Saturday, August 12, 2017

Working on species ID for our Leuctrids: they appear to be Leuctra sibleyi

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 another.

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.

Monday, August 7, 2017

Our common pronggilled mayfly appears to be Paraleptophlebia assimilis

One of the things that our new key has helped me ID is the pronggilled mayfly in the photo above.  This can now be ID'd with some confidence as Paraleptophlebia assimilis.  It's the most common pronggilled nymph that I see.  This one was found in South River (Greene County), but the Rapidan River is filled with these nymphs in the winter and spring.  And it seems to be a winter/spring species: I have photos of this one from December (young) to April (fully mature).

We can confirm this ID with our new key, and find the nymph fully described in the article by Randolph and McCafferty -- "First Larval Descriptions of Two Species of Paraleptophlebia," Entomological News 107 (1996), pp. 225-229.  We can also confirm it with a quick and easy deduction.   There are only four species of pronggilled mayfles with "branched" gills (very clear in our photos): swannanoa, assimilis, adoptiva and mollis.   Swannanoa is out -- it hasn't been found in Virginia.  As for adoptiva and mollis, both have posterolateral projections on segment 9, but not on 8: assimilis, on the other hand, has projections on both 8 and 9.  That's what we've got.

Now lets look at our key:  Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, p. 140.

263 Gills 2-7 forked at one-fourth or more length from base; gill trachea with distinctly pigmented branches at least in unforked region..........264
263' Gills 2-7 forked near base, usually not more than one-sixth length from base; gill trachea without distinctly pigmented branches, but often with very short and faint lateral branches.........267

Gills on our nymph fork about 1/3 the way down from the base, and, as we've seen, they're clearly branched above the fork.

On to couplet 264.

264  Mandibles relatively elongate, with about half length of angulate (left) mandible beyond angulate shelf.........Paraleptophlebia swannanoa
264' Mandibles not elongated as above, with less (usually much less) than half length of angulate mandible beyond angulate shelf.....265

At the moment, I don't have a specimen in my reference collection, so I can't examine this feature.  (I'll remedy this in the winter.)  However, as we've already seen, it can't be swannanoa since that species doesn't occur in our state.  Swannanoa has only been found in NC, SC, and GA.  On to 265.

265  Posterolateral projections present on abdominal segments 8 and 9.....Paraleptophlebia assimilis
265' Posterolateral projections present on abdominal segment 9 only ...... 266

Bingo.  P. assimilis.

Now let's look at the description in Randolph and McCafferty (pp. 225-227).  Unfortunately, I can't go through their description in a lot of detail: again, I don't have a specimen that I can examine.  But, there are features that help to confirm our ID.

1. "Head capsule brown with small, oval, pale medial spot bewtween antennal bases."  "Thorax brown with pair of pale, circular spots medially on mesonotum anterior to forewingpad bases."  Yes.

2. "Abdominal terga brown with pale, oval area sublaterally on terga 2-8, and with white, oval spot medially within posterior third to one-half and sometimes extending anteriorly on terga 2-8 or 2-9."  Here are the sublateral ovals.

But I think the medial ovals show up better on this mature specimen that I found in 2015.

I can't say that I'm crazy about calling those ovals, or ignoring the pale crescent shaped marks on some of the terga, but since everything else is a fit, I'm still content to accept our ID.  Paraleptophlebia assimilis.  However, I'll come back to this ID in the winter when I can examine the morphological features that Randolph and McCafferty describe.

What about this other pronggilled nymph that I found in May, 2012, also up at South River?  (Must be a spring/summer species, much like guttata and strigula.)  Any help from our new key?

Not really.  At the first couplet (see above, 263- 263'), we'd clearly move to 267 since these gills are forked near the base and lacking in pigmented branching.  But that leads us into a measurement of the maxillary palps, something I can't do without finding another nymph next spring.   That being said, I'm leaning towards an ID of Paraleptophlebia jeanae, based on the description in Randolph and McCafferty.

