Monday, January 24, 2011

Returning to the Depths: Submerged Oviposition

I suspect a lot of people don’t realize that some odonates deposit their eggs while submerged. I don’t mean that they simply stick the end of their abdomen below the water surface to lay their eggs (which is also done), but sometimes they completely submerge themselves.

An ovipositing River Jewelwing (Calopteryx
aequabilis) on the South Fork of the John Day
River, Oregon. Only the distal two-thirds of the
wings are above the water surface. 

The behavior is almost completely restricted to damselflies (suborder Zygoptera) which all oviposit endophytically—inserting their eggs into plant material instead of just dropping them from the end of the abdomen. It is rare among the dragonflies (suborder Anisoptera), probably because it’s difficult for them to break the surface tension with their broad wings sticking out to the sides. Damselflies must be better able to pierce the water’s surface with their wings folded together over the abdomen.

Friday, January 14, 2011

Insect Alchemy: The Maturing Female Western Forktail (Ischnura perparva)

Female damselflies of the genus Ischnura, the Forktails, often go through a pretty dramatic color change during their time as an adult. In many species they start out wearing some pattern of black-and-orange, and as they mature the orange areas dull to some less interesting hue like muddy yellow, and they may simply darken altogether. During this transition in color, females of some species start developing a coat of pale gray pruinescence—a fine, waxy, powdery film, which eventually covers much of the exoskeleton.

The Western Forktail (Ischnura perparva), abundant and widespread in the Pacific Northwest and much of western North America, is one of those species that performs this insect alchemy, changing from gold to silver. The orange stage is typically referred to as “immature”, but orange and pruinescent females are both seen copulating with males so they aren’t necessarily reproductively immature. If you didn’t know any better you might think each was a different species. Here’s a series illustrating the transition...

A classic immature female with orange areas on the head, thorax, legs, and abdomen.

This is intermediate with the orange areas on the thorax turned to dull grayish-yellow, the black shoulder
stripes broader, and the abdomen almost all black now. A very thin coating of pruinescence is starting to
show. Note the change in eye color.

Further along with lots of pruinescence—especially on the abdomen, but the thoracic stripes
are still clearly visible.

Fully mature with the thoracic stripes almost completely obscured.

Note how the eye color changes to vivid green with a sharply defined black “cap” and that the lower sides of the thorax turn pale green. The transition to the fully mature pruinescent stage seems to occur relatively rapidly since intermediates like the second individual are not often seen.

So here’s an exception (as I delved into the world of odonates I learned pretty quickly that there are exceptions to almost everything, and even some exceptions have exceptions—it get’s complicated): a very small number of female Western Forktails are androchromatic (or male-colored) and the pale areas on the upper part of the thorax are green instead of orange and there are blue rings near the tip of the abdomen. To continue the alchemy analogy, I guess this would be turning copper and cobalt into silver. I've only seen one example of an androchromatic Western Forktail which is pictured below.

An androchromatic immature female with a light coating of pruinescence. The dark red spots behind
the head are water mites—I’ll talk about those in a future post.

This variant is pretty rare, although it may not be quite as scarce as we think since they end up covered with pruinescence just like the typical gynochromatic (or female-colored) females, and by then they all look alike. It’s a matter of timing. Keep your eyes open for them.

Friday, January 7, 2011

Black Petaltail (Tanypteryx hageni), the Dragonfly of Soggy, Seepy, Slopes

One of my favorite odonates is the Black Petaltail, Tanypteryx hageni, and I presume it makes the top five on most Northwest odonatists’ lists for a number of reasons. It’s always a joy when I can see and handle these creatures, but especially so when I stumble across them unexpectedly during an outing.

This species’ family, Petaluridae, is a small one with only eleven species, nearly all of which are distributed around the Pacific Rim. Seven of these are found in Australia and New Zealand which seems unfair, and some of those are among the largest odonates in the world. The Petaluridae are known as petaltails because the males of most species have relatively broad, flat cerci (upper abdominal appendages) which are reminiscent of flower petals.

