Posts Tagged ‘instar equivalent’

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: 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.

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.

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.


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