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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It's only barely visible in one of my pictures, but I confirmed under the microscope that this one has a prosternal horn and the antennae are mid-way between the eyes and front of the head capsule.
I'm calling this one Pycnopsyche, but it's a bit perplexing. It seems to key definitively to at least Couplet 8 of the Key to Genera of Limnephilidae Larvae. That narrows it down to three genera, and the case seems wrong for the other two. The case looks right for Pycnopsyche, and it fits one of the key characteristics: "Abdominal sternum II without chloride epithelium and abdominal segment IX with only single seta on each side of dorsal sclerite." However, the characteristic "metanotal sa1 sclerites not fused, although often contiguous" does not seem to fit well. Those sclerites sure look fused to me, although I can make out a thin groove in the touching halves in the anterior half under the microscope. Perhaps this is a regional variation.
The only species of Pycnopsyche documented in Washington state is Pycnopsyche guttifera, and the colors and markings around the head of this specimen seem to match very well a specimen of that species from Massachusetts on Bugguide. So I am placing it in that species for now.
Whatever species this is, I photographed another specimen of seemingly the same species from the same spot a couple months later.
I'm calling this one Pycnopsyche, but it's a bit perplexing. It seems to key definitively to at least Couplet 8 of the Key to Genera of Limnephilidae Larvae. That narrows it down to three genera, and the case seems wrong for the other two. The case looks right for Pycnopsyche, and it fits one of the key characteristics: "Abdominal sternum II without chloride epithelium and abdominal segment IX with only single seta on each side of dorsal sclerite." However, the characteristic "metanotal sa1 sclerites not fused, although often contiguous" does not seem to fit well. Those sclerites sure look fused to me, although I can make out a thin groove in the touching halves in the anterior half under the microscope. Perhaps this is a regional variation.
The only species of Pycnopsyche documented in Washington state is Pycnopsyche guttifera, and the colors and markings around the head of this specimen seem to match very well a specimen of that species from Massachusetts on Bugguide. So I am placing it in that species for now.
Whatever species this is, I photographed another specimen of seemingly the same species from the same spot a couple months later.
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This topic is about the Mayfly Species Stenonema mediopunctatum
I have seen no mention of this species as a fishable hatch in angling literature, but it is noteworthy for the unusual coloration of the male spinners, which Malcolm and Knopp describe in Mayflies: An Angler's Study of Trout Water Ephemeroptera :...the male only exhibits this coloration on segments eight to ten, against an otherwise whitish to translucent body.
Example specimens
GONZO on Sep 1, 2012September 1st, 2012, 11:39 am EDT
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
Troutnut on Sep 2, 2012September 2nd, 2012, 11:54 pm EDT
Thanks for the detailed rationale, Lloyd! I moved those three to ithaca.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
Troutnut and salmonid ecologist
GONZO on Sep 3, 2012September 3rd, 2012, 9:34 am EDT
My pleasure, Jason. While on the subject of M. ithaca, I also noticed this male dun in the M. terminatum section that I suspect is probably another example of M. ithaca:
http://www.troutnut.com/specimen/780
Despite its coincidence with good examples of male and female terminatum adults, the overall appearance of this specimen is a good match for ithaca (cf. #733 and #765 above). Although there can be some dark-to-light variation in the subs/adults of the Eastern terminatum subspecies, I don't think they ever have the mid-dorsal stripes. When dorsal markings are apparent in darker terminatum duns, they usually consist of of a narrow band across the posterior margin of the terga or some suggestion of the twin sublateral dashes representative of the "bipunctatum" synonym. The femoral banding of M. terminatum subs/adults usually tends to fade into obsolescence on the rear legs, and they often have pale whitish unmarked (or faintly marked) tails. Although both of the following examples are of pale Western (female) terminatum duns, they show these (leg/tail) tendencies:
http://www.troutnut.com/specimen/886
http://bugguide.net/node/view/685336/bgimage
In addition, the eye separation in the male dun that I am proposing for ithaca (#780) is close to the width of the lateral ocellus while the separation of the male terminatum imago (http://www.troutnut.com/specimen/777) is considerably wider (which is typical of that species). I suspect that the the eye spacing on the dun will probably close slightly upon molting to the imago, but I don't think that space would ever widen.
http://www.troutnut.com/specimen/780
Despite its coincidence with good examples of male and female terminatum adults, the overall appearance of this specimen is a good match for ithaca (cf. #733 and #765 above). Although there can be some dark-to-light variation in the subs/adults of the Eastern terminatum subspecies, I don't think they ever have the mid-dorsal stripes. When dorsal markings are apparent in darker terminatum duns, they usually consist of of a narrow band across the posterior margin of the terga or some suggestion of the twin sublateral dashes representative of the "bipunctatum" synonym. The femoral banding of M. terminatum subs/adults usually tends to fade into obsolescence on the rear legs, and they often have pale whitish unmarked (or faintly marked) tails. Although both of the following examples are of pale Western (female) terminatum duns, they show these (leg/tail) tendencies:
http://www.troutnut.com/specimen/886
http://bugguide.net/node/view/685336/bgimage
In addition, the eye separation in the male dun that I am proposing for ithaca (#780) is close to the width of the lateral ocellus while the separation of the male terminatum imago (http://www.troutnut.com/specimen/777) is considerably wider (which is typical of that species). I suspect that the the eye spacing on the dun will probably close slightly upon molting to the imago, but I don't think that space would ever widen.
Entoman on Sep 4, 2012September 4th, 2012, 6:42 pm EDT
Thanks for the detailed analysis, Gonzo. VERY helpful for future reference.
"It's not that I find fishing so important, it's just that I find all other endeavors of Man equally unimportant... And not nearly as much fun!" Robert Traver, Anatomy of a Fisherman
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