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Ameletus (Brown Duns) Mayfly Nymph Pictures

 I spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.) ages trying to identify this one but ultimately couldn't narrow it down to species. I'm guessing it's either a species that has not yet been reported from Idaho or a species with some variation in characteristics not accounted for in the current key (Zloty 1997), which is only for Alberta but happens to contain all the species documented in Idaho except for one (which is rare and only in a different part o the state from this one).

Here are my raw notes from the microscope session:

8. Ameletus nymph (genus 100 % based on mouth parts under microscope)
1. This is probably a species with the nymph either not described yet or not reported in Idaho (or Alberta).
2. There is a key to the species of nymphs in Alberta (Zloty 1997) which includes all but one (A. tolae) of the species listed in Idaho by IDFG (https://idfg.idaho.gov/species/taxa/8607), and A. tolae is only listed from one drainage in north-central Idaho. So my specimen should be in that key. However, it doesn’t fit any of them.
1. The antennae are pale with brown at the apex (Apex: The uppermost, outermost, or culminating point; the tip.). This doesn’t fit any of the species they described.
2. The labrum (Labrum: The platelike structure forming the roof of the mouth of insects; the upper lip.) is almost completely dark brown, maybe a bit paler toward the apex (Apex: The uppermost, outermost, or culminating point; the tip.).
3. Following the key in Zloty 1997 basically rules out every species reported in Idaho except for tolae, which would be outside its range:
1. Couplet 1 : There definitely aren’t strong ganglionic markings on sternites (
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
Sternite: The bottom (ventral) part of a single segment on an insect's abdomen.
)
2-8 (100 % rules out similior and celer) —> 3
2. Couplet 3 : Posterior (Posterior: Toward the back of an organism's body. The phrase "posterior to" means "in back of.") margins of sternites (
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
Sternite: The bottom (ventral) part of a single segment on an insect's abdomen.
)
6-8 lack large spines (80 % sure) but other characteristics rule out the species if there were spines (validus, oregonensis, subnotatus) —> 6
3. Couplet 6 : Mesal (Mesal: Toward the middle.) gill extension present, but pretty slim… similar to Fig. 23B or 23G —> 7
4. Couplet 7 : Obviouly gos to 8
5. Couplet 8 : Small size and time of year rules out velox, tergite (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
)
patterna and gill shape rules out pritchardi (which is not reported in Idaho anyway). Additional features (antennae, labrum (Labrum: The platelike structure forming the roof of the mouth of insects; the upper lip.) color) rule out a small velox.
6. Backtrack to call the mesal (Mesal: Toward the middle.) gill extension “well developed” —> 9
7. Couplet 9 : Tail coloration obviously —> 10
8. Couplet 10 : Supposing it’s a small specimen of a “larger species” leads to 11, in which femora (
The femur of this Isonychia bicolor mayfly spinner is highlighted in red.
The femur of this Isonychia bicolor mayfly spinner is highlighted in red.
Femur: The main segment of an insect's leg close to the body, in between the tibia and the trochanter.
)
coloration and timing rule out vernalis, and color pattern rules out bellulus. Mesal (Mesal: Toward the middle.) extension on gills from species description very conclusively rules out bellulus. Therefore, calling it a “smaller species” is the correct path —> 12
9. Couplet 12 : Sternites (
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
Sternite: The bottom (ventral) part of a single segment on an insect's abdomen.
)
without well-defined longitudinal stripe —> cooki. However, tergite (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
)
color patterns don’t even come close to matching ANY of the 3 species from this point on (cooki, sparsatus, suffusus). From the species descriptions:
1. cooki: Antenna and labrum (Labrum: The platelike structure forming the roof of the mouth of insects; the upper lip.) colors don’t fit. Mesal (Mesal: Toward the middle.) extension should be larger and tracheation lighter, to be this species.
2. sparsatus: Antenna and labrum (Labrum: The platelike structure forming the roof of the mouth of insects; the upper lip.) colors don’t fit. Mesal (Mesal: Toward the middle.) extension should be larger. Postero-lateral (Lateral: To the side.) spines should be very prominent, not barely noticeable.
3. suffusus: Also bad fit to antenna and labrum (Labrum: The platelike structure forming the roof of the mouth of insects; the upper lip.) colors, mesal (Mesal: Toward the middle.) extension, and especially gill tracheation.


This mayfly was collected from Green Lake Outlet on August 4th, 2020 and added to Troutnut.com on August 20th, 2020.

In another specimen with intact tails, all three were about the same length.  Ameletus (Brown Duns) Mayfly Nymph from Green Lake Outlet in Idaho
In another specimen with intact tails, all three were about the same length.

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