Hatches Made Easy:

Light Cahill (Stenacron interpunctatum)


The Light Cahill common name is one of the most confused common insect (fly)
names used in the Smokies and elsewhere in the East for that matter. The
Stenacron genus, not to be confused with its sister genus, the similar sounding
Stenonema genus, includes one important species, the interpunctatum. This
mayfly is commonly called the Light Cahill.  
This mayfly is very similar to some of the
Stenonema species (that were recently
changed to
Maccaffertium species) which accounts for some of the confusion.
For example, the old
Stenonema ithaca (now the Maccaffertium ithaca) is often
called a Light Cahill. It is also called a Gray Fox, adding even more confusion
the Gray Fox common name. Because of the confusion, you will hear anglers
mention that Light Cahills are hatching all the way from April until the middle of
Most of the mayflies they are referring to are not Light
Some of the other mayflies the Light Cahill is confused with are the old
Stenonema (now Maccaffertium) mediopunctatum, carolina, and modestum
. These are usually and correctly (if there is such a thing as a correct
common name) called Cream Cahills.
The Light Cahills are also confused with the
Heptagenia group of mayflies or the
Little Yellow Quills that hatch later in the season.
That is why you will hear
anglers still taking about Light Cahills late in the Summer and early fall
There is a reason for all this confusion. The duns of these various species look
much alike.

Now I am sure all of the above scientific names are also confusing, especially to
those who are just getting started. Scientific names are necessary in order to
designate the insect I am referring too or otherwise I would not use them. If I
used common names only, there would be even more confusion. For example, if
I called a
Maccaffertium Ithaca a "Gray Fox" many anglers would think I was
referring to a
Maccaffertium varcium or the old March Brown common name. I
could give dozens of such examples. Common names vary from region to region,
book to book and angler to angler. In order to make it simple, what I and most of
the fly fishing community refer too as a "Light Cahill" is the
These mayflies hatch from the last week of April until the end of June, depending
mainly on the weather and elevation of the stream.
This hatch usually only
last two to three weeks at any one location
but the overall duration from the
streams at the lower elevations to the higher elevation, can last up to eight
weeks. These mayflies can be found in the tiny brook trout streams as well as
the larger watersheds at the point they exit the park.
As I said above, it is easy to understand some of the confusion in the common
name "Light Cahill". There is not a great deal of difference in the appearance of
some of the
Maccaffertium, Stenacron and Stenonema species. They are all
clinger nymphs that look fairly similar. However, there are differences in the
behavior of some of these various species that warrant attention.
Heptagenia group of mayflies (often confused as Light Cahills) are also
clingers but behave quite differently. When we review the "Cream Cahills" you
will find that some of those species are different colors. Some of them found in
the park are almost pure white. Some of them have heavily mottled wings. The
sizes of these mayfly species can vary a hook size of two and of course, the
hatch times vary greatly. For now, lets focus on the real "Light Cahill".

Coming Up Next:
Light Cahill (Stenacron interpunctatum) - Nymphs and Emergers

Copyright 2008 James Marsh