"Abdominal coloration variable: abdominal tergum 1 brown; terga 2-9 often with paired crescent-shaped pale markings submedially on each tergum; submedial tergal markings less often coalescing, forming larger pale area in posterior area of terga .... tergum 10 brown, pale medially." (p. 227)  Pretty good match.  (crescent-shaped marks on 4-6; coalesced on 7 and 8)

But two of the nymphs my friend found in a small stream in Sugar Hollow fill the bill even better.

Time will tell if this diagnosis is right.  Just have to find some more of these nymphs next spring!

Thursday, August 3, 2017

Is it possible now to identify Perlesta nymphs to the level of species?

I was pleased to see that in our new key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- six species of Perlesta (Perlidae, common stoneflies) have been described: shubuta, cinctipes, decipiens, teaysia, frisoni, and adena.   At last, I can see which species we have in our streams in May and June.

Let's work on that nymph in the photo at the top of the page, one that I found in Buck Mt. Creek.  The key begins in this way (p. 224).

120  Abdominal terga with dark, freckle-like spots around bases of intercalary setae (fig. 3.245)......121
120'  Abdominal terga without freckled appearance.....123

Lots of dark freckles on this one.  We proceed to 121.

121 Dark transverse pigment band through ocellar area bearing scattered dark spots (Fig. 3.246) .... Perlesta shubuta
121' Dark transverse pigment band on head uniformly colored, without darker spots (Fig. 3.247) .....122

No question that there are scattered dark spots in that band on our nymph.

Pretty darn easy.  However, to date, this species has only been found in AR, IA, IL, KY, MI, MO, MS, and OK -- i.e. not in the state of Virginia.  Still, it's shubuta that our key yields, and I've found nymphs that key out the same in the Rivanna,

and in the Doyles.

Great!  But what about those Perlesta that I've found in the small streams in Sugar Hollow?  Can we ID them as well?   Here's a pic of one of those nymphs.

If you look closely you'll see that there are some freckles on the abdominal terga, but nothing like our nymphs from Buck Mt Creek, the Rivanna, and the Doyles -- i.e. it doesn't have a "freckled appearance."  So, I think we should go from couplet 120 to couplet 123.

123  Pronotum with a pair of dark parenthesis-shaped bars (Fig. 3.249); head without pale M-line .... Perlesta teaysia
123' Pronotum without dark parenthesis-shaped bars (Fig. 3.251); head with pale M-line, but sometimes incomplete ....124

The dark band on our nymph goes completely around the pronotum, it's not limited to the "parenthesis" marks on the lateral edges, and, the M-line on this nymph is clearly present and well-defined.  On to 124

124 Most of dark pigment on head restricted to area of frons anterior to lateral ocelli (Fig. 3.251); known from the Appalachians and areas east of the mountains ... NC, SC, TN, VA, WV .....Perlesta frisoni
124' Dark pigment on head coverng most of frons anterior to ecdysial suture; known from the Cumberland Plateau and areas to the north and west ... IN, KY, OH, TN .....Perlesta adena

The dark pigment is clearly restricted to the "area of [the] frons anterior to [the] lateral ocelli".  We've got Perlesta frisoni.

Same ID for another nymph from the same stream.

So our new key allows us to identify at least some of the Perlesta we find in our streams.  Very exciting news.  But when I asked Steve Beaty about this this morning, he said it's not safe to use this key.

That sent me back to his descriptions in "The Plecoptera of North Carolina,"  (pp. 48-49)  where I found the following statement.

Notes: North Carolina has very high diversity of Perlesta, most of which are currently undescribed in the immature stage.  While many nymphs appear highly freckled, some nymphs have few or inconspicuous freckles.  It is inadvisable to use a new Perlesta key (Stark, 2016 in press) [that's the key we've been citing] as it contains only 6 of the 17 regional species (two of which are not known from NC).  Preliminary use of the key has led to immediate failure for most NC specimens.  It is not uncommon to get 2-3 nymphal habitus forms at one site.

So that's that. Have to follow his advice and leave our Perlesta ID at the level of genus.  Probably stuck at that level for a long time to come.