Posts Tagged ‘larvae’

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.

How to estimate instar using Photopea

December 3, 2021

In two recent blog posts (see Related Resources, below), I showed how Adobe Photoshop can be used to make measurements that enable you to estimate the instar of odonate larvae (nymphs).

The process works well, that is, as long as you have Photoshop. Does that mean you’re out of luck if you don’t? In a word, no.

Photopea” is a free alternative to Adobe Photoshop. Photopea is a Web-based clone of Photoshop — Photopea doesn’t do everything Photoshop does but it can be used to make measurements on photos using a workflow similar to the one I described in detail in a blog post entitled “How to estimate instar.”

Practical example using Photopea

Open Photopea in a Web browser: (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.

In order to make this tutorial as simple as possible I went to the Photopea menu bar, navigated to File / New… and created the default blank white canvas, shown below. Note: You should go to File / Open… and navigate to a photo of an odonate larva.

Use the Photopea “Ruler Tool” to measure the length (in pixels) of two line segments: HwL (Hind wing Length); and HW (Head Width). If you don’t know how to measure “HwL” and “HW” then please refer to “How to estimate instar” for detailed, step-by-step instructions.

Right-click on the Eyedropper Tool — located in the left sidebar of the main window, as shown below — and select the Ruler Tool.

Click and drag line segment HwL, such as the sample line shown below. Record the length of the line, in pixels. Click the “Clear” button (optional) and repeat the same process for line segment HW (not shown).

Do the math to calculate instar equivalent and voilà, the result is a number that can be used to estimate instar based upon Ken Tennessen’s average instar equivalents. Again, please refer to “How to estimate instar” for detailed, step-by-step instructions.

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

How to estimate instar, revisited

November 16, 2021

I’m the founder and co-administrator of the Odonate Larvae and Exuviae Facebook group. Friday, 12 November 2021 was the two-year anniversary of the group. As of this writing there are approximately 1,700 members in the group, including people from around the world.

For example, Abiodun Matthew Adedapo from Nigeria. Abiodun began posting to the group relatively recently, sharing information and photos related to his research. Sincere thanks to Abiodun for permission to repurpose two of his photos for another mini-lesson on how to estimate instar.

What is the instar? Not F-0.

The equation for instar equivalent is as follows.

Instar equivalent = HwL / HW

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


Photo used with written permission from Abiodun Matthew Adedapo.

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 collected and photographed by Abiodun.

HwL is ~132.6 pixels. HW is ~195.12 pixels.

Instar equivalent = 132.6 pixels / 195.12 pixels

The units cancel, so the answer is ~0.68 — close to Ken Tennessen’s  average value for F-1 (final instar minus one).

Abiodun reported the instar as F-2, based upon in situ observations of a cohort of larvae (nymphs) from Family Gomphidae (Clubtails).

For my purpose, it doesn’t matter whether the actual instar is either F-1 or F-2 — the important take-away is we know the instar is not F-0, the final instar. This provides an opportunity to mention a simpler way to estimate final instar.

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 closely at the first annotated image. Notice the length of the wing buds/pads doesn’t reach the fourth abdominal segment (S4) of the specimen therefore this larva is not in its final instar.

Thanks to Freda van den Broek for sharing this method with me!

F-0 (final instar)

The last annotated image shows part of a different larva also collected and photographed by Abiodun. Notice the length of the wing buds/pads does reach S4, therefore this larva is in its final instar.

Photo used with written permission from Abiodun Matthew Adedapo.

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

Anatomy & Functional Morphology of Dragonfly Nymphs

November 9, 2021

As a blogger I create and share content. Sometimes I share content created by others, such as the following YouTube video from the Dragonfly Society of the Americas (DSA).

“Anatomy & Functional Morphology of Dragonfly Nymphs,” DSA (48:58).

Marla Garrison, McHenry County College, Biology Faculty, was featured during a Zoom meeting on 24 September 2021 as part of a series of Virtual Lectures presented by the Dragonfly Society of the Americas.

Marla’s lecture is richly illustrated with spectacular still photographs and video clips. I think readers of my blog will enjoy Marla’s presentation.

Copyright © 2021 Walter Sanford. All rights reserved.

It’s all about habitat, habitat, habitat.

April 23, 2021

Seeps and small streams in the forest are perfect places to look for habitat specialist dragonflies such as petaltails and spiketails.

An old place revisited.

The following photograph of a forest seep has been featured in my blog at least once in the past. The seep feeds a small pond; Gray Petaltail dragonflies (Tachopteryx thoreyi) can be found feeding and perching in sunny spots around the pond during late-May and early-June.

23 MAY 2018 | Prince William County, VA | forested seep

The seep is the habitat where Gray Petaltail larvae live most of their lives, not the pond. I always wondered how so many adult petaltails could emerge from this relatively small seep.

Turns out Michael Powell, my good friend and photowalking buddy, must have been wondering the same thing because he explored the area upstream from the small seep shown above and discovered several more seeps located close to the one near the pond.

The next photo shows Mike resting on a log along the edge of one of the seeps, near the confluence of two small streams. Notice the patch of skunk cabbage (Symplocarpus foetidus) growing in the seep — a good sign that you might be looking at habitat suitable for Gray Petaltail.

13 APR 2021 | Prince William County, VA | forested seep

A new place worth exploring further.

Mike also discovered another small stream in the forest when he was exploring for Uhler’s Sundragon (Helocordulia uhleri). The stream is located at the approximate midpoint between two trails, so I nicknamed it “Middle Creek.” Clever, huh? Note the patch of skunk cabbage growing in a seep alongside the stream. Did an alarm just go off in your head?

