By Thomas Ames Jr.
Will the real
March Brown please stand up?
From May to early June we see a number of large
mayflies that go by names like "March brown," "gray fox"
and "light Cahill." Are these really separate insects, or
just separate names for the same thing? This is a good
time to consider the often over-emphasized relationship
between pattern names and insect names. For example, it
has been more than a decade since entomologists
determined that Stenonema fuscum, popularly known
as the gray fox, was not a separate species, but a
synonym for S. vicarium, and yet its ghost
continues to haunt us. Itâs time to lay its soul to
rest.
In the spring I keep a fish tank in my studio, and
every few days I load it up with larvae from one of the
many nearby streams and ponds. Several years ago I
gathered a dozen large Stenenoma nymphs from a
small brook and placed them in the tank. Within days I
had several adult male mayflies spanning hook sizes 10 to
12 and with overall body color ranging from creamy yellow
to grayish brown. When they had molted to the spinner
stage, without losing their diversity of size and color,
I sent them to Dr. Steven Burian, a mayfly expert, to be
identified. Dr. Burian informed me that I had three
"morphologically distinct" samples of the same species, Stenonema vicarium, alias the March brown.
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Light
Cahill or March Brown? |
I often come across an insect that resembles the March
brown in many ways, except that it is a full hook size
smaller, has a yellowish cast to its lightly marked wings
and almost none of the brown in the abdomen that gives
its larger relative its name. I used to think this was Stenonema ithaca (one of the light Cahills). Now
Iâm not so sure.
As anglers we are conditioned to believe that a mayfly
species is a certain size and color. At the same time we
accept the idea that all dogs are the same species, even
though we have different names for different breeds. My
border collie doesnât look anything like a German
shepherd. Both were bred for managing sheep, but each
serves a different purpose. Itâs a frustrated farmer
who confuses the two.
Why do we call an insect that hatches in May and June
the "March Brown"? The name is borrowed from an English
pattern that was designed to imitate their Rithrogena
germanica that hatches in late March and early April.
In the east weâve assigned the name to the largest
fly of the family Heptagenia, Stenonema vicarium.
Westerners have given the name to a close relative of the
English bug, Rithrogena morrisoni. In 1933 Preston
Jennings,finding the English pattern unsuitable for
American fishing, devised his own pattern which, with a
slight modification by Art Flick, has survived to this
day.
The light Cahill is a traditional name for the insect
that used to be called Stenonema canadense but is
now classified as a separate genus, Stenacron
interpunctatum (it has a different gill structure).
To anglers it is still a light Cahill, as it has been
ever since Jennings paired fly and pattern (together with S. ithaca) in A Book of Trout Flies.
Thatâs just too much Latin. Remembering all the
scientific names of this group of mayflies isnât
going to improve your fishing. Think of them all as
different breeds of the same insect (just remember the Stenonema group) and do your best to match size
and color. You canât do that by watching the insects
in the air. You have to get a sample in your hands.
By all means, keep your March browns, gray foxes and
light Cahills in your fly boxes. That includes any modern
variations with the same size and color characteristics,
such as Usuals, Comparaduns and parachutes. Use the one
that matches the hatch thatâs really going on, not
the one on the hatch chart.
Resquiescat in pace, Stenonema fuscum.
Thomas Ames Jr. is a commercial photographer and
author of Hatch Guide to New England
Streams.Signed copies of the book are available at
HRO. Tom lives in Norwich, Vermont and makes several
visits to the Housatonic each year.
