The new book on EPT of the southeast -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species (Clemson, 2017) -- provides us with a new key on the species of the flatheaded mayfly, genus Leucrocuta, so we can re-examine, with benefit, the four nymphs that we've found in local streams. The ID's that I've arrived at to date are L. hebe, L. thetis, L. aphrodite, and L. juno, that by using three different sources: 1) the key provided in Unzicker and Carlson, "Ephemeroptera" in Aquatic Insects and Oligochaetes of North and South Carolina (pp. 3.72-3.73, 1982), 2) photos provided on the website Discover Life (http://www.discoverlife.org/mp/20q?search=Leucrocuta), and 3) Knopp and Cormier's Mayflies: An Angler's Study of Trout Water Ephemeroptera (pp. 149-152). Let's see how those ID's hold up.
1. Leucrocuta hebe.
I just posted about this ID on 9/7, no need to repeat what I've already said. This keys out as L. hebe in all of my sources, including the new key. (Also, look at the photo on Discover Life -- http://www.discoverlife.org/mp/20q?search=Leucrocuta+hebe." The ID seems firm.
2. Leucrocuta thetis. (pictured here and at the top of the page)
Let's begin with the key in Unzicker and Carlson (p. 3.72). "Trachea of gills indistinct; pale markings on dorsum of abdominal segments 7 and 8 not coalesced; head without pale spots along frontal margin; lateral dark streaks present only on venter of abdominal segment 9."
All true, though I can't discount a bit of a dark streak on venters 8 and 10. Now here's what the new key has to say. "Gills darkened in upper half above midline of gill, with darkening sometimes extending over apex on both sides; abdominal tergum 9 brown; marginal pale areas on head present or absent." (p. 108) Yes, tergum 9 is brown, and in this case pale areas on the head margin are not present. Are the gills darkened in the "upper half above [the] midline of the gill"? Yes! But the darkening does slip over the midline near the the pointed tip.
Think we can stick with the L. thetis ID.
3. Leucrocuta aphrodite.
Unzicker and Carlson described L. aphrodite like this: "Dark areas on femora usually more extensive than light areas; dark areas of posterolateral and anterolateral angles of venter of segment 9 complete or almost complete, forming a dark margin, not divided into posterior and anterior areas."
Well, no question about the femora, but that's as far as I can go since I don't have a specimen in my collection at the moment so I could look at the venter. Knopp and Cormier claim that "tergites 9-10 [are] paler," which I don't see. Rather, the pale spots I normally see are on terga 1, 7, and 8 and a little on 9, as on this nymph.
Not sure where I got my ID, though I can say that other photos I've found of L. aphrodite match up with mine. (http://bugguide.net/node/view/1356607, and http://bugguide.net/node/view/1357415) How about our new key? Actually, this confirms our ID more than previous sources. We get to L. aphrodite by moving from "gills without distinct tracheation" to "gills with distinct tracheation [and] "pale marginal areas present on head" (from couplet 195 to 197). The alternative to 197 (i.e. 197') is "Head with pair of oblique dark bars between compound eyes; abdominal terga 2-9 with posterior margins with pair of purple-black transverse lines extending sublaterally...." the description goes on, but it clearly doesn't apply. We follow 197. (197', by the way, leads to L. minerva, a species I haven't seen.)
Couplet 197 leads us to a long paragraph on Leucrocuta aphrodite, L. hebe, L. maculipennis, and L. juno in which we find the following note applying to L. aphrodite: "L. aphrodite and L. hebe may have a pale medial V-shaped area on abdominal terga 7, 8, and most of the medial length of 9, or on tergum 9, three connected or unconnected pale spots." (p. 110)
Well, it's a bit of a stretch to say those markings are "V's" on terga 8 and 9, but for me that's close enough given everything else that we've seen.
I'm satisfied with L. aphrodite. Three for three on our ID's.
4. Leucrocuta juno.
Same insect in various stages of development. Once again, let's start with Unzicker and Carlson (pp. 3.72-3.73). "Gills heavily shaded with purplish-black with paler areas at extreme tip, on median space next to main trachea, and near base of outer margin; dark markings at anterior margin of sternites 8 and 9, in median."
Okay on the gills, but I can't see any "dark markings at [the] anterior margin of sternites 8 and 9, in median." So, not a whole lot to go on. However, the nymphs in the photos above do match the L. juno nymph pictured on Discover Life (http://www.discoverlife.org/mp/20q?search=Leucrocuta+juno).
But this time our ID is not confirmed in our new book. Let me quote what we find there on L. juno: "Diagnostics for the L. juno larva are unknown. It is unclear if larvae have been accurately associated with identifiable male adults, and thus the larva only is assumed to be similar to this grouping (i.e, the grouping of L. aphrodite, L. hebe, and L. maculipennis, and L. juno), and according to previous keys, to L. maculipennis in particular." Actually, the nymphs that I've found -- those I've been calling L. juno -- key out pretty well to what our new book labels Leucrocuta sp. 1 (p. 108). "Gills darkened along midline on both sides, then becoming infuscated, more so in the upper half of gill; abdominal terga, including 9, with extensive pale areas medially; marginal pale areas not present on head." Yes indeed, the gills are dark on both sides of the midline this time, turning blackish toward the margins, and there are "extensive pale areas medially, on the terga" (though much smaller on segments 8 and 9.
L. hebe, L. thetis, L. aphrodite, and possibly L. sp.1 -- that's where things stand at the moment. One final note, I suspect that the tolerance values will differ by species if anyone gets around to working that out. I've found L. hebe in medium streams to large rivers -- the Doyles, Buck Mt. Creek, and the Rivanna to be specific -- L. aphrodite, to date, I've only seen in Buck Mt. Creek. L. thetis and L. sp.1 (if that's what it is), on the other hand I've only seen in the high quality small mountain streams in Sugar Hollow.