Posts Tagged ‘exuvia’

Collecting odonate exuviae

January 7, 2022

Odonates (dragonflies and damselflies) are aquatic insects that spend most of their life as larvae (nymphs) that live in water; this stage of their life cycle can last from a few months to a few years. Finally, they emerge from the water and metamorphose into adults in order to reproduce; their offspring return to the water and the cycle begins again.

I think it’s safe to say less is known about odonates during the aquatic phase of their lives than during the terrestrial phase. In my opinion, there is a real opportunity to make a significant contribution to the body of scientific knowledge about odonates by collecting and identifying exuviae.

What can be learned from collecting odonate exuviae?

Here are two examples that illustrate why I think it’s important to collect and identify odonate exuviae.

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.

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

04 AUG 2016 | Fairfax County, VA | Stylurus spiniceps exuvia (ventral)

More recently, my good friend and odonate hunting buddy Mike Boatwright discovered a small breeding population of Zebra Clubtail dragonflies (Stylurus scudderi) at an undisclosed location in Amherst County, Virginia USA. For several years, Mike found exuviae but no adults. On 22 July 2021, years of searching the site finally came to fruition when Mike discovered a teneral female Zebra Clubtail.

13 JUL 2018 | Amherst County, VA | Stylurus scudderi exuvia (dorsal)

Do you need a permit in order to collect odonate exuviae?

A strict interpretation of the Code of Virginia might lead one to think a permit is required.

It is unlawful to collect animal parts, such as feathers, claws, and bones without a permit (4 VAC 15-30-10 and §§ 29.1-521 and 29.1-553). Source Credit: Overview: Collecting, Exhibiting, and Releasing Wildlife, Virginia Department of Wildlife Resources.

That being said, it appears there is an exception for Phylum Arthropoda.

At this time no DWR permit is required for the following: Phylum Arthropoda EXCEPT for the Superfamilies Astacoidea & Parastacoidea (Crayfish) – Arthropoda includes: Insects, arachnids, millipedes, centipedes and other crustaceans (EXCEPT Crayfish) such as: isopods, amphipods etc. … Source Credit: Overview: Collecting, Exhibiting, and Releasing Wildlife, Virginia Department of Wildlife Resources.

I interpret the Code of Virginia to mean a permit IS NOT required to collect odonate exuviae.

How to collect odonate exuviae

Before you collect an exuvia, please photograph the specimen in situ. Record the date, location, and species (if known). A photograph is especially valuable when both the adult and exuvia are shown in the same photograph.

17 JUN 2021 | PNC. Wm. County | Anax junius exuvia (lateral)

Here are my tips for collecting exuviae. Bear in mind, I’m a perfectionist. It’s a fault, but hey, I am what I am and that’s all that I am.

I carry several items in my camera bag: a small plastic spoon; narrow strips of heavy card stock (maybe 3/8″ wide and two inches long); a small pair of scissors (one of three tools in a simple Swiss Army knife); and small plastic containers.

“Go kit” for collecting odonate exuviae.

I’m guessing most people just grab an exuvia with their fingers, but whenever I do that I hear a crunching sound that makes me cringe and parts tend to break off, e.g., legs.

If the exuvia is on a sandy stream bank, then I scoop the specimen with a spoon. If the specimen is clinging to something like a rock or wooden dock, then I slide the handle of the spoon under the body and gently pry it off the surface. Those little grabbers on the end of their legs are very “grippy,” so “gently” is the operative word. If the handle of the spoon won’t fit under the body, that’s when I use the card stock.

If the exuvia is clinging to vegetation, e.g., a cattail, then make the “peace sign” with one hand and insert the stem up to the notch between your pointer- and middle fingers and then close those fingers. Put your hand BELOW the exuvia, palm up, like a cup (in case the specimen falls off the stem). Use scissors to cut the stem below your fingers/hand and a little above the exuvia. Put the specimen in a collecting container, including the stem.

I use large plastic pill bottles. (I take eye vitamins that come in a wide-mouth bottle, perfect for big specimens with long legs such as cruisers and Dragonhunter.) I also use the smallish plastic containers for Philadelphia cream cheese — they can be “nested,” allowing you to carry several containers without taking up much space.

There, now you know more about how to collect odonate exuviae than you ever wanted to know!

