northwest (depending on your definition of the region), and that pair of close-set, black-tipped spiky horns between the eyes point to just one of them: Ophiogomphus bison, the Bison Snaketail. The “bison” of both the English and scientific names is in reference to those horns, although you won’t find them on males of the species. I don’t know—when I look at those horns, native bovines of temperate North America and Europe don’t exactly leap to mind, but I guess it worked for Selys who named the species in 1854. The rather wide, almost solidly dark humeral stripe (aka shoulder stripe, or T1–2 stripes of Dennis Paulson [2009, 2011]) on the thorax, as well as the unmarked yellow face (no dark stripe across the middle of it) are also characteristic of the species.
On this individual, the dark thoracic humeral stripe envelopes a very thin pale line. That isn’t so typical of the Bison Snaketail and I think it threw some respondents off (in the post comments and elsewhere); normally it’s solidly dark with no internal pale markings. We can chalk this up to variation (remember, everything varies to some degree), or maybe it has to do with this individual being teneral (it had just emerged; note the very gray eyes lacking any hint of blue). That thin pale stripe is reminiscent of another snaketail in the region: O. occidentis, the Sinuous Snaketail, but there is a clear difference in the females’ occipital horns. They have two sets of horns instead of one, and their placement and orientation are different. Below is a comparison.
|Comparison of occipital structures: female Bison Snaketail (Ophiogomphus bison) on left, and female Sinuous Snaketail (O. occidentis). Note the single pair of spiky horns near the middle of the occiput on the female bison (normally they are not bent as they are on this specimen); the female occidentis has two pairs of horns—the curved pair on the posterior surface and the pair on the front which angle forward (the two white arrows point to them; the black spots are their tips). Images courtesy of Oregon State Arthropod Collection, Oregon State University, Corvallis, Oregon, USA; taken by Steve Valley.|
|Comparison of thorax patterns: female Bison Snaketail (Ophiogomphus bison) on left, and female Sinuous Snaketail (O. occidentis).|
Well, I think that covers everything. Our challenge subject is, in fact, a female Bison Snaketail (Ophiogomphus bison) which had just emerged on the Illinois River in southwestern Oregon, on 30 June 2012.
Paulson, D. 2009. Dragonflies and Damselflies of the West. Princeton, New Jersey: Princeton University Press.
Paulson, D. 2011. Dragonflies and Damselflies of the East. Princeton, New Jersey: Princeton University Press.