Monday, September 1, 2014

The "Acroneuria" common stones in the Rivanna appear to be Acroneuria arenosa

One of the most common insects I saw at the Rivanna today is this very brown common stonefly (Perlidae), which in the past I've identified as Acroneuria abnormis.  A. abnormis nymphs can show up in various forms.  Let's look at Beaty on this:

A. abnormis -- male nymphs 15-20 mm, female nymphs 25-30 mm; dorsum of head with a well defined M-shaped head pattern, sometimes with interruptions; posterior margins of abdominal tergites light, dark tergal bands irregular; or dorsum of head without M-shaped head pattern and abdomen uniformly brown; anal gills always absent.  ("The Plecoptera of North Carolina," p. 14)

We've looked at all of these patterns before, but to review...

1) well defined "M-shaped head pattern (terga banded)

2) "M" pattern "with interruptions" (terga banded)

and 3) "M-shaped head pattern" missing: abdominal terga brown

This last nymph was found in the Rivanna last year, and it's easy to see why I would have thought it was a "type 3" A. abnormis.

My entry today was to focus again on A. abnormis, pointing out that the A. abnormis type that I find in the Rivanna is one that I never see in our smaller, cleaner streams.  In the other direction, I've never seen A. abnormis types 1 and 2 nymphs in the Rivanna.  And then there's the question "will we ever know why?"

But then I ran into a problem.  Remember, Beaty says of A. abnormis, "anal gills always absent."
Take another look at that nymph from this morning.

Do you see what I see?  This nymph has anal gills!

And here's a microscope view.

I'll be damned.  It can't be A. abnormis.  So, is there an Acroneuria species that provides a good match for the Acroneuria nymphs that I find in the Rivanna?  Yes there is: Acroneuria arenosa.

Beaty: A. arenosa -- male nymphs 14-17 mm, female nymphs 20-24 mm; dorsum of head with M-shaped pattern, sometimes faint to absent; abdomen uniformly brown; anal gills present.  Uncommon but widespread during the spring through fall.  (p. 14)

Perfect fit.  "M" pattern faint: abdomen totally brown; anal gills present

 Acroneuria arenosa nymphs appear to be fairly common in the Rivanna.   I'll confirm this finding with Beaty, but at the moment, I can reach no other conclusion.

Two other surprises today at the Rivanna (Darden Towe Park in C'ville).  1) I saw many, many fingernet caddisfly larvae; I did not see a single common netspinner (common in here are the species H. venularis and H. rossi).   The "commons" appear to be gone (pupating?); the fingernets are here in force.

2)  I was amazed by the number of Heptagenia marginalis flatheads that I found on the rocks.  And I did get some  pretty good photos.


The tolerance value of Acroneuria arenosa is 2.4.

(For a second photo of A. arenosa, go to:

Sunday, August 24, 2014

A new one in Montana: the Perlodid stonefly, Kogotus

This is one that we don't get to see in the East: the Perlodid stonefly, genus Kogotus (K. modestus and K. nonus are both found in Montana).  I was totally stumped by the ID, so I did what we should always do with a new stonefly -- I keyed it out, using Stewart and Stark's Nymphs of North American Stonefly Genera (pp. 361-364).  The lack of thoracic gills, the unidentate lacinia, the lack of dark longitudinal bands on the abdominal segments, and the absence of a "transverse anterior suture" on the mesosternal Y-ridge, led me to Kogotus (pp. 413-416).

Let's note certain key features.  1) "Head with light M pattern anterior to darker M between antennae; light spot in ocellar triangle and 2 small light spots outside lateral ocelli."  Check.

2) Lacinia unidentate.  Yes.

3) "Pronotum light, encircled with brown band except light lateral margins; a few tiny marginal spinules, but mostly glabrous."  All easy to see in this photo.

4) "Y-arms of mesosternum meet posterior corners of furcal pits; no transverse anterior suture connecting furcal pits."  Yes.

And one more thing.  5) "Male and female 8th abdominal sterna with mesoposterior interruption of posterior setal row."

The Northwestern Perlodid stonefly, genus Kogotus.  I'll let you know if I'm successful in getting this down to the level of species -- but there are only two choices: nonus and modestus.


The other stonefly I found -- and photographed -- is one that I've found once before: Oregon, August, 2012 (entry posted on 8/22).  The common stonefly, Calineuria californica.

This Perlid is attested in Alberta, British Columbia, California, Idaho, Montana, Oregon, and Washington.  (See Stewart and Stark, p. 329)

A quick review of critical features.  (All quotes from Stewart and Stark.)  1) "Body 17-25 mm, dark brown patterned with yellow."  This immature nymph was just 17 mm.  2) "SL gills (subanal gills) absent."  We can see that in the photos.  3) "Lacinia bidentate, terminal tooth strongly curved."  Yes.

4) "Dorsum of body covered with fine, dark clothing hairs and scattered short, thick bristles."  Yes.

