The Skwala Perlodid stonefly: Back -- briefly -- to the Northwest

Just back from Oregon where we've been visiting our daughter in Portland.  On Sunday, we went to the North Fork of the Lewis River in southern WA where I just couldn't resist looking through leafpacks and turning over some rocks.  I found a number of stoneflies, all the same -- the Perlodid stonefly, genus Skwala: S. americana or S. curvata, I'm not really sure which.  You may recall that I found one in Montana in August as well, and as I noted in the entry I posted, the "Skwala hatch" is a major hatch in the Northwest in March and April.

For identification I've used Stewart and Stark, Nymphs of North American Stonefly Genera, pp. 448-451 (good illustrations on p. 450).   There they describe S. americana -- but remember they're describing a nymph that's fully mature: the nymphs I found were still very small.

1. head

"Head dark with lighter M mark forward of anterior ocellus and incomplete light mesal bar; 2 oval, light occipital spots inside eyes, broken with reticulate dark lines and bordered behind by narrow band of stout spinules."  Works very well.

2. lacinia

"Lacinia bidentate with tuft of setae on a low knob below subapical tooth and continuous inner row of marginal setae."  You bet.

3. mesosternal ridge

"Y-arms of mesosternum reach anterior corners of furcal pits."  For sure.

I hope I get to see some mature nymphs some spring.  But even these immature nymphs were colorfully patterned.

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I only had a short time to look for some insects, but I also managed to find a few mayflies -- the small minnow mayfly, Baetis tricaudatus, and what appeared to be a small Leucrocuta flathead.

1. B. tricaudatus, male

(and check out those eyes!)

Baetis tricaudatus, female

2. And the small Leucrocuta

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(Lucia Falls on the North Fork of the Lewis.  Spectacular!)

Off to Sugar Hollow: The first Perlodid stonefly of the young season, Malirekus hastatus

This is not something I expected to find in Sugar Hollow this morning, but it was the very first insect that I picked up.  The Perlodid stonefly, Malirekus hastatus.  The small stream that I went to is a stream in which I've found them before.  This is one that I collected there on February 13th, 2012.

Gorgeous!  But it's also a "killer."  Don't put these in a tray where you're keeping small mayflies!

A quick review of how this one is ID'd.  "Nymphs 15-19 mm; distinctive pale "M" pattern on head; conical submental gills; triangular lacinia with low marginal knob bearing a tuft of setae and venral surface with a cluster of approximately 50 clothing hairs near base; single curved row of spinules on back of head, obsolete near midline."  (Steven Beaty, "The Plecoptera of North Carolina," p. 26)  Those features look like this.

1. head pattern (for a detailed description, see Stewart and Stark, Nymphs of North American Stonefly Genera, pp. 416-417)

2. conical submental gills

3. triangular lacinia

4. clothing hairs on ventral lacinial surface

5. and the row of spinules on the back of the head

So, the Perlodid stoneflies are here -- my favorite time of the year!

But I was in for another surprise when I found this very small mayfly.

It's the spiny crawler, genus Eurylophella.  What on earth is it doing here now?  This is an insect I expect to see in the spring, but here it is in October.  The key feature marking the Eurylophella genus ID is already easy to see: the large operculate gills on segment 4, the gills that cover those on segments 5-7.

Given its size -- 2.5 mm -- I doubt that I'll be able to determine the species.  The species I see in Buck Mt. Creek is E. verisimilis.  But my friend who explores these small streams in Sugar Hollow in a regular way has found E. funeralis as well.  I'll have to give the ID a try.
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What else?

1. Giant stoneflies, Pteronarcys proteus.  They are plentiful in this stream, and some, like this one are starting to get pretty big.  (Note small "Roach-like" stonefly [Peltoperlid] riding on its side.)

2. Common stonefly, Eccoptura xanthenses.  Also a species we commonly see in these small mountain streams.

3. And a few common netspinner caddisfly larvae: Diplectrona modesta and Hydropsyche betteni.  (Note the low, rounded tubercle at the back of the head on H. betteni.)