Male Black Petaltail (Tanypteryx hageni)
at Eight Dollar Mountain, Oregon.
The Black Petaltail is the sole western North American representative of the family, distributed from northern California (and barely into Nevada) to coastal mainland British Columbia (but strangely not on Vancouver Island) as this map at illustrates. The only other member of the genus is Tanypteryx pryeri of Japan. The more distant cousin Gray Petaltail, Tachopteryx thoreyi, occupies woodlands in a large swath of the eastern US from east Texas to New Hampshire.

Adult Black Petaltails are largely black, as the name suggests, with a complex pattern of yellow spots on the thorax and abdomen, a mostly yellow face, and dark chocolate brown eyes—and if you get a close look note that there is a gap between the eyes on top of the head (only the clubtails, Gomphidae, share this characteristic among the dragonflies). It’s a relatively large species among the Pacific Northwest odonates, although it is near the small end of the spectrum among the Petaluridae and it doesn’t quite reach the size of most of our darners (Aeshnidae). They are fairly approachable as dragonflies go and it’s not unusual for them to perch on people—especially those wearing very light-colored clothing, and this imparts upon them a warm and fuzzy “personality”.

Look for Black Petaltails in forested areas with open or partially open seepy slopes with a soggy substrate of mud, moss, and herbaceous plants, fed by springs or small streams. In Oregon and Washington these sites are mostly in the Cascade and Olympic Mountains above about 2000 feet elevation, but they also use Darlingtonia (a pitcher plant) fens in southwest Oregon, some of which are as low as 1000 feet elevation. There was once a population on Mary’s Peak—the highest point in the Oregon Coast Range, but they have not been seen there in decades and I’m not sure anyone alive knows where the site was anymore. In 2009 Mike Patterson found a population on Onion Peak, Clatsop County, which is currently the only known Oregon Coast Range population. There must be others.

A Black Petaltail (Tanypteryx hageni) nymph at its
burrow entrance after dark. Photo by Cary Kerst.
Being aquatic insects, the nymphs of the Black Petaltail live in the saturated substrate on those soggy slopes. And they don’t just crawl around in the muck—they actually construct burrows into the mud and organic matter and this is what makes the species really unique among the North American odonate fauna (and this is true of most members of the family). The nymphs are partially terrestrial after dark, sitting at their burrow entrance where the ground is wet, but above standing water (see Cary Kerst’s photo at right). Here they wait for prey to amble by, but they may also wander a bit to hunt.

The burrow entrance of a full grown nymph is about a half inch or a little more in diameter and there is often a small pile of excavated mud just off the threshold if it’s in use. They can be really difficult to spot, but it gets easier after you've seen a few. During the day you can sometimes see the occupant hanging back in the shadows of their burrow, and at times you can even fish them out with a blade of grass. If you do fish one out, be sure to return it to its home if you don’t intend to collect it. The nymphs take five years or so to get to full size—maybe a bit more at the highest sites, maybe a bit less at the lowest Darlingtonia sites. Of course once they emerge and become adults, they don’t live beyond the season.

Adult females lay eggs by inserting the end of their abdomen into the muck and mosses while grasping vegetation or clinging to the side of a log. They are difficult to see while they’re ovipositing, especially when surrounded by taller plants, but you often hear their wings vibrate or rattle against vegetation periodically and that can help you zero in on their location. Adults are often found soaking up the rays on logs, boulders, and tree trunks, and they don’t appear to wander very far from nymph habitat—at least not as much as other dragonflies. If you happen to come across one in the hills, there is probably a soggy slope in the area.
Female Black Petaltail (Tanypteryx hageni) at Todd Lake, Oregon.

A female Black Petaltail (Tanypteryx hageni) ovipositing in a Darlingtonia fen at Eight Dollar Mountain, Oregon.

So keep an eye out for the Black Petaltail when you’re in the hills on warm sunny summer days. It’s a special dragonfly in my opinion. And if you find some soggy slopes, look for the little burrows of the nymph and go fishing!