Hex Mayflies
Hexagenia limbata
The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
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Mayfly Genus Stenonema (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. Caucci and Nastasi wrote of them in Hatches II:
By far the most important species is Stenonema vicarium, variants of which are known to anglers as the March Brown and Gray Fox. Of the others in the "Cahill" group, Stenonema ithaca is the next most important, along with Stenonema modestum and Stenonema pulchellum, which also produce localized fishable hatches. These lesser species together with a few species of Stenacron and the Stenonema femoratum represent the sporadic Light Cahill hatches on evenings in late Spring that often continue throughout the Summer.
"There is a matrimony between Stenonema flies and Catskill rivers that is as synonymous as ham and eggs."
By far the most important species is Stenonema vicarium, variants of which are known to anglers as the March Brown and Gray Fox. Of the others in the "Cahill" group, Stenonema ithaca is the next most important, along with Stenonema modestum and Stenonema pulchellum, which also produce localized fishable hatches. These lesser species together with a few species of Stenacron and the Stenonema femoratum represent the sporadic Light Cahill hatches on evenings in late Spring that often continue throughout the Summer.
Taxonomic History
For over a decade this page lamented the reclassification of iconic Stenonema species into the less storied and euphonious name Maccaffertium. During this time only Stenonema femoratum, a species of minor importance to anglers, remained in the famous genus. However, every once in a while, taxonomists throw anglers a bone. Zembrzuski & Anderson (2018) published results of a genetic analysis showing that there was not a clear basis for the way former Stenonema species were divided into genera, and in fact it makes more sense for now to group them all together in Stenonema.
One species of light cahill known in the old angling literature as Stenonema canadense is now Stenacron interpunctatum, and that has not changed with the new analysis. Another one, Stenonema fuscom, known to some anglers as the Gray Fox, has for many years been recognized as a lighter and slightly smaller variant of the March Brown Stenonema vicarium.
One species of light cahill known in the old angling literature as Stenonema canadense is now Stenacron interpunctatum, and that has not changed with the new analysis. Another one, Stenonema fuscom, known to some anglers as the Gray Fox, has for many years been recognized as a lighter and slightly smaller variant of the March Brown Stenonema vicarium.
Where & when
This genus is only important in the East and Midwest, where they are often the most noticeable members of the Heptageniidae family due to their medium-large size and abundance. In the spring there are superhatches of Stenonema vicarium. The lesser species are primary players on the barren stage of summer evening mayfly hatches. Most of them emerge sporadically for months. Anglers know them as Light Cahills and Cream Cahills, and they often reinforce hatches of the closely related Light Cahill species Stenacron interpunctatum.In 247 records from GBIF, adults of this genus have mostly been collected during June (32%), July (25%), May (15%), August (13%), and April (9%).
In 85 records from GBIF, this genus has been collected at elevations ranging from 282 to 9318 ft, with an average (median) of 2904 ft.
Genus Range
Hatching behavior
The early-season species of Stenonema crawl to slow, shallow water before emerging. Apparently they lose their grip or otherwise end up in the drift during this movement, because they are common in the trout's diet in the weeks before they emerge. Common attractor nymphs like the Hare's Ear are good likenesses of this genus.Most of them are good dry fly insects because they take a long time to emerge and ride the water for a while before getting airborne. They are generally sporadic emergers, but their high populations put at least a few of these insects on the water most nights of the spring and summer.
Nymph biology
Stenonema mayfly nymphs are very adept clingers, but most species are at home in all stream habitats. They (and their close relatives in Stenacron) do not have the high oxygen requirements of the other genera in Heptageniidae.Specimens of the Mayfly Genus Stenonema
6 Male Duns
This dun emerged from a nymph in one of the bowls on my table while I was sorting. I photographed it with the nymphal shuck, and it seems to be of the same species as a nymph I photographed from the same sample.
I 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 Stenonema vicarium which anglers call the "Gray Fox."
I collected this male dun together with a female.
Based on the identical size, shape, and relative proportions of different body parts, I'm pretty sure this dun is of the same species as two spinners I collected at about the same time. The color is dramatically different, but that's to be expected with many of these species in the transition from dun to spinner.
This is the only Heptageniid mayfly I saw hatch on May 24, 2004, and luckily it floated right by me.
Edit: Gonzo confirmed most likely vicarium 06.21.12
Edit: Gonzo confirmed most likely vicarium 06.21.12
7 Female Duns
This female looks very much like a male I collected a few hundred miles away a few days later, so I'm guessing it's the same species, which I believe is Maccaffertium mediopunctatum.
Here's a classic March Brown dun from the Catskills.
I collected this female dun together with a male. She's fairly damaged but at least you can see the colors.
9 Male Spinners
I positively identified this specimen under a microscope with a good key. It was one of the more common species in a generally light mixed bag of mayflies over a big Catskill tailwater last night.
I swooped this one out of the air while it was mating with a female, so I can be certain about her ID as well.
I swooped this one out of the air while it was mating with a female, so I can be certain about her ID as well.