What are the take-aways?

Hey, I get it — building a collection of odonate exuviae and learning to identify them might not interest you. But I can assure you there are many people like me who are interested in odonate exuviae who would love to have specimens that you find and collect.

I’m not necessarily saying you should go out hunting specifically for exuviae, but I am saying when you go hunting adult dragonflies and damselflies please be on the lookout for exuviae and collect them when you find them (and I predict you will).

In this way, there is a multiplier effect that will result in the collection of more specimens than a single individual is likely to find. In turn, this should help to advance our understanding of the odonates of Virginia.

Copyright © 2022 Walter Sanford. All rights reserved.

Determining final instar the Cham way

December 14, 2021

Did you notice I added a new text label to the annotated image of an exuvia from a Comet Darner dragonfly (Anax longipes) featured in my last two blog posts? I added the label in order to make the connection between this image and related ideas discussed in two other relatively recent blog posts (See Related Resources, below).

Remember that all dragonflies and damselflies have a 10-segmented abdomen, numbered from front to back. “S4” stands for abdominal segment four.

Counting odonate abdominal segments can be challenging sometimes. A good strategy to avoid mis-numbering is to begin counting abdominal segments from S10 (located toward the posterior end of larvae (nymphs)/exuviae) and work toward the thorax.

Final instar, the Cham way

There is a simpler way to estimate final instar than calculating instar equivalent.

Larvae in the final stage can be recognized by the length of the wing buds which cover the fourth abdominal segment. Source Credit: Field Guide to the larvae and exuviae of British Dragonflies, by Steve Cham, p. 30.

Look at the preceding annotated image. Notice the tips of the wing pads reach the fourth abdominal segment (S4), indicating the dragonfly larva that emerged from this exuvia had reached final instar. And that leads to the other idea I mentioned at the outset of this blog post.

Every odonate exuvia is a cast skin of the larva at F-0, the final instar, before it emerges to become an adult.

Turns out that’s another nugget of gold paraphrased from Steve Cham’s beautiful little book.

Post Update

The beauty of the Cham way of determining final instar is it’s simplicity. That’s the upside. The downside is there’s no way to determine the actual instar when it isn’t F-0.

For example, the following composite image shows dorsal views of a Common Green Darner (Anax junius) nymph (larva) and exuvia. As expected, the exuvia is F-0 because its wing pads cover S4. On the other hand, the nymph is F-? because its wing pads only reach S2.

Image used with written permission from Freda van den Broek.

Photo Credit: Both specimens (shown above) were collected by Freda van den Broek. The nymph was collected on 06 April 2020 from the Milwaukee River in Ozaukee County, Wisconsin USA, photographed, and released unharmed. The exuvia was collected from Ozaukee County too.

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

Proof of concept, revisited

December 10, 2021

In this blog post I’m going to show you how I used Photopea to annotate one of my photos. The workflow using Photopea is virtually identical to the way I annotate photos using Adobe Photoshop.

The original photo was edited using Apple Aperture (a discontinued photo editor) and Photoshop (for spot removal and sharpening). I could have attempted to edit the photo using Photopea but that wasn’t the point of my “Proof of concept” blog post.

Finished project

The first screen capture shows the finished project — I used it to reverse-engineer the steps that I took to create the annotated image and the settings I made along the way.

The “Layers” panel (located in the right sidebar) shows the finished project is compromised of seven layers, listed from bottom to top in the order they were created.

The “Background” layer is the photo itself. Go to File / Open… and navigate your computer to find a photo you would like to annotate.

View / Rulers

Before proceeding, it’s helpful to turn on “Rulers.” More about that later. In the Photopea menu bar, go to View and check Rulers.

Tech Tips: command-R (macOS) is a keyboard shortcut that can be used to toggle Rulers on/off.

The “View” menu also includes “Zoom In” and “Zoom Out” plus the associated keyboard shortcuts.

Press and hold the spacebar. Notice the arrow-shaped cursor turns into a hand that can be used to drag the canvas around the work space.

