Two similar odonate exuviae were collected by Michael Boatwright on 11- and 13 July 2018 along a medium-size stream in Amherst County, Virginia USA. Mike and I collaborated to identify the specimen collected on 13 July.
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 (prementum) that doesn’t cover the face (not mask-like), as shown in Image No. 1, 3-5.
- Antennae are club-like (not thin and thread-like, as in Aeshnidae larvae), as shown in Image No. 1.
- Eyes not exceptionally large compared to the size of the head (not large, as in Aeshnidae), as shown in Image No. 1.
It’s relatively simple and straightforward to recognize this specimen is a clubtail. Determination of the genus and species is more challenging.
Step 2. Genus and species
Several dichotomous keys were used to determine the genus and species of the exuvia. (See Related Resources.) By far the easiest keys to use are found in Identification Keys to Northeastern Anisoptera Larvae, compiled by Ken Soltesz. Markers that match this specimen are highlighted in boldface green text. Three boldface green asterisks (***) are used to highlight the thread for identification of this specimen.
p. 7 – Key to the Genera of the Family Gomphidae
p. 9
***8a. Tibial burrowing hooks vestigial or lacking; 1-4 palpal teeth [see Image No. 5]; abdomen slender, no wider than head [see Image No. 2]. [Stylurus]
p. 16
[diagrams (showing palpal lobe)] … traced from Walker (1928)
This specimen is a species from the genus Stylurus, confirmed by Kenneth J. Tennessen, Ph.D. in Entomology (personal communication).
p. 17 – Key to the Species of the Genus Stylurus
***1b. Abdominal segment 9 less than twice as long as wide at base [see Image No. 2]. [2]
2a. Vestigial dorsal hook, represented on segment 9 by small flattened triangular rearward projection on mid-dorsal line, not hooklike in form. [3]
***2b. Nothing representing a hook on 9. [4]
3a. … [notatus] ← Elusive Clubtail
3b. … [plagiatus] ← Russet-tipped Clubtail
4a. Total length less than 34 mm; median lobe of labium very convex. [amnicola] ← Riverine Clubtail
***4b. Total length more than 34 mm [see Image No. 2]; median lobe of labium only slightly convex [see Image No. 5]. [scudderi] ← Zebra Clubtail
Note: See Walker (1928) for a complete description of the nymphs of Stylurus.
[diagrams (showing posterior abdomen)] … traced from Walker (1928)
The exuvia is ~37 mm (~3.7 cm) long, measured from end-to-end, including the antennae that extend ~1-2 mm in front of the face. The overall shape of the posterior abdomen most closely matches S. scudderi, as shown by tracings in the Soltesz key. The length of abdominal segment nine (S9) is less than its basal width. S9 is only slightly longer than S8. S10 is wider than its length.
Notice the flat labium (prementum) doesn’t cover the face of the specimen, as shown in Image No. 1, 3-5. The basal width of the lateral spine on abdominal segment nine (S9) in lateral view is approximately equal to the basal width of the spine in dorsal view.
The lateral spine on abdominal segment nine (S9) is nearly as long as abdominal segment 10 (S10).
The overall shape of the palpal lobes most closely matches S. scudderi, as shown by tracings in the Soltesz key. Note the median lobe of the labium (prementum) is only slightly convex, as shown by both the tracings and Image No. 5.
What it is
According to the identification key for the genus Stylurus compiled by Ken Soltesz, this specimen is Zebra Clubtail (Stylurus scudderi).
Soltesz’s key is based, in part, on the excellent work of E. M. Walker in 1928. (See Related Resources.) Neither Walker’s paper nor Soltesz’s identification key includes Laura’s Clubtail (Stylurus laurae), described in 1932 (Benjamin Coulter, personal communication). There is considerable overlap in the characters used to identify S. laurae and S. scudderi that cannot be ignored.
We referred to two more identification keys, one of which is known to contain misinformation in the keys for S. laurae and S. scudderi (Kenneth J. Tennessen, personal communication) — if one is flawed, then so is the other since both keys refer to many of the same characters. That being said, the greatest number of matching characters indicates S. scudderi although S. laurae cannot be 100% ruled out.
Further evidence
One of the two virtually identical odonate exuviae that Mike Boatwright collected included a teneral adult that failed to emerge successfully.
After careful examination, Mike tentatively identified the adult as a male Zebra Clubtail (Stylurus scudderi) based on the following characteristics.
Face yellow with crossbars forming a dark “X” with a light center. Front half of frons yellowish; rear of frons and vertex dark. Top of thorax dark brown with two vertical yellowish white stripes, and a light colored mid-dorsal stripe narrowing to a point at the yellowish-white collar stripe. Humeral and anti-humeral stripes (T1 and T2) dark brown and almost completely fused. Area between T2 and T3 a fairly broad yellow stripe. T3 and T4 dark brown, mostly fused, and bordered by light colored broad oval area below.
Abdomen slender with dark blackish-brown segments. Two pale yellow spots on the sides of S2 with one completely covering the auricle. Anterior end of S3-6 and posterior end of S9 with thin pale yellow to whitish ring. Lateral edges of S7-9 widely expanding forming a club with broad irregular whitish-yellow markings on lateral surfaces. Source Credit: Michael Boatwright.
Significance
Either Zebra Clubtail (S. scudderi) or Laura’s Clubtail (S. laurae) are new species of odonates for Amherst County, Virginia USA, as shown by the Dragonfly Society of the Americas species checklist for Amherst County.
Zebra Clubtail is not within range of Amherst County; Mike’s discovery could indicate expansion of its range farther eastward into Virginia. Laura’s Clubtail is within range of Amherst County, although unknown to occur there.
According to records for the Commonwealth of Virginia maintained by Dr. Steve Roble, Staff Zoologist at the Virginia Department of Conservation and Recreation, Division of Natural Heritage, the adult flight period for Zebra Clubtail is 13 July to 02 October; Laura’s Clubtail is 20 June to 26 September.
Related Resources
- Identification Keys to Northeastern Anisoptera Larvae, compiled by Ken Soltesz.
- Walker, E. M., The Nymphs of the Stylurus Group of the Genus Gomphus with Notes on the Distribution of this Group in Canada, 1928.
- Needham, J.G., M.J. Westfall, and M.L. May. March 2014. Dragonflies of North America, 3rd Edition. Scientific Publishers, Gainesville, Florida, pp. 310-312.
- Stylurus: Needham, 1897 – Hanging Clubtails
Tech Tips
The following equipment was used to shoot Image No. 1-4: Canon EOS 5D Mark II digital camera, in manual mode; Kenko 20mm macro automatic extension tube; Canon EF100mm f/2.8L Macro lens (set for manual focus); Canon MT-26EX-RT Macro Twin Lite set for “Master” mode, and Canon 580 EX- and Canon 580EX II Speedlites set for “Slave” mode. Image No. 5: Canon MP-E 65mm Macro lens (manual focus only, set for ~2x magnification) plus the multiple-flash setup.
Image No. 1-5 are focus-stacked composite images created and annotated using Adobe Photoshop CC 2017: Image No. 1 (13 photos); Image No. 2 (14 photos); Image No. 3 (11 photos); Image No. 4 (18 photos); Image No. 5 (six photos).
Mike Boatwright’s photos, taken soon after the specimens were collected, were shot using a Samsung Model SM-G950U cell phone.
Copyright © 2018 Walter Sanford. All rights reserved.
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