This spinner comes from a dun I collected and photographed earlier.
7 Female Spinners
Although I couldn't key out this female spinner, I'm fairly comfortable calling her Stenonema modestum based on the markings on the abdomen and their similarity to those on the males of this species. This one probably came from the Namekagon River, although there's a possibility it came from the West Fork of the Chippewa River, because it was found in the truck after fishing those streams on back-to-back days.
I was able to identify this female because I captured her while she was mating with an identifiable male.
This one may look a bit strange, because one of her main wings came off in my container.
I 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."
This one was collected in in association with this male. It looks quite different, but that's often the case between spinner genders.
37 Nymphs
This specimen seems to be of the same species as a dun I photographed which emerged from another nymph in the same sample.
I keyed this nymph to Stenonema fuscom, which is now synonymized with Maccaffertium vicarium. However, the size, markings, and time of year make me very skeptical of this identification.
I photographed this Maccaffertium vicarium nymph shortly after it molted to a new instar in my holding container. Many nymphs take on drastically different coloration immediately after molting, and this species is no exception.
Every picture of this Maccaffertium nymph was taken with my old C740UZ through a microscope.
I looked at this nymph closely under my crappy microscope. It has truncate as opposed to rounded gills, postero-lateral spines only on abdominal segments 7-9, and oblique crossbars through the center of each sterna, not on the posterior edge like on some of the other Maccaffertium species. As best I can tell from the keys this is either modestum or ithaca and from the distribution maps I've seen I'm guessing it's modestum.
I saw under a microscope that the gills on this specimen aren't tracheated.
This is a subspecies M. terminatum terminatum, formerly called Stenonema. Found mostly in larger rivers in western Montana. This is an immature nymph.
2 Streamside Pictures of Stenonema Mayflies:
1 Underwater Picture of Stenonema Mayflies:
Discussions of Stenonema
M. ithaca in M. mediopunctatum section?
3 replies
Posted by GONZO on Sep 1, 2012 in the species Stenonema mediopunctatum
Last reply on Sep 4, 2012 by Entoman
Hi Jason,
In this section, the Midwestern nymphs (#573 & #574) with the dark irregular ventral bars across the anterior portion of the sternites look like Maccaffertium mediopunctatum arwini (the Midwestern ssp.), but the two Eastern duns (#733 & #765) and the associated shuck (of #765) and nymph (#764) look more like Maccaffertium ithaca to me.
Three Maccaffertium species can have very similar ventral markings in the nymph—dark, sinuate, chevron-shaped bars on many of the sternites and dark lateral marks (sometimes connected to form an inverted U-shaped mark) on segment 9. The Eastern mediopunctatum subspecies, M. m. mediopunctatum, has these markings, as does M. ithaca. Similar markings also appear as a less-common variant marking of M. modestum (or the M. modestum species complex). However, these species differ in the length and location of posterolateral projections, leg markings, the appearance of the subs and adults, and size.
Although interpretation of posterolateral projections can be tricky, those projections should help to separate the nymph (and husk) from mediopunctatum. On mediopunctatum, projections are usually on segments 3-9, 4-9, or 5-9, and those on 8 and 9 are fairly long. On ithaca, projections are usually on segments 6-9 or 7-9, and those on 8 and 9 are somewhat shorter (when compared to mediopunctatum). The twin brown bands on the femora of the nymphs should also help to separate them from modestum and mediopunctatum (usually three or four in those species).
The brown posterior margins and median dorsal stripes of the duns (similar to those found in M. vicarium) are typical of ithaca. In McDunnough’s original description of mediopunctatum (1926), he mentions that some of his (paratype) specimens were reared from subimagos, and he describes those subimagos as “quite pale whitish in coloration.”
Size might also be somewhat helpful in distinguishing these specimens from M. m. mediopunctatum (about 7-10 mm at maturity) and modestum (about 8-11 mm at maturity). M. ithaca is about 9-14 mm at maturity. The relatively mature nymph (#764) is at least 11 mm, the female dun (#733) is about 13 mm, and the male dun (#765) is about 11 mm.
When all of these factors are considered, it seems to me that M. ithaca is a more likely ID for the Eastern specimens. (See Bednarik and McCafferty 1979 and Lewis 1974.) I would suggest the following placement for specimens currently in this section:
M. ithaca nymph: http://www.troutnut.com/specimen/764
M. ithaca female dun: http://www.troutnut.com/specimen/733
M. ithaca male dun: http://www.troutnut.com/specimen/765
M. mediopunctatum arwini nymph: http://www.troutnut.com/specimen/573
M. mediopunctatum arwini nymph: http://www.troutnut.com/specimen/574
Best,
Lloyd
In this section, the Midwestern nymphs (#573 & #574) with the dark irregular ventral bars across the anterior portion of the sternites look like Maccaffertium mediopunctatum arwini (the Midwestern ssp.), but the two Eastern duns (#733 & #765) and the associated shuck (of #765) and nymph (#764) look more like Maccaffertium ithaca to me.
