March Browns and Cahills
This common name refers to only one genus.
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.