Yes, another rainy, gray, November day in Charlottesville.
I've mentioned in recent entries that the one mayfly I continue to find in almost every stream that I visit is the flatheaded mayfly, genus Maccaffertium. The one in the photo above was found at the Rapidan River last week. I've also mentioned (entry posted 10/6) that Maccaffertium species ID is no easy thing. But, it's a good day for microscope work, and there are two nymphs that I've found recently, both in very good streams, that look to me like they might be the same species. So, I'd really like to nail down what species that is. I'll give it a try in this entry, but you must keep in mind my "amateur" status -- my conclusions might be totally wrong.
The two nymphs I want to look at in greater detail are the one in the photo above, and this one, found on Saturday, 11/5 in the upper Doyles River.
I was struck right away with one feature they share: tergites 7, 8, and 9 on both nymphs are orange; tergite 6 is dark brown. Now, the two nymphs might not be the same species. But at the moment I think there's a good chance they are, and I've narrowed it down to either M. mediopunctatum or M. meririvulanum, both of which are found primarily in mountain streams.
What anatomical features do these nymphs have in common, features that point to the two species I've mentioned? There are five things at which we have to look, beginning with the maxillary crowns (put the nymph on its back and remove the labium).
Rapidan Mac:
I count 6 spines on the top edge of the maxilla; there are no "hairs" that follow the spines. The same is true on the nymph from the Doyles River -- 6 spines (there are hairs on the maxilla, but not on top.) The lack of hair on the crown of the maxilla eliminates about half of the Maccaffertium species we might expect to see in this part of the country (North Carolina has 15) from contention.
Doyles River Mac:
Secondly, if you look at the claws on the legs of the nymphs, there are no "teeth," no "denticles" (they're "edentate").
Rapidan Mac:
Doyles River Mac:
True, there is a "spur" on the tarsal claw, but that is not the same -- I think I'm right in saying this -- as denticles/teeth.
Number three, we have to determine if there are "lateral projections" (spiny points) on the abdominal segments preceding segment 6. In our case there are; were they not present we'd be looking at some different species.
Rapidan Mac:
Doyles River Mac:
The other things that are sometimes relevant to Maccaffertium species ID are the ventral and dorsal patterns. With M. meririvulanum, it's the dorsal pattern that matters. On Tergite 5 in nymphs of this species there is a pale colored "V"; this can also be seen on tergite 7, or even 7-9. I could convince myself that we do have "V's" on both of our nymphs. And keep in mind that these nymphs are still immature: colors and patterns are not yet fully developed.
Rapidan Mac:
Doyles River Mac:
But M. mediopunctatum nymphs have dark ventral bands on segments 2-8 and an inverted "U" on segment 9. Those features seem to be very pronounced on our nymph from the Rapidan River; not so much on the Doyles River nymph. (Have another look at the photos above showing the "lateral projections.") That could mean that we are looking at two different species (with a lot of anatomical features in common); it could also mean, however, that the nymph from the Doyles River is not as mature as the Rapidan nymph. Again, the ventral colors and patterns are not as developed.
Well, my results are inconclusive. But I do think M. mediopunctatum and M. meririvulanum are two good choices for identifying these fall/winter nymphs. And at least we were able to see what the experts have to look at when they work on Mac ID.
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