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March Browns and Cahills

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This common name refers to only one genus.

Mayfly Genus Maccaffertium

These are pretty much always called March Browns and Cahills.
Many of America's traditional flies, like the March Brown and the Light and Dark Cahills, originated in the Catskills to imitate the mayflies of this genus. Back then they were classified in the genus Stenonema, the name by which many anglers still know them. Caucci and Nastasi wrote of them in Hatches II:

"There is a matrimony between Stenonema flies and Catskill rivers that is as synonymous as ham and eggs."

A lot has changed in the intervening years. Excepting a handful of species moved to Stenacron, most have been moved to the new genus Maccaffertium. By far the most important species is Maccaffertium vicarium (March Brown & Gray Fox). Of the others in the "Cahill" group, Maccaffertium ithaca is the next most important, along with Maccaffertium modestum and Maccaffertium pulchellum that also produce localized fishable hatches. These lesser species together with a few species of Stenacron and the last remaining species in Stenonema (femoratum) represent the sporadic Light Cahill hatches on evenings in late Spring that often continue throughout the Summer.

Anglers looking for information on the popular species Stenonema fuscum (Gray Fox) should know that it has been combined with Maccaffertium vicarium (March Brown) and is now only considered a lighter and slightly smaller version of the same species.
Maccaffertium ithaca (Light Cahill) Mayfly NymphMaccaffertium ithaca (Light Cahill) Mayfly Nymph View 9 PicturesThis specimen seems to be of the same species as a dun I photographed which emerged from another nymph in the same sample.
Collected May 29, 2007 from in
Added to by on June 4, 2007
Male Maccaffertium vicarium (March Brown) Mayfly DunMale Maccaffertium vicarium (March Brown) Mayfly Dun View 10 PicturesI collected this mayfly on the same trip as a female of the same species. After these photos it molted into a spinner. This is the form of Maccaffertium vicarium which anglers call the "Gray Fox."
Collected May 28, 2005 from in
Added to by on May 24, 2006
Female Maccaffertium vicarium (March Brown) Mayfly SpinnerFemale Maccaffertium vicarium (March Brown) Mayfly Spinner View 7 PicturesI collected this mayfly on the same trip as a male of the same species. They are Maccaffertium vicarium mayflies of the type formerly known as Stenonema fuscom, the "Gray Fox."
Collected May 28, 2005 from in
Added to by on May 24, 2006
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