Post Update: View / Screen Mode > Fullscreen will scale up the Photopea window to fit the size of your computer screen. Fullscreen mode makes it much easier to work on fine details, especially when used in combination with the “Zoom Tool” and “Hand Tool”; the icon for both tools appears in the lower part of the left sidebar. Try it — you’ll like it! Uncheck “Fullscreen” in order to return to the desktop view. By the way, did you notice I added another guide line and Text layer for a follow-up blog post?

Text Tool settings: Title

The title is the first layer I added using the following selections: IBM Plex Sans (font type); SemiBold (font style); Size = 150 px; Color = Black; and “Aa” = Sharp.

Tech Tips: Text size is measured in pixels, not points. I’m not sure Photopea is calibrated correctly to convert points to pixels. Test it yourself using one of many online points-to-pixels calculators such as PT to PX Converter.

No matter, click the down arrow beside text “Size” and simply use the slider to select a text size that you like. You can also highlight the pixel size and type the exact number you prefer.

To begin adding text to your project, select the “Text Tool” (located in the left sidebar), click an insertion point on the photo and start typing. The default name for the new layer is “Text layer 1, 2, 3, etc.”

When you are finished, either delete the layer by clicking the “Cancel” button (X icon located near the right end of the Photopea menu bar) or save changes made to the layer by clicking the “Confirm” button (checkmark icon located to the right of the X) and the layer name changes so it is the same as the text you typed. In this case, I shortened the name of the layer to “Comet Darner dragonfly.”

With the layer selected, click on the “Move Tool” (left sidebar, at the top) and click-and-drag the text to reposition it exactly where you like.

Text Tool settings: Labels

Next I added a layer for “eye (1 of 2),” one of several labels for parts of the anatomy, using the following selections: IBM Plex Sans (font type); Regular (font style); Size = 100 px; Color = Black; and “Aa” = Sharp.

Tech Tips: The “Sharp” setting is used to avoid text with “jaggies,” that is text with saw tooth edges.

As an aid for aligning text, I created a blue guide line by clicking on the ruler along the top of the screen and dragging down to position the guide line on the photo. The guide line can be repositioned using the “Move Tool.”

Line Tool settings

The next layer I added is an arrow that points from the label to the anatomical part, in this case an eye. The settings I used for “Arrow 1” are shown in the following screen capture.

Tech Tips: Add a new vertical guide line for Arrow 1 by clicking on the ruler along the left side of the screen and dragging to the right to position the guide line on the photo. Right-click on the “Rectangle Tool” and select the “Line Tool.” I used shift-click-and-drag to draw a straight arrow that is aligned with the new blue guide line.

Add more layers

I followed the same steps described above to add a Text layer for “wing pads,” a new guide line for Arrow 2 and an arrow that points from the label to the anatomical part, and another Text layer for “prementum.”

Saving / Exporting

I saved the final image as a Photoshop document by selecting File / Save as PSD. The resulting PSD file can be reopened in Photopea in order to continue working on the project. Post Update: .psd files created by Photopea can be opened in Adobe Photoshop, although it is likely the font(s) you chose to use in Photopea are not available in Photoshop. Photoshop will prompt the user to resolve the problem.

I also saved the file as a PNG by selecting File / Export as.

What are the take-aways?

Open Photopea in a Web browser: www.photopea.com (For what it’s worth, I prefer “Google Chrome.”) Since Photopea is Web-based, it runs on desktop computers, laptop computers, tablets, and smart phones. Having said that, I think it would be challenging to annotate a photo with a tablet or phone unless those devices are used in combination with an external keyboard and mouse. Not impossible, but definitely challenging.

My dear friend Phil Wherry passed away too long ago. Phil was my tech guru for many years. I’m the type of person who suffers from “approach avoidance” — I like to know what will happen BEFORE I do something. One of the best bits of tech advice Phil shared with me is “Don’t be afraid to experiment and see what happens.” That’s exactly what I did when I used Photopea to annotate one of my photos and I encourage you to do likewise. Good luck!

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

Proof of concept

December 7, 2021

Can Photopea be used to annotate photographs? Yes!

Do the results look as good as photos of the same subject that were annotated using Adobe Photoshop? You be the judge.

Comet Darner (Anax longipes) | exuvia (lateral)

In my opinion the results are of comparable quality and that’s saying a lot considering this is my first attempt at using Photopea to annotate a photo.

As expected, the workflow flowed slowly since this was the first time I used many of the tools featured in Photopea.

