[Odonata-l] Tail-dipping Female Octogomphus (Grappletail)

[Jim Johnson]

Greetings,

I've been corresponding with a few individuals about this subject, but I
thought the wider community would find it interesting. Sorry for
cross-posting...

A little background: 

First, the Gold Lake outlet/Salt Creek head in the Oregon Cascades near
Willamette Pass harbors a large population of Octogomphus specularis
(Grappletail). At their peak, the density of individuals at the outlet and
immediate area is staggering for a dragonfly, matched in my experience only
in certain circumstances such as heavy migrational concentrations and when
large numbers seek shade in extremely hot, sunny conditions. It really has
to be witnessed to be appreciated. The high density of individuals at this
location in and of itself is very interesting and I'd like to know if there
are other locations with similar densities of Octogomphus.

Second, during the Dragonfly Society of the Americas annual meeting last
year in Oregon, Nick Donnelly visited Gold Lake and witnessed previously
unreported behavior: a female Octogomphus perching on the side of a rock
with only the tip of her abdomen in the water. She appeared to be
ovipositing in this way although the normal mode for this species (and most
gomphids and many other dragonflies) is to drop eggs on the surface of water
while in flight.

Last Friday I visited Gold Lake to photograph some odonates and spent time
at the outlet to see the Octogomphus. I think there were more there than
during any other visit that I have made. I'd say there were easily four
dozen right at the outlet flying around and sitting on rocks and logs, plus
countless more in the nearby parking lot, campground, on the bridge, on my
vehicle, on my hat, etc. Seemed like one or two copulating pairs flushed
from every tree that I walked past. I photographed boulders in the outlet
with multiple resting Octogomphus--the highest number in one shot was 13.

It didn't take long to start noticing females perching on the sides of rocks
and logs with just the tip of the abdomen in the water, just like Nick
Donnelly reported last year. I saw around 15 instances and photographed ten
of them. I have created a page on my web site to display some of these:
http://odonata.bogfoot.net/photo-pages/tail-dipping-Octogomphus.htm . 

I *may* have seen eggs drifting away from the abdomen in one case, but
because of the suspended material in the water flowing out of the lake and
because of the viewing conditions, I couldn't be sure. I don't see any
readily apparent eggs in any of my photos. I observed one female make a
short typical ovipositing flight, tapping her abdomen on the water surface
right after "dipping" from the side of a log. Several times I was able to
predict this behavior when I saw females making low, short, back-and-forth
flights along the side of a log or boulder before she perched on the side.

There are a couple of obvious questions for which there are no apparent
answers at this time:

1. Are these females ovipositing, and if not, what are they doing?

2. Is this behavior a product of living in such high density (e.g. covert
oviposition in order to avoid harassment by the many males nearby) or is it
normal behavior which was only noticed at Gold Lake simply because there are
so many readily observable females in one location?

Dennis Paulson mentioned that many gomphids eggs have threads which adhere
them to the substrate as they drift downstream, and perhaps this behavior is
related to that (loosening up the threads). I was so focused on
photographing these females that I didn't net any of them to see if they
were extruding eggs or not. In hindsight, that would have been a good idea.
There is still plenty of time this season to go back and do that.

Something for those who live near Octogomphus to watch for.

Cheers,

Jim Johnson
Vancouver, Washington
jt_johnson at comcast.net
http://odonata.bogfoot.net/