That's an excellent explanation, and I copied it and put it in the info section for the species. Thanks again for all your help.

Dark bars across the sterna that become thicker and more distinct toward the rear of the abdomen are typical of this species. Sometimes these bars develop fainter (somewhat triangular) projections extending toward the anterior edge of the segment, but these are nearer the sides of the abdomen than the more distinct projections often found in sterna 8/9 of luteum. Your excellent photos also show the long posterolateral spine on segment 9 (as long or longer than the one on Ab8 in vicarium). That spine is shorter in luteum, and luteum is usually 10mm or less at maturity.

The dark ventral markings at the rear of the abdomen show one of several patterns that are found in this species. These were sometimes used as species distinctions before vicarium, fuscum, and rivulicolum were synonymized and show variously as:

1. A solid dark bar (classically attributed to vicarium).
2. Three connected dark blotches (sometimes attributed to rivulicolum).
3. As 1 or 2 above, but with a pale spot near the base of each cercus.
4. Three separate dark blotches, usually with the middle blotch somewhat smaller or paler (as in your specimen). McDunnough mentions this variation in his 1933 description of the nymph of Ecdyonurus fuscus (=Stenonema fuscum=Maccaffertium vicarium), and I see it regularly in both Eastern and Midwestern specimens.
5. Two dark lateral blotches (classically attributed to fuscum).

In addition, the immature specimens that you collected earlier in NH (176293 & 176294, 176295 & 176296, 176298 & 176299) are also vicarium. They reflect McDunnough's observation that (unlike luteum) immature specimens tend to be darker in coloration than more mature ones.

I would ordinarily expect M. vicarium, but it just doesn't look quite right. The ventral shot should be very helpful, but the color pattern is midway between the typical pattern for each species.