Maccaffertium ithaca: the flatheaded nymph of the moment

While I've made my way out to some streams, I haven't posted an entry since the photos I've taken just aren't that good.  Cloudy skies.  Nonetheless, what I've been finding does warrant comment.

The flatheaded mayfly at the moment in a lot of our streams -- mid-sized streams -- is Maccaffertium ithaca.   That's one at the top of the page, photo taken on 9/4/13, and here's one of several I found at the Rapidan River on Sunday.

M. ithaca, as Beaty points out ("The Ephemeroptera of North Carolina," p. 19), is a "mountain taxon": we're finding them in the Moormans River, the Doyles River, and the Rapidan River.  I.e. they're in moutain streams, but not the small headwater streams that I visit in the winter and spring.  This is an important mayfly to fly fishermen to whom it's known at the "Light Cahill."  There are evening hatches throughout the summer (June through August).  I remember them well from my years in Vermont.

For a detailed description of the species ID, we use the well-known work of A. F. Bednarik and W. P. McCafferty, "Biosystematic Revision of the Genus Stenonema" (Bulletin 201 of the Canadian Bulletin of Fisheries and Aquatic Sciences).  See page 22.  Here's what they say.

Head -- Brown with pale markings; freckled, pale dots present; large pale areas laterad of compound eyes each divided by brown band extending from eye to lateral margin.  We can see those features in the photo at the top of the page.

They continue by noting the setal patterns on the mandibles and maxillae.  I can see those patterns, but I can't get really good photos.

Thorax -- Brown to grey-brown ... Fore femora with armature in apical 1/2--2/3 dorsal surfaces; posterior margins of fore femora with spinelike setae and hair setae; anterior margins with variable armature; hair setae, if present, shorter than hair setae on posterior margins.

In our picture, the spinelike setae and the long setae on the posterior margin of the fore femora is very clear: hard to see any setae on the anterior margin.

Fore tarsal claws usually not denticulate.

No "teeth" on the claws of this nymph though there are basoventral hooks.

Abdomen -- Dorsally light brown to greyish brown, ventrally pale yellow with dark maculations.  Terga with variable pale areas; at least some with lateral pale areas.

The lateral pale areas are easy to see on this nymph from the Rapidan River.

Sterna 2-8, 3-8, 4-8, or 5-8 with median, sinuate posterolaterally directed, transverse maculations with vertices reaching anterior margins of segments only on posterior sterna; transverse maculations often reduced on anterior sterna; 9 with pair of sublateral to lateral, oblique, dark brown bands, often connected anteriorly on 9, forming an inverted-U.  Lateral projections on segments 7-9 (rarely 6); those on segment 9 shorter than on 8 and shorter than ventral lateral margins posterior to projection bases.    That's a lot to look for, but you can see all of those features in the following photo.

Caudal filaments yellow basally, apically often with alternating light and dark segments.  Yes.

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I plan to focus on Maccaffertium nymphs this summer, and I hope to see a number of species.  But I need to get to some streams that aren't in the mountains.  Need a break from the rain.