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Pteronarcys californica

The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.

Mayfly Species Ecdyonurus criddlei (Little Slate-Winged Dun)

Where & when

Time of year : June to July

This is one of the most widely distributed of the former Heptagenia species in the West, including the rivers along the Pacific Northwest coast.

Hatching behavior

Water temperature: 54°F for good hatches

Specimens of the Mayfly Species Ecdyonurus criddlei

1 Female Spinner
Female Ecdyonurus criddlei (Little Slate-Winged Dun) Mayfly Spinner
This one was collected close to a male which, based on size and general structure, was likely the same species. Females are supposed to be lighter-colored and with less distinct markings than the males, although the difference in this pair is particularly stark.
1 Male Spinner
Male Ecdyonurus criddlei (Little Slate-Winged Dun) Mayfly Spinner
This one was very puzzling to identify. It's clearly Heptageniidae. It keys pretty easily to couplet 58 in Merritt, Cummins, & Berg 5th Edition, but that's where things get tricky.

58: Male penes distinctly L-shaped; Segment 1 of foretarsi usually 1/3 to 2/3 the length of segment 2
58': Males penes not distinctly L-shaped; Segment 1 of foretarsi usually 1/5 to 1/2 the length of segment 2

The characteristics conflict here, as I'd say the penes ARE L-shaped, but the fore tarsal ratio is less than 1/3. Maybe the weasel word "usually" allows for that, and it's some kind of Stenonema, for which I'm not aware of any records in Montana, let alone western Montana.

Going by the tarsal ratio and assuming it's not Stenonema, everything in couplet 60 points to Afghanurus: contiguous compound eyes, genitalia similar to fig 13.222 in MC&B, and weakly developed basal costal crossveins in the forewings. But I looked up the original descriptions of the three Afghanurus species, and the body sizes (flowersi 5-7 mm, inconspicua 4 mm, joernensis 6-7 mm) are way below this one's 10 mm.

This led me to notice that the closely related Ecdyonurus is not included in the key at all, so I checked the descriptions of those species and found that Ecdyonurus criddlei fits very well: the color of the legs, the markings on the tergites and sternites, the short fore tarsus and ratios of the segments, etc. It was originally described as 7 mm, smaller than the 10 mm body of this specimen, but synonyms have been described with larger sizes including wing lengths up to 11 mm for females for Heptagenia salvini.

This specimen was found with a female nearby, which I think is probably the same species, although it is difficult to be certain.
1 Nymph
Ecdyonurus criddlei (Little Slate-Winged Dun) Mayfly Nymph
This is an interesting Heptageniid mayfly since in western Montana it is only found in cold lakes, especially those in Glacier National Park.

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