Posts Tagged ‘ventral’

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!)

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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.

Uhler’s Sundragon dragonfly (emergent female)

April 27, 2021

Uhler’s Sundragon (Helocordulia uhleri) was spotted during a recent photowalk with Michael Powell along a mid-size stream at an undisclosed location in Prince William County, Virginia USA.

The dragonfly was perched on the exuvia from which it emerged; its cast skin was still clinging to a grass stem approximately six inches (6″) above the ground. The patch of grass was located two-to-three feet from the shoreline, where the stream current was slow-to-moderate. The stream bed was sandy and gently sloped near the site on land where the dragonfly larva stopped to eclose.

This individual is an emergent female, as indicated by her teneral appearance, rounded hind wings, terminal appendages, and subgenital plate.

13 APR 2021 | Prince William County, VA | Uhler’s Sundragon (emergent female)

The subgenital plate is located on the ventral side of the abdomen beneath segment eight.

Underneath Segment 8 there is either an ovipositor or a subgenital plate, depending upon the species [of dragonfly]. Both structures are for laying eggs and extend over Segment 9 and possibly beyond. Source Credit: Dragonflies of the North Woods, by Kurt Mead.

Remember that “Segment 8 and 9” refers to abdominal segments eight and nine (of 10), numbered from front to back.

13 APR 2021 | Prince William County, VA | Uhler’s Sundragon (emergent female)

The following graphic from Dragonflies and Damselflies of the East by Dennis Paulson shows the shape of the subgenital plate on our emergent female matches Uhler’s perfectly.

Graphic used with written permission from Dennis Paulson.

True Detective

Generally speaking, teneral odonates are more challenging to identify than most mature adult dragonflies and damselflies. Full disclosure: Neither Mike nor I were certain of the identity of the emergent female when we were shooting photographs in the field. “Shoot first and ask questions later” is one of my mantras for wildlife photography.

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.

In this case, I used the exuvia to reverse-engineer the identity of the teneral dragonfly. Look closely at a lateral view of the exuvia, as shown in the following photo taken by Mike Powell. Notice there are dorsal hooks on abdominal segments seven, eight, and nine (S7-9). This distinctive field mark confirms the identity of the species as H. uhleri.

Photo used with written permission from Michael Powell.

Also notice Mike’s photo shows a clear view of the emergent female’s terminal appendages, as well as the subgenital plate on the underside of her abdomen.

How it all began, and ended.

The sky was almost completely overcast and it was cool and windy when I spotted the emergent female. Mike Powell and I observed the dragonfly for quite a while and there was little noticeable progress. According to Michael Boatwright, founder and administrator of the Virginia Odonata Facebook group, the process of emergence can take longer when the air temperature is cooler.

When the sky began clearing, Mike Powell and I decided to go to a nearby place where he had seen adult Uhler’s recently. Later the same day, we returned to the site of the emergent female; she was gone, so I collected the exuvia. I plan to publish a photo set of the exuvia in an upcoming blog post. To be continued.

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Copyright © 2021 Walter Sanford. All rights reserved.

Dromogomphus spinosus exuvia (ventral view)

February 16, 2021

A dragonfly exuvia was collected by Joe Johnston on 22 May 2019 at Aquia Creek, Stafford County, Virginia USA. This specimen is the cast skin from a Black-shouldered Spinyleg (Dromogomphus spinosus) larva (nymph). D. spinosus is a member of Family Gomphidae (Clubtails).

The specimen is approximately 3.3 cm (~1.3 in) long, measured from head to tail.

15 FEB 2021 | BoG Photo Studio | D. spinosus exuvia (ventral)

Male odonates have two sets of sex organs: primary genitalia located on abdominal segment nine (S9); and secondary genitalia located on abdominal segments two-to-three (S2-3).

This male specimen has too much dirt on S9 to see the primary genitalia; the secondary genitalia are visible on S2-3. I plan to annotate the image in order to highlight select anatomical features.

Related Resources

Tech Tips

The subject was photographed against a pure white background (255, 255, 255) using the “Meet Your Neighbours” (MYN) technique. The exposure was increased by 0.3 stop during post-processing in order to attain a pure white background.

A composite image was created from several photos taken using a Fujifilm X-T3 digital camera, Fujifilm MCEX-11 extension tube, Fujinon XF80mm macro lens, and an array of external lights.

One external flash unit was used to create the white background and two others to light the subject. A Sunpack LED 160 was used as a focusing aid.

RAW FILE CONVERTER EX 3.0 was used to convert six (6) RAW (RAF) files to TIFF files. In a departure from my usual workflow, the TIFF files were loaded into Adobe Photoshop CC 2017 in order to create the focus stacked composite image. The composite image was edited using Apple Aperture and sharpened using Photoshop.

Tech Take-aways

Notice there is some color fringing (mostly reds) that is especially noticeable on the front legs and antennae, as well as along the edges of the posterior abdomen. I have been experimenting with a new technique for backlighting the subject. It appears I need to add another layer of diffusion between the light source and the subject.

There is also some “softness” along the edges of the abdomen that could be the result of too few focus points for the shape of the body, which in turn, could have caused artifacts in the composite image.

Copyright © 2021 Walter Sanford. All rights reserved.

Veiled ventral view

October 2, 2020

Several Black and Yellow Argiope (Argiope aurantia) spiders were spotted during a photowalk with Michael Powell at Occoquan Bay National Wildlife Refuge (OBNWR), Prince William County, Virginia USA.

15 SEP 2020 | OBNWR | Black and Yellow Argiope (female)

This individual is a female, as indicated by her size. The preceding photo shows a ventral view of the spider, slightly obscured by the thin veil of her web. Contrast this ventral view with a recent blog post featuring a photo of the dorsal view of another A. aurantia.

Related Resource: Arachtober, a Flickr group that’s sure to make your spider senses tingle!

Copyright © 2020 Walter Sanford. All rights reserved.

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