Our new key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- keys out 8 different species of Leuctra (family Leuctridae: genus Leuctra), so I thought I'd see if I could ID the specimens I have in my vial, including the nymph at the top of the page, one that I collected this spring. As with a number of taxa that we find in our streams, not all species are covered in this key, so any conclusions we reach are bound to be tentative. But let's see what we have.
I'd best begin by verifying that our nymph is indeed genus Leuctra. It's fairly easy to do using this key (pp. 184-186). We can eliminate Megaleuctra which has "body form robust": all of the Leuctrids I find have "body form slender." We can also take Zealeuctra out of the mix: Zealeuctra is "without long setae on [the] corners [of the pronotum]"; those setae are present here.
That leaves Paraleuctra or Leuctra. On Paraleuctra nymphs "abdominal segments 1-6 [are] divided by a ventrolateral membrane" and the "mesosternal Y-arms [have a] double stem and [a] median dark band"; on Leuctra, the ventrolateral membrane only divides segments 1-4, the Y-arms have a single stem and they lack the dark median band. (p. 186)
We have a Leuctra.
Good. We can proceed. Our first couplet reads:
79 Cercal segments bearing prominent, long, bushy whorls consisting of 8 or more lateral setae on most segments.....Leuctra sibleyi
79' Cercal segments with less prominent whorls usually consisting of 5 or fewer lateral setae on most segments..........80
Since I have a number of nymphs preserved in a vial, I just grabbed one and looked, assuming all of my nymphs would key out to the same species. Here's what I saw.
And a third.
Just no doubt about it. Those are "long bushy whorls" with more than 8 lateral setae. End of story: our Leuctrids are Leuctra sibleyi.
But to be sure of our nymph at the top of the page, I looked at those cerci as well.
Certainly not "prominent, long, bushy whorls of setae." I'll proceed on that assumption to see where it leads, but not without noting before I begin that on closer inspection, I do think there are more than 5 setae in each of those whorls: I think I can see 8-10. So in the end, this one is probably L. sibleyi as well. But let's explore option one.
If there are "5 or fewer lateral setae" in the cercal whorls of our nymph, we proceed to couplet 80.
80 Abdominal terga 7 to 10 or 8 to 10 covered with short, stout hairs, more anterior terga with hairs reduced.......... 81
80' All abdominal terga covered with short, stout hairs......82
I see the former, but I admit I'm largely going by what I can see at the sides of the terga: this.
There are clearly short, stout setae on terga 7-10; none on 6 or 5. So I went on to couplet 81.
81 Mesonotum with numerous short, stout setae on anterolateral angles......Leuctra tenella
81' Mesonotum with few short, stout setae on anterolateral angles.......Leuctra truncata
Judging by that, I'd call this Leuctra truncata.
Worth noting as well that L. tenella has not been found in VA; L. truncata is here.
At the moment, the safe thing to say is that many of the Leuctrids I find are -- using this key -- Leuctra sibleyi in terms of the species. Some could be Leuctra truncata. But there is a third possibility as well, one that I find very appealing. The nymph that could be L. truncata could also be a species for which the nymph has not yet been described, one not associated with any adults. For that species, the setae on the cercal whorls number more than 5 in lateral aspect, but they're on the short side, not long, and they're also far from bushy.
A final note for those who monitor streams. I'm sure that all of you have struggled with distinguishing Leuctrids (rolled-winged stoneflies) from Capniids (small winter stoneflies) in your samples, especially when dealing with very small nymphs. The usual feature we use -- the ventrolateral folds. On Capniids, those folds are present on segments 1-9: on Leuctrids, at the most, on segments 1-7. E.g. here's a Capniid:
And those on our Leuctrids only present on 1-4.
But as you know, those folds are not always clear. Our new key might be a help in this regard, providing another distinction.
8 Mentum [sic. should be submentum] small, not extending over base of maxilla....Capniidae
8' Mentum [= submentum] extends forward partially covering inner margins of maxillary base......Lecutridae
A nice view of this feature is also provided -- Fig. 3.29 on p. 183. But here are two photos I took.
That's pretty clear. We might actually use the shape of the submentum to distinguish the two.