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Mayfly Species Siphlonurus alternatus (Gray Drake)

Pictures Below
This species occasionally produces important spinner falls. Its spinners may join the swarms of Siphlonurus quebecensis or Siphlonurus rapidus, amplifying the importance of all three species.  

Where & When


Regions: East, Midwest

Time Of Year (?): Early May through early September; peaking from late June through July


Spinner Behavior


Time Of Day: Evening

Habitat: Riffles

Pictures of 2 Mayfly Specimens in the Species Siphlonurus alternatus:

Female Siphlonurus alternatus (Gray Drake) Mayfly SpinnerFemale Siphlonurus alternatus (Gray Drake) Mayfly Spinner View 7 PicturesI'm tentatively classifying this one as Siphlonurus alternatus, because it seems to fit that species best in the old keys in Needham's Biology of Mayflies, but I'm hesitant because I can't find documentation that southwest Montana or the surrounding area is within the species' range, although they are widely distributed throughout eastern North American and western Canada.
Collected July 2, 2019 from the Gallatin River in Montana
Added to Troutnut.com by Troutnut on July 18, 2019

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