I saved the final image as a Photoshop document by selecting File / Save as PSD. The resulting PSD file can be reopened in Photopea in order to continue working on the project. I also saved the file as a PNG by selecting File / Export as.

The Backstory

The preceding photo shows an exuvia from a Comet Darner dragonfly (Anax longipes) that was collected by Stanley Caveney on 19 July 2021 from a pond at MeadowWoods in West Elgin, Ontario, Canada. Sincere thanks to Stan for kindly sharing this beautiful specimen!

Related Resource: Learn Photopea

Copyright © 2021 Walter Sanford. All rights reserved.

Dark and moody

November 19, 2021

I spotted an emergent Uhler’s Sundragon (Helocordulia uhleri) during a photowalk along a mid-size stream at an undisclosed location in Prince William County, Virginia USA. The following photograph shows the exuvia from which the teneral adult emerged.

13 APR 2021 | PNC. Wm. County, VA | Uhler’s Sundragon | exuvia (ventral)

In the opinion of the author, larvae (nymphs)/exuviae from Family Corduliidae (Emeralds) and Family Libellulidae (Skimmers) can be challenging to differentiate and identify to the family level.

One way to differentiate Emerald from Skimmer larvae/exuvia is to look for a “ventromedial groove” in the prementum: it’s probably Corduliidae (Emeralds) if there is a ventromedial groove; it’s probably Libellulidae if there isn’t.

Look closely at a version of the preceding photo that was reformatted, rotated, and cropped to show an enlarged view of the prementum. You should notice a ventromedial groove on the basal half of the prementum, indicating this specimen is a member of Family Corduliidae (Emeralds).

13 APR 2021 | PNC. Wm. County, VA | Uhler’s Sundragon | exuvia (ventral)

Three raised structures on the underside of the prementum remind me of the hood ornament on a 1949 Lincoln automobile. (No, I wasn’t alive in 1949!)

Related Resources

Tech Tips

One reason I underexposed the photo is to add definition to the ventromedial groove and avoid overexposing the black background.

I prefer a white background for photographing odonate exuviae. Using a black background proved to be more challenging than I expected. More later in a follow-up blog post.

Copyright © 2021 Walter Sanford. All rights reserved.

Mouth parts – Tiger Spiketail exuvia

November 12, 2021

I revisited a photograph featured in a blog post published several years ago. I wanted to annotate the image to include information that I learned recently.

The following annotated image shows the face and mouth of an exuvia from a Tiger Spiketail dragonfly (Cordulegaster erronea) after its scoop-shaped face mask was pulled away from the head in order to count setae on the inner side of the prementum.

Cordulegaster erronea | exuvia (face and mouth)

There are two mandibles, one located on each side of the labrum. And there are two maxilla, one located below each mandible. Coarse setae make it challenging to see all of the parts clearly.

Sincere thanks to Marla Garrison for verifying my tentative identification of these mouth parts. And of course, thanks to Mike Boatwright for collecting and sharing the specimen with me.

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

Post update: What is it — emerald or skimmer?

November 5, 2021

An exuvia from a Stygian Shadowdragon dragonfly (Neurocordulia yamaskanensis) was collected by Freda van den Broek on 10 June 2019 along the St. Croix River in Interstate Park, Polk County, Wisconsin USA.

The presence of a ventromedial groove in the prementum suggests this specimen is a member of Family Corduliidae (Emeralds) and in fact it is.

Congratulations to Douglas Mills, who correctly identified the family of this specimen.

Going with corduliidae for the groove. It’s got impressive crenulations — I had to double check they weren’t jagged and this was a trick question 🙂 Source Credit: Douglas Mills.

Douglas successfully avoided the trap that was set when I chose to use a specimen that features deeply-scalloped crenulations along the margins of the palpal lobes. According to Kevin Hemeon, member of the “Odonate Larvae and Exuviae” Facebook group, crenulations like these are a characteristic field mark for Genus Neurocordulia (Shadowdragons) in the Family Corduliidae (Emeralds).

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

What is it — emerald or skimmer?

November 2, 2021

Ladies and gentlemen, children of all ages. It’s time for another exciting episode of “What is it?

