I spent 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 of the state from this one).
Here are my raw notes from the microscope session:
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 doesnâ€™t fit any of them.
1. The antennae are pale with brown at the apex
. This doesnâ€™t fit any of the species they described.
2. The labrum
is almost completely dark brown, maybe a bit paler toward the apex
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 arenâ€™t strong ganglionic markings on sternites
2-8 (100 % rules out similior and celer) â€”> 3
2. Couplet 3 : Posterior
margins of sternites
6-8 lack large spines (80 % sure) but other characteristics rule out the species if there were spines (validus, oregonensis, subnotatus) â€”> 6
3. Couplet 6 : Mesal
gill extension present, but pretty slimâ€¦ similar to Fig. 23B or 23G â€”> 7
4. Couplet 7 : Obviouly gos to 8
5. Couplet 8 : Small size and time of year rules out velox, tergite
patterna and gill shape rules out pritchardi (which is not reported in Idaho anyway). Additional features (antennae, labrum
color) rule out a small velox.
6. Backtrack to call the mesal
gill extension â€œwell developedâ€ â€”> 9
7. Couplet 9 : Tail coloration obviously â€”> 10
8. Couplet 10 : Supposing itâ€™s a small specimen of a â€œlarger speciesâ€ leads to 11, in which femora
coloration and timing rule out vernalis, and color pattern rules out bellulus. Mesal
extension on gills from species description very conclusively rules out bellulus. Therefore, calling it a â€œsmaller speciesâ€ is the correct path â€”> 12
9. Couplet 12 : Sternites
without well-defined longitudinal stripe â€”> cooki. However, tergite
color patterns donâ€™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
colors donâ€™t fit. Mesal
extension should be larger and tracheation lighter, to be this species.
2. sparsatus: Antenna and labrum
colors donâ€™t fit. Mesal
extension should be larger. Postero-lateral
spines should be very prominent, not barely noticeable.
3. suffusus: Also bad fit to antenna and labrum
extension, and especially gill tracheation.