This is the small minnow mayfly I found yesterday at South River, the one I ID'd as Baetis tricaudatus in terms of the species. This is a call that I'm always uneasy about, for a couple of reasons: 1) Steven Beaty ("The Ephemeroptera of North Carolina," p. 6) implies that B. tricaudatus and B. brunneicolor can be hard to tell apart, and 2) some of the features we need to see to distinguish the two are difficult for me to see with the microscope that I use.
So, I think it's worthwhile to look at the pertinent evidence. However, let me say at the outset that having looked at multiple sources this morning, I'm more certain than ever that this is indeed a Baetis tricaudatus nymph. It's a male (big, red eyes), fairly mature, but not yet fully mature. In the photo below is a fully mature female that I found at the Rapidan River last winter (2/24).
Let's begin by reading Beaty's descriptions of the two species in question, B. brunneicolor and B. tricaudatus. But first, here are some key photos to which we'll need to refer. All of these photos show anatomical features on the nymph that I found yesterday.
1. antennal scape and pedicel:
2. labial palpi:
3. tails -- showing the shading:
Off we go.
B. brunneicolor -- nymphs 6-10 mm; antennal scape and pedicel without robust setae; labial palpi with segment 2 less than twice as long as basal width of segment 3; inner margin of segment 2 concave and medial lobe moderately developed; pronotum uniformly shaded; paraproct surface without robust setae; tail shading gradual. Baetis brunneicolor CAN be separated from B. tricaudatus (with the lack of robust setae the best character) although most BAU records are likely B. tricaudatus. (Beaty, p. 6)
The size is all right: our nymph yesterday measured 6.5 mm (and males tend to be smaller than females). The presence or absence of setae (hairs) on the pedicel and scape is a problem for me. I think I can see some setae on the scape -- but I'd need greater magnification to decide this issue for sure. On the labial palpi -- segment 2 on our nymph is clearly much longer -- more than twice as long -- as the basal width of segment 3, the segment on which we find the "thumbs." So, one strike against this species ID. Are the inner margins of segment 2 of the palpi "concave"? Perhaps -- but slightly at best. Is the pronotum "uniformly shaded"? I might say that's true for the female in the pictures above, but I would not say it's true for the male. Tail shading gradual? Yes it is: well, at least the tips of the lateral tails are darker than the rest.
On the "paraproct surface" -- our nymph does seem to have "robust setae," though this is also a difficult call with the microscope view that I can get.
Conclusion: B. brunneicolor for our ID seems unlikely, but we'd have to leave the door open.
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B. tricaudatus -- nymphs 5-8 mm; distinct palpal "thumb"; antennal scape and pedicel with robust setae; gill margins without large robust setae and serrate; caudal filament shading gradual, less dark than in B. intercalaris; middle caudal filament less than half as long as lateral filaments. Primarily a Mountain species. (Beaty, p. 6)
Again the size seems right, at 6.5 mm. Are there distinct "thumbs" on the palpal lobes? There sure are: look at photo 2 above. Do the scape and pedicel have "robust setae"? We've already noted that this is hard to determine. I can see some setae -- I'd hardly call it "robust." Are the gill margins serrate and lacking large setae? Again, I have trouble determining this with the microscope that I use, but the edges do look serrated to me. This is the best I could do for a photo.
We've already noted that the caudal filament (tail) shading is gradual, and the middle filament is indeed less than half as long as the lateral filaments. (Look again at out initial photos of the male and female nymphs.)
Conclusion: B. tricaudatus is the likely ID, but I'd sure like a good look at the pedicels and the scapes.
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Now, part two. Let me turn to two other sources. First, a look at the descriptions of both of these species in Knopp and Cormier, Mayflies: An Angler's Study of Trout Water Ephemeroptera, pp. 48-49. 1) Knopp and Cormier note that "The mature nymphs of...B. brunneicolor and B. tricaudatus are recognized by the presence of faintly striped abdominal tergites (pale striped marking along the dorsal or topside region of the abdomen.) That faint stripe is very clear on both the male and female nymphs in our photos at the top of the page, but since such a stripe is also present on B. brunneicolor nymphs, we can't use that as a definitive feature. But Knopp and Cormier make an important distinction on p. 49.
B. tricaudatus: [abdomen] -- "grayish olive thorax and olive brown abdomen with abdominal segments 5, 9, 10 paler: rounded 7th gill," [and tails] -- "3, grayish olive and lacking banding."
A very good description of our nymphs, especially of the male I found yesterday.
There may be dark tips on the tails, but there is no "banding." For "banding," look at this photo of a B. intercalaris nymph. Dark bands at the base, middle and tips of the tails.
By way of contrast, Knopp and Cormier say the following things about B. brunneicolor:
[abdomen] -- "brown with a faint pale dorsal stripe," [but, tails] "3, banding at base and tip." Aha!
So on B. brunneicolor tails, banding can be detected.
Using Knopp and Cormier, I'd readily conclude that we're looking at B. tricaudatus nymphs.
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One final source. Donald Chandler has posted photos of both of these species on "Discover Life."
I can't show you those photos without getting permission -- but you can look at them on your own.
The difference in the tails is very clear.
B. tricaudatus: http://www.discoverlife.org/mp/20p?see=I_DSC215&res=640
B. brunneicolor: http://www.discoverlife.org/mp/20p?see=I_DSC207&res=640
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That does it for me. I'll no longer hesitate to call these B. tricaudatus nymphs.
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