10s, maybe 100s of adult Cobra Clubtail dragonflies (Gomphurus vastus) were spotted on 16 May 2016 during a photowalk along the Potomac River at Riverbend Park, Fairfax County, Virginia USA. Cobra Clubtails were the only species of odonate observed in a period of several hours.
A single exuvia (shown below) from an unknown species of dragonfly was collected with permission from park staff. If adult Cobra Clubtail dragonflies were common, then the specimen is probably a Cobra Clubtail exuvia, right? Let’s test our hypothesis.
A two-step process was used to identify the genus and species of the exuvia.
- Determine the family.
- Determine the genus and species.
Step 1. Family
First, determine the family of the specimen. For reference, watch the excellent Vimeo video, Identifying dragonfly larva to family (8:06). Here’s the decision tree used to identify the exuvia as a member of the Family Gomphidae (Clubtails).
- The specimen has a flat labium that doesn’t cover the face (not mask-like). [See Photo No. 3.]
- Antennae are club-like (not thin and thread-like, as in Aeshnidae). [See Photo No. 3.]
- Eyes not exceptionally large compared to the size of the head (not large, as in Aeshnidae). [See Photo No. 3.]
(See a full-size version of the original photo, without annotation.)
Step 2. Genus and species
Gomphidae is the second largest family of dragonflies, so it can be challenging to identify some specimens to the genus and species level.
Characters from two dichotomous keys were used to identify the exuvia, in part, due to confusion caused by the fact that the name for the genus to which Cobra Clubtail belongs was changed recently from Gomphus to Gomphurus. As a result, the workflow for identifying this specimen was a little “jumpier” than usual.
dichotomous key: a key for the identification of organisms based on a series of choices between alternative characters. Source Credit: Merriam-Webster Dictionary.
Dichotomous Key 1
Identification Keys to Northeastern Anisoptera Larvae, compiled by Ken Soltesz. Cobra Clubtail (Gomphus vastus), p. 15, Key to the species of the genus Gomphurus.
- 1a – Strongly hooked palpal lobes with few teeth (3-5). Gomphurus group I (2) [See Photo No. 8.]
- 2a – Length 27-30 mm. (vastus) [See Photo No. 4, below.]
(See a full-size version of the original photo, without annotation.)
Dichotomous Key 2
“Gomphus complex” (= Gomphini) – Clubtails, Odonata Nymphs of Michigan.
- 1b. Middorsal length of Ab9 [S9] less than half its basal width (fig); length of Ab10 [S10] < 0.50x its width (fig) – 6 [See Photo No. 6.]
- 6b. Lateral spines of Ab9 close to Ab10, markedly longer than those on Ab8 (fig) [S8]; abdomen appears dorsoventrally flattened (fig) – Gomphurus, 8 [See Photo No. 6.]
- 8b. Apical margin of median lobe of prementum straight or slightly convex (fig) – 9 [See Photo No. 8.]
- 9a.(8b). End hook of lateral lobe of labium strongly incurved, extending far past apex of the truncate, 3 to 4 lateral teeth next to it (fig) – Gomphurus vastus
The preceding photograph shows a wider view of the posterior end of the abdomen. A closer view of the anal pyramid helps to illustrate several of the morphological characters described in Dichotomous Key 2.
(See a full-size version of the original photo, without annotation.)
The basal width of abdominal segment nine (S9) was measured in a straight line from edge-to-edge across the abdomen. The same distance would be longer if it were measured along the joint between abdominal segments eight and nine (S8 and S9). Either way the basal width is measured, the middorsal length is less than half of the basal width.
Since the middorsal length of abdominal segment 10 (S10) is clearly less than its basal width, the character wasn’t illustrated in the preceding annotated image.
Looking carefully at the anal pyramid, notice the cerci (sing. cercus) are slightly shorter than the epiproct, and the epiproct is almost as long as the paraprocts.
Photo No. 7 shows a wider view of the ventral side of the specimen. Zooming in on the prementum helps to illustrate some of the morphological characters described in Dichotomous Key 2.
(See a full-size version of the original photo, without annotation.)
Tech Tips:
The following equipment was used to shoot all of the preceding photographs: Canon EOS 5D Mark II digital camera, in manual mode; Canon EF100mm f/2.8L Macro lens (set for manual focus) plus a Kenko 20mm macro automatic extension tube; Canon 580EX II external flash, off-camera, in manual mode; Canon 580EX external flash, off-camera, in manual mode; and a Yongnuo YN-622C-TX E-TTL II Wireless Flash Controller for Canon plus a two-pack of Yongnuo YN-622C II E-TTL Wireless Flash Transceivers for Canon.
A Raynox DCR-250 close-up filter was used for Photo No. 3, 6, and 8. A Canon Extender EF 1.4x II was used for more magnification in Photo No. 8. Adding the tele-extender results in a 1 f/stop loss of light; additional backlight was added to the scene using a Nissin i40 external flash unit (off-camera, in SF mode).
Adobe Photoshop CC 2017 was used to annotate selected images.
The following photograph of another dragonfly exuvia was taken in-situ along the shoreline of the Potomac River using a Panasonic DMC-FZ150 superzoom bridge camera and Canon 580EX Speedlite, my go-to kit for photowalking. There were many exuviae clinging to the concrete retaining wall shown in the photo. Photo No. 1 was taken using the same camera-flash combo.
Related Resources:
Editor’s Note: Sincere thanks to Sue Gregoire, Kestrel Haven Migration Observatory, for her kind mentorship. Sue patiently provides guidance regarding the scientific jargon that can make it either challenging (at best) or impossible to understand many dichotomous keys for the identification of odonate larvae/exuviae. Like every good teacher, Sue doesn’t “give me a fish” — she teaches me how to fish. Thanks again, Sue!
Copyright © 2017 Walter Sanford. All rights reserved.
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