Sunday, November 4, 2012

Olive Clubtail (Stylurus olivaceus): The Lower Columbia's Gomphid


 
Pacific Northwest members of the dragonfly family Gomphidae—the clubtails, are mostly species of the height of the summer season. Even though daylight hours are waning after the summer solstice, daytime temperatures continue to rise and sunny, dry days are increasingly expected. During this time most gomphids are out in force and doing what gomphids do. One species, however—the Olive Clubtail, Stylurus olivaceus—is just starting to emerge while other gomphids are at the zenith of their adult activity.

Let’s consider Oregon and Washington as a single region and the ten clubtails which are found there collectively (see the graph below). Half of the species have started emerging by the end of May; June through August are the peak months with all ten species recorded in July; things are winding down by the latter half of August with six species recorded in September—and most of those are nowhere to be found by the middle of the month. But look at October and November, each with one species recorded. That’s the tail end of the Olive Clubtail season in this area.
 
The number of clubtail species (Gomphidae) recorded as adults in Oregon and Washington per month. The lone species recorded in October and November is Olive Clubtail, Stylurus olivaceus. Sources: Johnson (2012) for Oregon; Paulson (2011) for Washington.
 
The Olive Clubtail occurs widely west of the Rocky Mountains, but its distribution is patchy and it does not seem to be especially numerous in many places where it is found. Here’s a map of known records at OdonataCentral.org. There are a few strongholds, however, and I happen to live in the middle of one of them: the lower Columbia River west of the Cascade Mountains; if you view the OdonataCentral map, the cluster of dots along the west end of the Oregon–Washington border is the area that I’m talking about. Zoom in and you can also see how close to the Pacific Ocean exuviae have been found—well into what would be considered brackish waters, I assume, although I haven’t yet seen any relevant data regarding that.

What’s interesting is that the species was not even known to be on the lower Columbia River, or anywhere in Oregon for that matter until 1997 (Johnson, 1998), but I’m sure that was just a matter of no one checking its shores during the right time of year. It is the only gomphid you can expect to see on this part of the Columbia River and the adjacent reaches of its bigger tributaries.
 
Male Olive Clubtail (Stylurus olivaceus) along the Columbia River in Vancouver, Washington.
 
What puts the “olive” in Olive Clubtail is the unimpressive background color of much of the thorax, although to my eyes it usually looks more gray than olive. In sharp contrast to this are the rich blue eyes which almost seem out of place. Like many gomphids, there’s an attractive pattern of yellow on the abdomen, particularly on the sides of the male’s club. If you look closely at the thorax, there are even little spots of yellow near the legs which stand out against the dull olive/gray.

Emergence in this area starts in July, and the exuviae are not hard to find along the high water line where fine debris is deposited. Adults are quite common and easy to find during August and September, at least, and usually into October as long as the weather stays nice. I have twice found them in Vancouver during the month of November—in 1999 and again in 2010. That requires unusually pleasant conditions at that time of year.
 
Male Olive Clubtail (Stylurus olivaceus). This shot shows off the male's club a bit better. This was taken in Vancouver, Washington, on 5 November 2010—the latest date for the species in the state thus far—during unseasonably summerish weather.
 
The male's club.
Any place along the lower Columbia where you can wander among young cottonwoods, willows, blackberry brambles, and shrubbery at the top of a beach, you are apt to find Olive Clubtails perching on the vegetation. You have to move slowly, however, and be very observant because they frequently flush before you notice them. Even when they are sitting right in front of you, they are often not easily spotted. I suppose the thoracic and abdominal pattering break up their outline and make them a bit cryptic. Even those beautiful blue eyes are not very obvious from a distance. If they don’t immediately fly away when spotted, they can be very approachable. Just don’t make any sudden moves!

Females are similar in appearance to males, although the pale coloration on the thorax is bit more yellowish, the abdomen has more extensive pale patterning, and their eyes are often not quite as vibrant. Her abdomen has just a tiny bit of expansion where the male’s club is located, and, of course, her abdominal appendages (cerci and epiproct) are far less substantial.
 
Female Olive Clubtail (Stylurus olivaceus), Vancouver, Washington.
 
Copulation for Olive Clubtails can be an extended affair, as it is with most gomphids. When a copulating pair comes buzzing in from the open waters of the river (I think that’s their version of the singles’ bar), they often fly up to the bigger cottonwoods—well out of camera range—to perch for . . .    . . . well, who knows how long. I’ve never taken the time to watch exactly how long they will copulate, but I understand that up to an hour or more is not uncommon for many gomphids. I was lucky to spot the pair below perching at eye level in small willows.

Pair of copulating Olive Clubtails (Stylurus olivaceus), Vancouver, Washington.
 
Olive Clubtails do occur in other areas, of course, but I’ve only seen the species twice away from the lower Columbia River. Once instance was on the Humboldt River outside of Winnemucca, Nevada. In arid areas like that with very hot summers, Olive Clubtails look different. The dark markings on the thorax and abdomen are reduced, and as a result they appear noticeably paler overall. The photo below isn’t stellar, but it illustrates what they look like in the deserts of the Great Basin. Note the series of pale rings on the abdomen and the nearly solid yellow area on the side of the club, almost completely lacking dark markings along the lower edge.

Male Olive Clubtail (Stylurus olivaceus), Humboldt River near Winnemucca, Nevada. Note the less extensive dark markings compared with other males shown here.
 
The Olive Clubtail is the Pacific Northwest’s only member of the genus Stylurus. These are commonly known as “hanging clubtails” because they typically perch with the abdomen angled down, sometimes to the vertical. I frequently see Olive Clubtails assuming this orientation on leaf surfaces within several feet of the ground; sometimes they use twigs or the sides of logs—still with the abdomen angling down, but they rarely perch on the ground. This contrasts with many other gomphids which often perch on the ground or on cobbles and twigs with the abdomen elevated above the horizontal. Don’t go around identifying gomphid genera based on this alone, since there is a lot of overlap in this behavior, and dragonflies don’t always do what is expected of them!

Here are a few final close-ups of the lower Columbia River gomphid . . .

Male Olive Clubtail (Stylurus olivaceus) feeding on a beetle, Vancouver, Washington.
 
Male Olive Clubtail (Stylurus olivaceus) doing some neck exercises, Vancouver, Washington. Note the gap between the eyes on top of the head— a characteristic of all gomphids.

 
References

Johnson, J. 1998. Stylurus olivaceus in Washington and Oregon [PDF]. Argia 10(3): 20–22.

Johnson, J. 2012. Oregon Odonata Early/Late Flight Dates [PDF]. Accessed 03 Nov 2012.

Paulson, D. 2011. Washington Odonata [WWW page]. Accessed 03 Nov 2012.
 
 

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