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Perlodid stonefly, Malirekus hastatus.

Hitching a ride -- the first spiny crawler (Ephemerella subvaria) of the season at the Rapidan River

It's the most colorful spiny crawler we see -- and at 8.5-9.5 mm -- one of the largest, and the only place that I've seen it is the Rapidan River up in Madison County: Ephemerella subvaria.  Beaty says that it's "collected summer through fall"..."mountains only"... and that it's "rare."  ("The Ephemeroptera of North Carolina," p. 28.)  I see them in fall through winter, and this is about as small as they get.

This one was collected last year on September 22:

This one on December 6:

And these -- male and female -- on March 29:

I'm glad that the season has started, and I look forward to seeing them more and more on future trips.
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With this "tiny spiny" as the exception, the insects I saw today were much the same insects I saw on my last trip: Giant stoneflies, Common stoneflies, Humpless casemaker caddis, some flatheads, common netspinners, and fingernets.  Oh, and the occasional small minnow mayfly -- looked to be B. intercalaris.

1. Giant stonefly, Pteronarcys biloba.  Getting bigger and bigger.

2. Giant stonefly, Pteronarcys proteus.

And next to a recently molted E. vitreus flathead.

3. Common stonefly, Acroneuria abnormis.  (Note how different this is from the one that I found yesterday in the Rivanna.)

4. And one of the many Brachycentrids (humpless casemaker caddisflies) that I saw clinging to rocks.  Brachycentrus appalachia.

The common stonefly, Acroneuria abnormis, Rivanna River style

The Rivanna River is my "river of choice" at the moment since there isn't much water in a lot of the streams that I visit.   In the spring and summer, we had way too much rain; now we're getting nothing.  But the Rivanna River has water.

I saw the same insects this morning that I've seen in other recent visits to the Rivanna -- common stoneflies (Acroneuria abnormis and Agnetina annulipes), Giant stoneflies (Pteronarcys dorsata), a sprinkling of small minnow mayflies (Baetis intercalaris, mostly), and the occasional fingernet caddisfly larva.  This lack of variety is common for this time of year, but before long we'll start to see large winter stoneflies in here (Taeniopteryx burksi/maura) and some the mayflies of spring.

The A. abnormis common stones that I see in the Rivanna, you may recall, are different than those that I see in smaller streams that are in or close to the mountains.  Most A. abnormis nymphs that we see have a well-defined yellow "M" on the head -- which is sometimes "interrupted" -- and the abdominal terga are banded; the posterior margins are light, the anterior margins are dark.    As an example, we can look at the nymph that I found last week in the Rapidan River.

Every A. abnormis nymph that I've seen in the Rivanna is "type B," as it were; the terga are brown with no banding, and there is no "M" on the head -- at least not an "M" that we can see.   (Note that there is an "M" crease or indentation on the head, it's just that it's not colored yellow.)

There is something I want to explore, and I'll let you know if I find anything.  Could it be that the "type B" that we see in the Rivanna is a sub-species that is somewhat more tolerant of warmer water than those with the well-defined M?   Could it be that habitat is relevant to the issue of which type of A. abnormis we find?  I'd like to understand why I see this type nowhere else.  I'll dig around in the literature and may check with Steven Beaty to see if any work has been done on this issue.
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And the other stoneflies in the Rivanna this morning were...

1. The Giant stonefly, Pteronarcys dorsata.  The rocks are covered with them.  Very odd.  The "striped" abdomen of which Beaty speaks ("The Plecoptera of North Carolina," p. 28) is very clear on this particular nymph.

2. And the common stonefly, Agnetina annulipes.  Note that the posterior edge of the wing pads is starting to curve.

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And another look at the A. abnormis that we find in the Rivanna.  This was a spectacular nymph that measured, I would guess, at least 15 mm.