The Main Story Was Stoneflies -- Young and Old -- at the Rapidan River

Obviously one of the "old" ones with those very black wing pads: Paragnetina immarginata.  This one was small, 15 mm.  This one was large, 25 mm.

What accounts for the differences in size and abdomen color?  I don't really know, but I wonder if it's a matter of gender.  In any event they're both the same species.

P. immarginata -- nymphs ?? mm; head M-pattern with medial pale line extended anteriorly, often connected to pale frontoclypeal margin; yellow femora distinctively patterned with dark brown longitudinal bar extending about 2/3 the length; abdominal terga banded, anterior half dark; anal gills absent.  (Steven Beaty, "The Plecoptera of North Carolina," p. 18)

We can see those features on both of the nymphs, but let me use the big nymph for purposes of illustration.

Both of these stones have "missed the boat" when it comes to the peak hatch of the Golden Stones: that took place in July and August, and stonefly "shucks" were all over the place on the rocks in the river.

Note how the head and thorax splits open to allow the adult to emerge.   Most of the mature nymphs have hatched, but the leaf packs were jammed with small and mid-sized Perlids, both P. immarginata and A. abnormis.  Some of those will be hatching next summer; some have two years to go.
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Other insects around?  Some small minnow mayflies, mostly Acentrella nadineae, and still some E. vitreus flatheaded mayflies,  lots of common netspinners and fingernet netspinners (Dolophilodes), and caddis casemakers -- Saddle-case makers (Glossosomatids), Humpless case-makers (Brachycentrus appalachia), and some Strong case-makers (Odontocerids), the case-maker we commonly see here in the fall.

Odontocerid, Psilotreta labida

Brachycentrus appalachia (quite a few of their "log cabin" cases attached to the rocks)

And then there was this Giant stonefly.

Do you recognize it?  It's Pteronarcys proteus, the same species we see in the small streams in Sugar Hollow.   This is the first time I've seen it up here.  Actually, I've probably seen it before and mistakenly thought it was the species I'm used to finding in the Rapidan River, Pteronarcys biloba.  This one.

Note how the lateral abdominal hooks point sharply away from the body on P. biloba while they're "appressed"  -- i.e. tucked in close to the body -- on P. proteus.  Also, while there are sharp hooks on segments 1-8 on P. biloba, on P. proteus, the hook on segment 7 is small and virtually non-existent on segment 8.

So last week we discovered two Giant species in Buck Mt. Creek (P. dorsata and P. biloba), and today we find the same thing at the Rapidan River (P. proteus and P. biloba).

Always a treat to make this trip to Madison County.

And the answer is "yes": Pteronarcys biloba and Pteronarcys dorsata in the very same stream

Checked with Steven Beaty on this one, and "yes," it is possible for two different species of Giant stoneflies to live in the same stream -- more, in the very same set of riffles in the same stream, the riffles pictured in the photo above.

Buck Mt. Creek: Pteronarcys dorsata, June 20, 2013

Buck Mt. Creek: Pteronarcys biloba, September 5, 2013

That's the most tolerant Giant, -- P. dorsata, TV of 2.4 -- alongside the most intolerant Giant -- P. biloba, TV of 0.0.

Beaty added that this is most common in "good" streams.  People should pay more attention to Buck Mt. Creek.  I've said it before: I've found a greater variety of aquatic insects in this particular stream -- orders, families, genera, species -- than in any other stream I explore, ranging from very intolerant to very tolerant.  (For the up-to-date taxa list for Buck Mt. Creek, see the entry posted on 6/20/13).

Buck Mt. Creek Continues to Amaze

My destination this morning was the Rivanna River at Crofton where I found some very cool insects in September last year.  I went to the river, and I looked for insects, but I found very little.  The Rivanna is still abnormally high; the boulders that provide me with "tables" on which I can scrutinze rocks are all underwater.

But the morning sun was too good to waste, so I headed off to Buck Mt. Creek, my expectations low I might add.  Surprise, surprise -- I found some very nice insects including this young Giant stonefly, Pteronarcys biloba, tolerance value, 0.0.  If you look back to my posting of 6/20, you'll see another Giant that I found in this creek, one that appears to have been Pteronarcys dorsata (on which see the posting of 8/16).  Two Giant species in the same stream?  That's the way it looks at the moment.  Note the long tails, the "hooks" forming on the anterior corners of the pronotum, and, critically, the sharp lateral projections on the abdominal segments.

