Stenacron, Acroneuria arenosa -- and other things at the Rivanna

On the first rock I turned over this morning -- at the Rivanna at Crofton -- there were 3-4 Stenacron nymphs; took me by surprise.  As you can tell by the wing pads on this one, it was fairly mature which made for some very nice photos.

The genus call on these nymphs -- Stenacron -- is fairly easy to make:  they're long and slender, and the gills on terga 1-6 are sharply pointed with the gill on segment 7 being no more than a string.  (See Steven Beaty, "The Ephemeroptera of North Carolina," p. 42)  Here's a photo of one that I found some years ago.

For that matter, the species ID isn't too difficult either.  This is Stenacron interpunctatum.  Two things give it away.  1) The "mid-dorsal streaks" on the abdomen are very distinctive, and 2) the "pale spot anterior to [the] median ocellus [is] approximately as long as wide [and] may be triangular to irregularly shaped."  (Beaty, p. 42)  Bingo.

Beaty notes that S. interpunctatum is "the most common and tolerant Stenacron species in NC": probably the same here in VA.  Still, I think it's one of the more attractive flatheaded mayflies.
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I also picked a couple of common stoneflies (Perlidae) both of which turned out to be Acroneuria arenosa.

I've gone back and forth on the species ID for these nymphs:  A. arenosa or A. evoluta?  The difference is slight.  While both have anal gills -- clearly visible in both of those photos -- the terga on A. evoluta are "uniformly brown" (Beaty, "The Plecoptera of North Carolina," p. 37), but on A. arenosa there's  a "narrow, pale anterior band" on each of the segments.  In the past, I've never seen that "pale, narrow band" so I've been tempted to go with A. evoluta.  But I could easily see it on both of these nymphs.

Acroneuria arenosa, I'm a believer.
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I made one other find, not something I've seen before in the Rivanna.

The dragonfly nymph -- Macromiidae.  Might have to key this one out to the level of genus.  This was a BIG, BIG nymph.
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Need rain!

Somehow I missed this: Clubtail dragonfly, genus Ophiogomphus

I was looking through my dragonfly photos this morning when it suddenly dawned on me that this nymph -- which I found in the Doyles on 9/2 of last year -- was not the clubtail dragonfly that I normally see.  The nymphs that I normally see have wingpads that are parallel to the body: they look like this.

Perplexed, I decided to key out both types and here's what I found.

17a  Naked antennal segment 4 generally about 1/2 as long as hairy segment 3..........Progomphus
17b  Segment 4 of antennae vestigal or nearly so.......18

We've got the latter as I'll show in a moment.  So on to 18.

18a Wingpads strongly divergent........... Ophiogomphus
18b Wingpads laid parallel along back........19

Here you go.

Ophiogomphus for sure.  Hmm...  But I'm quite sure that I thought the other nymph was Ophiogomphus.  Time to explore.  On to 19.

19a Body very flat: abdomen nearly circular in dorsal view; paired tubercles on top of head....... Hagenius
19b Abdomen more nearly cylindrical; no tubercles on head....... 20

19a describes genus Hagenius, and I've found quite a few of those over the years.

We move on to 20.

20a Antennal segment 3 flattened and oval, nearly as wide as long.... 21
20b Long antennal segment 3 more or less cylindrical....... 22

Definitely "nearly as wide as long."  On to 21.

21a Antennal segment 3 widest proximally..... Stylogomphus
21b Antennal segment 3 widest near middle, about 2 times as long as wide and rounded apically....Lanthus

Lanthus.  Got it.  And I find that one a lot.  But for me, Ophiogomphus is a new one.  Have to add that one to my list of taxa.

Macrostemum carolina: no doubt about it

It's the weirdest common netspinner (Hydropsychidae) we see in our streams and I've long wondered about the species ID.

Macrostemum is a netspinner that I've found a number of times in the North Fork of the Rivanna and just last year, I found one in the Rivanna at crofton.  I've posted entries on this one in July, 2011 and August 2012, where I've discussed the genus ID, but to review, there are two features that are fairly dramatic:  1) "the flattened head with a sharp U-shaped carina," and 2) the "dense fringe of setae on the fore tibia and tarsus."  (Glenn B. Wiggins, Larvae of the North American Caddisfly Genera, p. 110 in the 1977 edition)   These.

In the past, using Beaty's "The Trichoptera of North Carolina" (p. 77) I've resisted speculating on species ID, divided between M. carolina and M. zebratum.  The distinction is small: on carolina the tubercles near the eyes are large; on zebratum they're little.   But, our new key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- seems to resolve the issue in a definitive way.

115  Anterior tubercles above each eye conspicuous ........ Macrostemum carolina
115' Tubercles above each eye inconspicuous .......... Macrostemum zebratum

On the larvae I've found, those tubercles are very, very conspicuous.

Actually, no microscope photo is needed.

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I'd say that mystery's solved.  Macrostemum carolina for sure.

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.
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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.  

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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.

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.
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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!