5) "Occipital setae scattered near postocular fringe but arranged in a close set row behind ocelli."  Also, "Head with pale, oval spot surrounding lateral ocelli."  Both features are clear in this photo.

Common stonefly, Calineuria californica.


And one more insect -- sort of.  I was hoping to get a live photo of an "October Caddis" -- the Northern case-maker, genus Dicosmoecus.   Every case that I found, save one, was sealed up with the larva in pupation.  (In Montana, the "October" caddis tends to hatch in November.)  But in the Blackfoot River, I did find one that was open.

This is a large caddis: the case was 25 mm.  But, I could not get the larva to come out of its case!  Maddening.  And I waited a very long time.  To see it, I had to preserve it.  And here it is.

Wiggins' description reads as follow.  "These larvae are large and stout-bodied; sclerotized parts of the head and thorax are mostly uniform dark brown to black."  For sure.  "The tibiae of all legs have several pairs of stout spurs

and there are metanotal setate on the membrane between the primary sclerites."

Finally, "On segment IX a band of 20-40 setae extends ventrad from each side of the dorsal sclerite."  I didn't count them, but 20-40 seems right.

On the case, Wiggins notes, "Final-instar larvae have a case of fine gravel, regular in outline, slightly curved, and often somewahat flattened."  Like the case in the photo above.  "In the early instars cases are largely of plant materials."  You might recall that I found one of those immature larvae in such a case in Oregon in May.  (See post of 5/1/14.)

All quotes from Glenn B. Wiggins, Larvae of the North American Caddisfly Genera, University of Toronto Press, 1977.)

I wish I could have found time to get some more photos of insects: the streams that we fish are healthy with good populations.  Unfortunately, the trout fishing just got in the way!  Below, an 18" "cutbow" (Cutthroat and Rainbow hybrid) from the Upper Clark Fork.  What a beauty!

Friday, August 15, 2014

Case-maker day at the Rapidan River

(Obviously "not" a case-maker caddisfly larva -- but still one of the prettiest common stoneflies we find: Paragnetina immarginata.  It's a small one.  Even so, it measured -- I'd guess -- about 20 mm.  The Rapidan is one of the few places I see them.)

It was another day when I expected to find lots of small minnow mayflies, and I did find a couple.  But of much greater interest, I found 5 different case-maker families: Goeridae (Goera calcarata), Limnephilidae (Pycnopsyche scabripennis), Brachycentridae (Brachycentrus appalachia), Odontoceridae (Psilotreta labida), and Glossosomatidae (Glossosoma nigrior).

1. Weighted-case maker, Goera calcarata

The Goerid was the first thing I found, not something I ever expected to see.

I thought it was a Glossosomatid (Saddle-case maker) until I dropped the case into my bowl.  The two large pebbles on either side of the case immediately gave it away: Goeridae, the "weighted-case maker."   I thought these showed up in late fall and winter -- thinking I have to revise.

A defining trait of Goera -- in addition to the conspicuous case -- the "pronotum [is] produced anterolaterally into wide, sharply pointed processes." (Beaty, "The Trichoptera of North Carolina," p. 87)  Easy to see in the first photo.

This appeared to be Goera calcarata.  It was about 8 mm long with 3 pairs of sclerites on the metanotum.  Also, I did not see any sternal thoracic plates (which are distinct on Goera fuscula -- see the entries posted on 10/27/13 and 11/27/13), and there were "conspicuous spicules" on the anterior edge of the pronotum.  (For all of these traits see Beaty.)

By the way, if you find one of these cases but don't see the larva, turn the case on its back.  This larva only emerged when its case was flipped over.


2. Northern case-maker, Pycnopsyche scabripennis

I usually see these in the spring, so this too appeared to me to be out of season.  But there it was.  This is a large larva: the case was 25 mm, the larva about 22.  Nothing special I thought -- until I got home and downloaded my photos.  Then I spotted what looked to me like a tiny case on the side of the case.

And it was!  Under the microscope, I peeled this little case off and pulled it apart, finding this inside.

It's a midge!  How about that.  Actually, there's a whole world of "micro" midges, and my friend in Sugar Hollow has found them before.  Chironomids of any size, are difficult to ID to the level of genus, let alone species (over 20,000 species world-wide), so this is as far as I go.  Still, a very cool find.

(Is that the midge head and proleg I see sticking out?)


3. Humpless case-maker Brachycentrus appalachia

This was not a surprise.  This is a caddis that is plentiful in this stream, especially at this time of year -- and I saw a lot of cases today.

But do you notice what's on the side of this case as well?  It was another midge case.

When I peeled this one off it measured about 2 mm.  This one, however, was not occupied.

4. Strong-case maker , Psilotreta labida

Not a good photo, but it was a very small larva.  It's the start of the season for these larva: more common in the next 2-3 months.

5. Saddle-case-maker, Glossosoma nigrior

And by the time I got around to photos of the Glossosomatids, they had all climbed out of their cases!

A fun day -- and oh yes, I did photograph one of the small minnow mayflies.  It was a male Plauditus dubius.  Tiny: 3-4 mm.