Three Maccaffertium species can have very similar ventral markings in the nymph—dark, sinuate, chevron-shaped bars on many of the sternites and dark lateral marks (sometimes connected to form an inverted U-shaped mark) on segment 9. The Eastern mediopunctatum subspecies, M. m. mediopunctatum, has these markings, as does M. ithaca. Similar markings also appear as a less-common variant marking of M. modestum (or the M. modestum species complex). However, these species differ in the length and location of posterolateral projections, leg markings, the appearance of the subs and adults, and size.
Although interpretation of posterolateral projections can be tricky, those projections should help to separate the nymph (and husk) from mediopunctatum. On mediopunctatum, projections are usually on segments 3-9, 4-9, or 5-9, and those on 8 and 9 are fairly long. On ithaca, projections are usually on segments 6-9 or 7-9, and those on 8 and 9 are somewhat shorter (when compared to mediopunctatum). The twin brown bands on the femora of the nymphs should also help to separate them from modestum and mediopunctatum (usually three or four in those species).
The brown posterior margins and median dorsal stripes of the duns (similar to those found in M. vicarium) are typical of ithaca. In McDunnough’s original description of mediopunctatum (1926), he mentions that some of his (paratype) specimens were reared from subimagos, and he describes those subimagos as “quite pale whitish in coloration.”
Size might also be somewhat helpful in distinguishing these specimens from M. m. mediopunctatum (about 7-10 mm at maturity) and modestum (about 8-11 mm at maturity). M. ithaca is about 9-14 mm at maturity. The relatively mature nymph (#764) is at least 11 mm, the female dun (#733) is about 13 mm, and the male dun (#765) is about 11 mm.
When all of these factors are considered, it seems to me that M. ithaca is a more likely ID for the Eastern specimens. (See Bednarik and McCafferty 1979 and Lewis 1974.) I would suggest the following placement for specimens currently in this section:
M. ithaca nymph: http://www.troutnut.com/specimen/764
M. ithaca female dun: http://www.troutnut.com/specimen/733
M. ithaca male dun: http://www.troutnut.com/specimen/765
M. mediopunctatum arwini nymph: http://www.troutnut.com/specimen/573
M. mediopunctatum arwini nymph: http://www.troutnut.com/specimen/574
Best,
Lloyd
Stenonema femoratum male spinner
3 replies
Posted by Jmw975 on May 21, 2012
Last reply on May 21, 2012 by Entoman
Male Stenonema femoratum collected from the Grand River, Ontario. Note the 3 spots at the hind margins of each of the abdominal segments.
2 tails or 3
1 replies
Posted by Snagy on Feb 5, 2010 in the species Stenonema vicarium
Last reply on Feb 6, 2010 by Taxon
I notice that while the dun, nymph, and spinner photos on the page are all listed as March Brown (maccaffertium vicarium). While the coloration patterns all seem to follow other March Browns I have seen, I notice that the winged flies pictures all have 2 tails, but the nymphs in the photos have 3. I was under the impression this species was a 2 tailed mayfly. Is the nymph mislabelled?
Anybody know more about Maccaffertium terminatum?
3 replies
Posted by Troutnut on Jun 8, 2007 in the species Stenonema terminatum
Last reply on Jun 11, 2007 by Konchu
I just wrote up what I found on the page for this species. I was mildly surprised to see so many of a species I've never seen mentioned in an angling context -- at least a few dozen throughout the evening. They piqued my curiosity.
diff between march brown &gray fox
7 replies
Posted by Jrcald on Mar 12, 2007 in the species Stenonema vicarium
Last reply on Mar 13, 2007 by Troutnut
i know that gray foxs and march browns are classified in the same family but what are the differences ? what is the color differences are both bodies the same color tan ?
Start a Discussion of Stenonema
References
- Caucci, Al and Nastasi, Bob. 2004. Hatches II. The Lyons Press.
Mayfly Genus Stenonema (March Browns and Cahills)
Taxonomy
Species in Stenonema
Stenonema exiguum
0
0
Stenonema femoratumCream Cahills
1
6
Stenonema ithacaLight Cahills
4
34
Stenonema luteum
1
4
Stenonema mediopunctatumCream Cahills
2
6
Stenonema mexicanumCream Cahills
0
0
Stenonema modestumCream Cahills
10
51
Stenonema pudicum
1
6
Stenonema pulchellumLight Cahills
0
0
Stenonema terminatum
5
28
Stenonema vicariumMarch Browns
20
93
Species in Stenonema: Stenonema exiguum, Stenonema femoratum, Stenonema ithaca, Stenonema luteum, Stenonema mediopunctatum, Stenonema mexicanum, Stenonema modestum, Stenonema pudicum, Stenonema pulchellum, Stenonema terminatum, Stenonema vicarium
7 species (Stenonema bednariki, Stenonema carlsoni, Stenonema flaveolum, Stenonema lenati, Stenonema meririvulanum, Stenonema sinclairi, and Stenonema smithae) aren't included.