Wednesday, May 25, 2011

A Fetch of Dragonflies

In case you've ever wondered what a group of dragonflies is called ...

About a kilometer east of Tweed "the Trail" crosses a sand bottomed creek. Pools of standing or slowly moving water in shady forested areas are combined with open sunlit patches created by the trail. This medley of habitats, very different than the river, lake or marshes, has its own collection of odonate species.

While it was on the wing I mistook this immature female Chalk-fronted Corporal for a species of Baskettail. Although this one is perching on a twig these small dragonflies (40 mm long) are more often seen sitting on the ground or on rocks.

Habits can be a good way to identify or at least narrow down dragonfly species. Springtime Darners (Basiaeschna janata) prefer to hang vertically when resting. At about 60 mm in length it's relatively small for a darner. The colors and markings of the males are the same as the female's.

The Stream Cruiser (Didymops transversa) is about the same length as the darner above but much more robust. This dragonfly's habit of perching on an angle, like the female in the next two photos, is typical of its species.

Both of our two local species of cruiser have a pale facial stripe and only one lateral thoracic stripe, making them easy to spot in the field ... other large dragonflies, such as darners, have two thoracic stripes (if any).

Male and female Stream Cruisers are similarly patterned. The male can be distinguished by its slightly flared abdomen and pale claspers.

A mating pair of Stream Cruisers

This is an immature female Mustached Clubtail (Gomphus adelphus). The markings and length (at 45 mm it's small for a clubtail) agree with the species account in my field guide. The colors threw me off at first, but eyes will turn green and the bright yellow on the body will change to grayish-green as the dragonfly ages.

The "7"-shaped pale marks on the upper thorax, the broken middle thoracic stripe, and particularly – the heavy black markings on the face – are the characteristic field marks of this species.

Many species of clubtails prefer to stay low when at rest, adopting a horizontal position on the ground, rocks or leaves like these two immature males. In the field males can be differentiated from females by their clubbed tails and claspers, visible even from a distance on the male to the right.

A closeup of an immature male's facial markings. Clubtail species can be hard to tell apart and it's often necessary to get a detail shot of the terminal appendages for a positive I.D. In this case the bold black cross-stripes on the face are sufficient.

An older male Mustached Clubtail ... this photo was taken about one month later than the others. Note the changes in color.

Obvious field marks aren't good enough to sort out three of our local Baskettail species, all are similarly patterned and about 45 mm long. The only noticeable difference between them is the amount of pigmentation at the bases of the hindwings (which can vary) and the presence or absence of a "T"-shaped spot on the top of the frons (no images of the latter, the dragonflies weren't co-operating). So let's have a closer look ...

Baskettail # 1
The hindwings have a generous amount of black at their bases.

Looking at the terminal appendages: we see that the cerci are constricted at their bases but there's no evidence of any teeth. Taken in conjunction with the relatively large amount of hindwing pigmentation we have to conclude that this dragonfly is a Common Baskettail (Epitheca cynosura).

Baskettail #2
Some black pigmentation is visible at the bases of the hindwings.

Views of the claspers from a couple of slightly different angles. A ventral tooth is barely visible in the first image, but this isn't necessary for an I.D. What is clear in both photos is that the cerci turn down at the ends, and – they definitely possess dorsal teeth. This is sufficient to identify this dragonfly as a Beaverpond Baskettail (Epitheca canis).

Baskettail #3
The hindwings have a small amount of pigmentation at their bases.

A dorsal view would be helpful but it's unneccessary because luckily I was able to obtain a good image of the claspers – the cerci definitely have a sharp ventral tooth. As always, the claspers are the final word as to species. This dragonfly is Spiny Baskettail (Epitheca spinigera).

A teneral male Brush-tipped Emerald (Somatochlora walshii), length about 45 mm. As a rule it's not a good idea to handle a teneral odonate, especially by its delicate wings. But this one is gravely injured (likely by a beaver, a dog or a hiker) and won't survive much longer. A pity this dragonfly in such rough shape, as it would have made an excellent subject with its amber-tinted wings and a thorax that looks like it's made out of colored metal foil.

The hairy claspers (which sustained injury as well) are visible in the field and leave no doubt as to the identity of this dragonfly.