Fly Fishing for Trout Home
User Password
or register.
Scientific name search:

> > Size Variation in March Browns

Fishskicano has attached these 2 pictures to aid in identification. The message is below.
Shown Full Size
Shown Full Size
FishskicanoMay 19th, 2009, 7:59 am
Merrill, WI

Posts: 2
I was fishing yesterday in north central Wisconsin. At the end of the day there was a sparse hatch of mayflies. They resembled the March Browns that hatch later in the year on this same river but they were a good 2 to 3 hook sizes smaller than the March Browns (#14/#16 as opposed to #10/ #12) that come off in early June. Add to that that the season is running a week or two behind schedule and that makes it even harder to reconcile an early emergence of March Browns. The fish did not seem particularly interested in the bugs (they go nuts over the MB hatch in June). Any ideas?

GONZOMay 19th, 2009, 5:30 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
Hi George,

Other than the small size, I can't see anything about your specimen that suggests that it is not a March brown (M. vicarium). That species, as currently described, is notorious for wide variation in size and color, and often has a rather lengthy and spotty emergence. I find sizes for this species (or its synonyms) ranging from 10-18mm in the scientific literature. The lower end of that range is close to what you describe, about a #14. (There is a fair amount of subjective interpretation involved in hook size translations.) Your specimen is also a male, and males run smaller than females. Although some unusual influence may have played a role, I would guess that you witnessed a limited emergence of smallish males.
KonchuMay 19th, 2009, 7:59 pm
Site Editor

Posts: 496
If this is Maccaffertium (formerly Stenonema), vicarium is an excellent guess. It is a fairly early Maccaffertium. My source suggests a range of 9-14mm for the male adults (10-18 for larvae). The longitudinal striping on the abdomen makes me lean towards vicarium, in addition to the size. I'd thought Leucrocuta earlier (deleted the post) based on the smaller size and shading of the wing veins, but M. vicarium is better, due to coloration, leg striping, etc. Good call from the Bear Swamp.

M. integrum is even smaller, 5-7mm, and has a broken stripe, but it occurs later in the year.
FishskicanoMay 20th, 2009, 4:40 am
Merrill, WI

Posts: 2
Well, I just picked up a ruler and held it against the same spot I took the second picture and 10mm is just about what the bug would have measured. So mystery solved. Interestingly enough a friend was fishing a river last night 20 miles from where I was Monday and also ran across a hatch of smaller than average March Browns. In his case though the eyes were the moss green that the males have in Jason's photos. I'll post a link to his response and his picture below.

WiflyfisherMay 20th, 2009, 5:09 am

Posts: 612
IMHO, the wing is dead on M. vicarium. I have seen a lot of size variation as well. Although in early June in the NW corner you get a lot of much larger March Browns. If I remember right, "Konchu and gang" lumped Stenonema fuscum (which is smaller) with Maccaffertium vicarium now.
John S.
GONZOMay 20th, 2009, 9:45 am
Site Editor
"Bear Swamp," PA

Posts: 1681
If I remember right, "Konchu and gang" lumped Stenonema fuscum (which is smaller) with Maccaffertium vicarium now.

I don't think Konchu was part of that "gang," John. As best I can reconstruct it, that particular "lumping" went something like this: The former Stenonema fuscum was synonymized under Stenonema vicarium by Bednarik and McCafferty in 1979. That same year, Bednarik also proposed "Maccaffertium" as a subgenus of Stenonema. Maccaffertium was then elevated to genus level by Wang and McCafferty in 2004, leaving only Stenonema femoratum in the genus Stenonema. That combination produced Maccaffertium vicarium as we know it now. (I'm sure Konchu can correct me if I've made any mistakes in tracing that process.)

Quick Reply

You have to be logged in to post on the forum. It's this easy:
Username:          Email:

Password:    Confirm Password:

I am at least 13 years old and agree to the rules.

Related Discussions

TitleRepliesLast Reply
Re: Little J
In General Discussion by Wbranch
3May 17, 2009
Re: Females duns can be tough !!!
In the Identify This! Board by Brookyman
6Feb 22, 2013
by Entoman
Male + female ... Maccaffertium mediopunctatum adults
(13 more)

In the Identify This! Board by Brookyman
Re: Finally my "Stenonema" section isn't empty
In Stenonema femoratum Mayfly Nymph by Troutnut
2May 20, 2007
by Troutnut
Re: 2 tails or 3
In the Mayfly Species Maccaffertium vicarium by Snagy
1Feb 6, 2010
by Taxon
Re: Not sure what this is...
In the Photography Board by Freepow
3Jun 6, 2008
by Freepow
Re: Insect photos on CatskillFlies website
In the Mayfly Genus Ephemerella by Jpsully
4May 24, 2008
by Softhackle
Re: Need help with mayfly nymph ID
In the Identify This! Board by RustySpinnr
8Apr 27, 2012
by Brookyman
Re: Maccaffertium spinner, maybe??
In the Identify This! Board by Wiflyfisher
9Jun 13, 2008
by Wiflyfisher
Re: What genus this mayfly ?
In the Identify This! Board by And
5Jun 21, 2012
by Brookyman
Most Recent Posts
Re: I hope all of you doing well!
In General Discussion by Partsman (Martinlf replied)
Re: Thunder Creek flies
In Fly Tying by Dbinmt (Falsifly replied)
Re: Trout Unlimited, who are they?
In General Discussion by Red_green_h (Billfish3r replied)
Re: My wild & crazy Flordia trip part 3: hotel room and other fun stuff
In the Photography Board by Jmd123 (Martinlf replied)
Re: Furled Leaders
In Gear Talk by Ov10fac (Martinlf replied)
Re: Personal Best Largemouth
In the Photography Board by Summer_doug
Re: My wild and crazy trip to Florida & back while the world went crazy in my "absence"
In the Photography Board by Jmd123
Re: My wild and crazy trip to Florida & back, part 2: fly fishing & bottom bouncing
In the Photography Board by Jmd123 (Martinlf replied)
Re: Wading Staff suggestions?
In General Discussion by NMiAngler (Gene_PA replied)
Re: FINALLY heading to Florida!
In the Photography Board by Jmd123