Now I'm intrigued.  Do P. biloba and P. dorsata co-exist?  You can be sure that I'll be carefully checking every Giant I see in BMC from now on.

P. dorsata? on 6/20.

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But another pleasant surprise.  I found three different species of flatheaded mayflies in the short while I was wading the stream: Maccaffertium ithaca (the "Light Cahill"), Heptagenia marginalis, and a very small Epeorus vitreus (so there are still some around).

1. M. ithaca.  I didn't take diagnostic photos -- though I saw what I needed to see in my microscope study.  "Sterna 5-6 or 7-8 with transverse sternal bands with anterolateral extensions" [and] "no denticles on tarsal claws."  (Steven Beaty, "The Ephemeroptera of North Carolina," p. 19)  Note that the thorax seems to be splitting in preparation for hatching.

2. Heptagenia marginalis.  This is one that we normally see in late summer -- August and September -- though this is a small one that will probably hatch in October.  Remember that this genus differs from Leucrocuta in having a fibrilliform portion on gill 7 (Leucrocuta does not), and that the species ID is determined by the oblique black lines on the sides of the abdominal segments.  (Beaty, "The Ephemeroptera of North Carolina," p. 18)

3. And the very small Epeorus vitreus, tiny, tiny wing pads.

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One more surprise -- though I've seen a few of them in here before: common netspinner, Macrostemum.   You'll recall that these larvae are common in the North Fork of the Rivanna below the bridge at Advance Mills.   In some of my photos, the "carina," the rim that forms the edge of the head, is easy to see.

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It was a quick trip to Buck Mt. Creek, but one that I'm glad that I made.

The Best of Montana, 2013

Time to get back to my streams in Virginia -- which I'll be doing on Thursday -- but one last look at my favorite photos from my trip to Montana last week.

1. In the photo above, a Trico nymph (family, Leptohyphidae; genus Tricorythodes).  I find them in the Rivanna all summer, but I never seem to find one that's "clean"!  This was a beauty, and the large oval/triangular operculate gills couldn't be clearer.

2. The spiny crawler mayfly Drunella coloradensis: they're so photogenic with those different colors and patterns.  (For the keys to species ID, by the way, look back to the posting of 8/31/12.  This includes the distinction between D. coloradensis and D. flavilinea: they're closely related.)

3. Flatheaded mayfly, Epeorus deceptivus.

4. A lovely small minnow mayfly, Baetis tricaudatus Montana style.

5. Perlodid stonefly, genus Skwala.  Finally found one.

6. Our free-living caddisfly larva, part of the R. brunnea/vemna group.

7. And a handsome common netspinner caddisfly larva, Arctopsyche grandis.

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I may make two trips to Montana next year, one in the spring where I'll do nothing but look for cool insects.  (The Bitterroot range of the Rockies seen from the Bitterroot River.)

(And the "Box Canyon" in the Blackfoot.)

Rhyacophila Brunnea-Vemna Group: Working out the ID of our Montana freeliving caddisfly larva

For the data that follows, I have relied on two sources: 1) the very thorough study of Rhyacophilids -- "Annotated list of Rhyacophila of North America with larval key and descriptions," compiled by Joe Giersch and Robert Wisseman, and 2) the "BugLab Western Rhyacophila Distribution Summary," available on line at: http://usu.edu/buglab/Content/Western%20Rhyacophila%20Distributions.pdf.  Many thanks to Roger Rohrbeck and Dave Ruiter for bringing these sources to my attention.
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The Rhyacophilid that I found in Grant Creek in Missoula, MT one week ago belongs to the Rhyacophila Brunnea-Vemna Group.  That means it is one of seven possible species: R. brunnea, R. inculta, R. starki, R. vao (R. brunnea Gr.),  R. gemona, R. robusta, or R. vemna (R. vemna Gr.).    However, if we rely on the "BugLab Distribution Summary," we can narrow that down to two possible species: R. brunnea and R. vao.  Those are the only species from either group that are attested in the state of Montana.  (But we must note that Giersch and Wisseman in their more recent study -- published in 2012 -- attest R. gemona, R. robusta, and R. vemna in Montana as well, complicating the picture.)