I wrote about the “ventromedial groove” in a recent blog post. Based upon what you learned, is the following odonate exuvia a member of Family Corduliidae (Emeralds) or Family Libellulidae (Skimmers)?

Odonata (Suborder Anisoptera) | exuvia (face-head)

If you think you know the family, then please leave a comment. The answer will be revealed in a post update.

Copyright © 2021 Walter Sanford. All rights reserved.

How to estimate instar

October 29, 2021

Most odonate larvae (nymphs) go through 10-13 stages of development known as “instars.” F-0 is the final instar, F-1 the preceding instar, and so forth.

Sidebar

The “F” in the name for every instar stands for Final. “F-0” is the final instar. “F-1” means final instar minus one, that is, the stage that precedes the final instar. “F-2” means two stages before the final instar.

Ken Tennessen, author of Dragonfly Nymphs of North America: An Identification Guide, devised a method for determining instar by examining hind wing length and head width.

Calculating the ratio between hind wing length and head width results in a number that is approximately equivalent to instar, that is, assuming you know how to interpret the result¹.

The equation for instar equivalent is as follows.

Instar equivalent = HwL / HW

Where HwL is Hind wing Length and HW is Head Width.

Math Tips

Fractions are read from top-to-bottom, or when written as shown above, left-to-right. The equation literally says “Instar equivalent equals Hind wing Length divided by Head Width.”

The equation is units independent, meaning any units of measurement can be used as long as the same units are used above and below the dividing line. Instar equivalent is a dimensionless number because the units cancel during division. (Remember “dimensional analysis” from chemistry and physics?)

For most of the life of an odonate larva (nymph) its head is wider than the length of its wing pads. Therefore instar equivalent is calculated by dividing a smaller number by a larger number, resulting in a decimal fraction. As the wing pads grow, the instar equivalent increases until the ratio is approximately 1:1 (or slightly larger) at F-0, the final instar.

¹According to empirical data collected by Tennessen, average instar equivalents are as follows: ≥1.00 for F-0; 0.66 for F-1; 0.50 for F-2; 0.33 for F-3; and 0.25 for F-4. Remember, these numbers are averages — your mileage might vary.

Theory into practice

Cordulegaster sp. larva (female) | dorsal view

I used the Adobe Photoshop “Ruler Tool” to measure the number of pixels along the two double-tipped white arrows shown in the preceding annotated image of a preserved specimen.

Tech Tips

60s ‘shop: Using the ruler tool to measure distances in Photoshop CC, by Photoshop for the Scientist (1:00) provides a clear and concise explanation of how it’s done.

HwL is ~920.81 pixels. HW is ~911.15 pixels.

Instar equivalent = 920.81 pixels / 911.15 pixels

The units cancel, so the answer is ~1.01 — close enough to the average value for F-0 (final instar). Easy, huh?

What are the take-aways?

  1. An instar of F-0 indicates the spiketail larva featured in this blog post was nearer the end of the larval phase of its life than the beginning. Time is dilated for larvae in the Family Cordulegastridae (Spiketails), so it’s difficult to say how much longer it would have been until the larva metamorphosed into an adult.
  2. Every odonate exuvia is a cast skin of the larva at F-0, the final instar, before it emerges to become an adult. Therefore the instar equivalent for all exuviae should be ≥1.00. Try it and see!

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

Anal pyramid

October 26, 2021

The “anal pyramid” is a more-or-less triangularly-shaped group of five appendages on the posterior end of dragonfly larvae (nymphs) and exuviae, including one epiproct, two cerci (sing. cercus), and two paraprocts.

The author suggests either “posterior pyramid” or “posterior triangle” as a less offensive sounding collective name for these anatomical parts. The author is just saying.

Although the anal pyramid isn’t always shaped like a perfect equilateral triangle, as shown above (in white), it always features the same five component body parts. These parts are often critical for identifying odonate larvae (nymphs) and exuviae.

Glossary

 


The Backstory

The Eastern Ringtail dragonfly (Erpetogomphus designatus) exuvia shown above was collected along the Potomac River at Riverbend Park, Fairfax County, Virginia USA. This species is a member of Family Gomphidae (Clubtails). I created a photo-illustrated identification guide for E. designatus using the same specimen.

Copyright © 2021 Walter Sanford. All rights reserved.


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