Stylurus spiniceps exuvia

I’ve never seen an adult Arrow Clubtail dragonfly (Stylurus spiniceps). That’s not surprising, since many experienced odonate hunters classify them as uncommon to rare.

But I know a place along the Potomac River in Fairfax County, Virginia USA where I am certain Arrow Clubtail dragonflies live. How do I know? Because I collected a Stylurus spiniceps exuvia from that location on 04 August 2016. In a nutshell, this is one of many reasons I’m learning to identify odonate exuviae.

An Arrow Clubtail dragonfly (Stylurus spiniceps) exuvia collected at Riverbend Park, Fairfax County, Virginia USA.

No. 1 | Arrow Clubtail (Stylurus spiniceps) | exuvia (face-head)

A two-step process was used to identify the genus and species of the specimen.

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 I 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), as shown in photo No. 2 and 3.
  • Antennae are club-like (not thin and thread-like, as in Aeshnidae).
  • Eyes not exceptionally large compared to the size of the head (not large, as in Aeshnidae).
An Arrow Clubtail dragonfly (Stylurus spiniceps) exuvia collected at Riverbend Park, Fairfax County, Virginia USA.

No. 2 | Arrow Clubtail (Stylurus spiniceps) | exuvia (face-head)

(See a full-size version of the original photo, without annotation.)

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. As it turns out, Arrow Clubtail exuviae are easy to identify because their abdominal segment nine (S9) is unique among Gomphidae: S9 is more than twice as long as it is wide at its base, as shown in the following annotated image.

The length of the exuvia is approximately 4.2 cm (~1.7 in).

An Arrow Clubtail dragonfly (Stylurus spiniceps) exuvia collected at Riverbend Park, Fairfax County, Virginia USA.

No. 3 | Arrow Clubtail (Stylurus spiniceps) | exuvia (ventral)

(See a full-size version of the original photo, without annotation.)

More photos of the exuvia are shown below.

An Arrow Clubtail dragonfly (Stylurus spiniceps) exuvia collected at Riverbend Park, Fairfax County, Virginia USA.

No. 4 | Arrow Clubtail (Stylurus spiniceps) | exuvia (dorsal)

An Arrow Clubtail dragonfly (Stylurus spiniceps) exuvia collected at Riverbend Park, Fairfax County, Virginia USA.

No. 5 | Arrow Clubtail (Stylurus spiniceps) | exuvia (dorso-lateral)

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 (Master); Canon 580EX external flash, off-camera, in manual mode (Slave); and a coiled six-foot Vello Off-Camera TTL Flash Cord for Canon Cameras. A Raynox DCR-250 close-up filter was used for Photo No. 2.

Adobe Photoshop CC 2017 was used to annotate selected images.

Related Resources:

dichotomous key: a key for the identification of organisms based on a series of choices between alternative characters. Source Credit: Merriam-Webster Dictionary.

In long form, the decision tree is as follows (assuming the convention of labeling the two branches of each dichotomy as “a” and “b”, e.g. 1a, 1b, etc.):

p. 17, Key to the species of the genus Stylurus
1a – Abdominal segment 9 twice as long as wide at base. (spiniceps)

Editor’s Note: This is the 1,000th post on my photoblog. That’s a major milestone, and quite candidly, one I never expected to reach. Who knew I have so much to show-and-tell?

Copyright © 2017 Walter Sanford. All rights reserved.

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2 Responses to “Stylurus spiniceps exuvia”

  1. Mike Powell Says:

    Fascinating post, Walter. Congratulations on your newest milestone–1,000 posts is a big number.

    • waltersanford Says:

      Thanks, Mike. I think it’s cool to know with certainty a location where Arrow Clubtail dragonflies live despite the fact that I never saw a single adult. Bear in mind, I didn’t recognize the species of clubtail when I collected the exuvia — I hypothesized it might belong to one of the adult Black-shouldered Spinyleg dragonflies that I saw the same day. Turns out the exuvia verifies the presence of one of the hard to find species of clubtails. I assure you I will return to the same location next year ISO the elusive Arrow Clubtail!

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