In any event, Giersch and Wisseman note that "Larvae of the Brunnea and Vemna Groups cannot be separated at this time" (p. 110), adding that "species of the Brunnea Group are exceedingly common in western montane streams, from small streams to hard-bottomed rivers," and  "Final instar larvae of the Vemna Group are significantly larger than Brunnea Group species," and "Species of the Vemna Group are found in colder, higher elevation, small to mid-size streams in mountains of the Pacific Northwest." ("Annotated list of Rhyacophila of North America," p. 47)  Our larva is a member of the Rhyacophila Brunnea-Vemna Group -- that's as far as we can take it.
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What is it that R. Brunnea gr. and R. Vemna gr. larvae have in common?  All species in both of these groups have "Three pairs of gills on abdominal segments II-VII." (Giersch and Wisseman, p. 10)  The gills are in clusters -- or multi-branched -- and in the following photo I've tried to point out one set of 3 on segment 5 of the larva I found.

R. Brunnea gr. and R. Vemna gr. larvae have only one pair of gills on abdominal segment I and two pairs of gills on segment 8.  In the following microscope photo I've done my best to show those gills on segments 7 and 8.

Giersch and Wisseman note two other diagnostic features that we can also see in this photo: 1) "Basoventral hook: Present, but not strongly developed," and 2) Anal claw, ventral teeth: One large and one small ventral tooth." (p. 47)

Rhyacophila Brunnea-Vemna Group -- but this is a small one, 10 mm. Final instar larvae of R. Brunnea can be as much as 21 mm (p. 47) while R. Vemna larvae tend to be slightly longer, 25 mm. (p. 110)
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Montana Part II: Perlodid Stonefly "Skwala"

Last Monday we floated -- and fished -- the Upper Clark Fork River, east of Missoula, and I found an important stonefly in this part of the country, the Perlodid stonefly, genus Skwala (probably Skwala americana).   The "Skwala hatch" is one of the first major fly fishing events for guides working out of Missoula: it normally happens in April.   Most guides focus on the Bitterroot River, but obviously there are Skwalas in other local rivers as well.

This is not a stonefly that's found in the East, so to make this identification I turned to Stewart and Stark's Nymphs of North American Stonefly Genera, pp. 448-451.  The critical features for us to see are 1) the pattern on the head,  2) the morphology of the lacinia,  and 3) the shape of the mesosternal "Y-arms".

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." (Stewart and Stark, p. 448)  In this photo, we can easily see everything but the band of stout spinules behind each of the spots (though they're easy to see with the microscope).

Lacinia: "Lacinia bidentate with tuft of setae on a low knob below subapical tooth and continuous inner row of marginal setae; complete band of submarginal setae...; terminal tooth about 0.4 times total outer lacinial length and subapical tooth about 0.6 times length of terminal tooth."  (p. 448)
Here's what that looks like in a microscope view.

Shape of Y-arms:  "Y-arms of mesosternum reach anterior corners of furcal pits." (p. 451)  They look like this, as illustrated by Stewart and Stark on p. 450.

I'd note one other feature for confirmation.  On the abdomen, the medial longitudinal line consists of a series of dots and slashes --

that feature, too, is illustrated by Stewart and Stark, p. 449.  The Perlodid stonefly, Skwala.

Now if I can only get out to Montana in April when these nymphs mature and hatch: it's supposed to be fantastic fishing!
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Naturally, I found other insects on Monday as well, some of which require more work for species identification.  More on that later, but here are some photos.

1. A "Trico" (family, Leptohyphidae; genus, Tricorythodes).  We had prolific hatches of Tricos every morning, no matter where we were fishing.

2. The flatheaded mayfly Nixe, species unknown.

3. A pronggilled mayfly, Paraleptophlebia bicornuta.  The "tusks" at the front of the head give this one away.

4. Some small minnow mayflies that I need to ID.  (Very small, 4-5 mm)

5. And a very large, very black,  common netspinner larva.  The genus is Arctopsyche.  I think it's A. californica, but I need to look into that further.