13 APR 2021 | Prince William County, VA | forested stream and seep

Mike and I are eager to explore the stream further, mainly looking for Gray Petaltail during late spring. Several species of spiketails might be found there as well.

Copyright © 2021 Walter Sanford. All rights reserved.

Tramea carolina exuvia

January 22, 2020

I’m a man on a mission to demystify identification of odonate exuviae, as I’m fond of saying. Easier said than done. In my experience, the process of identification can be challenging at best and impossible at worst.

For example, it’s nearly impossible to differentiate exuviae from the Genus Tramea (Saddlebags) to the species level.

The search for reliable larval characters to distinguish species of Tramea has generated considerable confusion in the literature. Source Credit: Landwer, Brett & Sites, Robert. (2006). Diagnostic efficacy of morphological characters of larval Tramea lacerata Hagen and Tramea onusta Hagen (Odonata: Libellulidae). Great Lakes Entomologist. 38. 155-163.

More recently, Tennessen cited the preceding research as well as a follow-up article by the same authors in 2010, and wrote…

…specific identification is still problematic. Source Credit: Tennessen, Kenneth. (2019). Dragonfly Nymphs of North America – An Identification Guide. Springer International Publishing. 567.

The fact of the matter is the most reliable way to identify odonate larvae to the species level is to rear them to maturity and emergence, that is, unless you’re fortunate to find a larva emerging in the field. Since an exuvia is essentially a nearly perfect shell of the last instar, it can be used to identify other specimens of the same species by pattern matching.

Rearing an unknown species of larva from genus Tramea

A larva from the Genus Tramea (Saddlebags) was collected by Andy Davidson near Richmond, Virginia USA, and reared to maturity. The following photo shows the adult dragonfly soon after emergence.

Photo used with permission from Andy Davidson.

The image was rotated in order to get a better look at the shape of the “saddlebags” on the rear wings of the dragonfly.

Photo used with permission from Andy Davidson.

The following composite image — created by Ed Lam, author and illustrator of Damselflies of the Northeast — clearly shows the difference in the shape of the saddlebags for Carolina- versus Red Saddlebags. Look closely at the saddlebags in the full-size version of the preceding photo and you can see the pattern perfectly matches the Carolina Saddlebags in Ed’s image, shown below.

Composite image used with permission from Ed Lam.

Look-alikes: Genus Pantala and Genus Tramea

Two genera from the Family Libellulidae (Skimmers) look similar: Genus Pantala (Rainpool Gliders); and Genus Tramea (Saddlebags). If you find an exuvia with long “tail fins,” then it might be a member of one of these two genera.

A Spot-winged Glider dragonfly (Pantala hymenaea) exuvia was collected by Andy Davidson near Richmond, Virginia USA.

A Carolina Saddlebags dragonfly (Tramea carolina) larva was collected by Andy Davidson near Richmond, Virginia USA, and reared to maturity. Andy saved the exuvia after emergence.

The following couplet from Identification Keys to Northeastern Anisoptera Larvae, compiled by Ken Soltesz, can be used to differentiate exuvia from Genus Pantala and Genus Tramea.

p. 37, Key to the Genera of the Family Libellulidae
12a – Superior abdominal appendage (epiproct) as long as, or longer than inferiors [paraprocts]. Pantala
12b – Superior abdominal appendage (epiproct) shorter than inferiors [paraprocts]. Tramea

What are the take-aways?

If you find an exuvia from the genus Tramea, then you might be unable to identify it to the species level.

I collected a Tramea sp. exuvia during Fall 2016 from a water retention pond located at a small park in Fairfax County, Virginia USA. I keyed out the specimen and identified it as Tramea carolina, in part, because Carolina Saddlebags had been observed in numbers at the same location. Am I certain of the identity? Yes and no. I’m certain the specimen is from genus Tramea; I’m reasonably sure (but not certain) it’s T. carolina. That said, my rationale is fairly good.

Related Resources

Copyright © 2020 Walter Sanford. All rights reserved.

“Odonate Larvae and Exuviae” Facebook group

November 29, 2019

I created a new Facebook group called “Odonate Larvae and Exuviae.” Readers of this blog who enjoy my photographs of odonate exuviae might be interested in following the new group. Please join us!

Copyright © 2019 Walter Sanford. All rights reserved.

Macromiidae larvae/exuviae are horny

October 4, 2018

A prominent horn on the face is a key field mark for all larvae/exuviae in the Family Macromiidae (Cruisers).

The first image shows the top of the head of an exuvia from an Allegheny River Cruiser dragonfly (Macromia alleghaniensis), collected by Mike Boatwright on 07 June 2018 at Otter Lake in Amherst County, Virginia USA.

07 June 2018 | Amherst County, VA | exuvia (head-horn)

The next photo shows the top of the head of an Allegheny River Cruiser larva reared by fellow Virginian Bob Perkins, providing an excellent view of both the horn and antennae (2).

Image used with permission from Bob Perkins.

Tech Tips

The following equipment was used to shoot the composite image of the exuvia: Canon EOS 5D Mark II digital camera, in manual mode; Canon MP-E 65mm Macro lens (set for 3x); Canon MT-26EX-RT Macro Twin Lite set for “Master” mode, and Canon 580 EX- and Canon 580EX II Speedlites set for “Slave” mode.

Adobe Photoshop CC 2017 was used to create the focus-stacked composite image from eight photos.

Bob Perkins’ photo of the larva, taken on 03 October 2018, was shot using a Canon EOS Rebel T3i camera body and Canon EF-S 60mm macro lens.

Copyright © 2018 Walter Sanford. All rights reserved.

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