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Like most common names, "Adams" can refer to more than one taxon. They're previewed below, along with 5 specimens. For more detail click through to the scientific names.

Mayfly Species Siphlonurus alternatus

These are often called Adams.
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.
Siphlonurus alternatus (Gray Drake) Mayfly NymphSiphlonurus alternatus (Gray Drake) Mayfly Nymph View 2 Pictures
Collected June 8, 2005 from in
Added to by on May 26, 2006
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 in
Added to by on July 18, 2019

Mayfly Species Siphlonurus quebecensis

These are often called Adams.
This is the Siphlonurus species I have encountered most frequently on Eastern trout waters. I often find its spinners swarming in a mix with Siphlonurus alternatus, and I have read that Siphlonurus rapidus may join these groups as well.
Siphlonurus quebecensis (Gray Drake) Mayfly NymphSiphlonurus quebecensis (Gray Drake) Mayfly Nymph View 9 Pictures
Collected May 13, 2007 from in
Added to by on May 18, 2007
Female Siphlonurus quebecensis (Gray Drake) Mayfly DunFemale Siphlonurus quebecensis (Gray Drake) Mayfly Dun View 3 PicturesThis one hatched in my house after I brought some nymphs home to photograph.
Collected May 18, 2004 from in
Added to by on January 25, 2006
Male Siphlonurus quebecensis (Gray Drake) Mayfly SpinnerMale Siphlonurus quebecensis (Gray Drake) Mayfly Spinner View 10 Pictures
Collected June 2, 2005 from in
Added to by on May 24, 2006

Mayfly Species Siphlonurus rapidus

These are sometimes called Adams.
This species may reinforce spinner flights of Siphlonurus quebecensis and Siphlonurus alternatus.
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