Hitchin' a ride: fun photos from the past

I'll be off to the Rapidan River on Tuesday, but every so often I check through my photos to see what I'm most likely to see at a certain time of year.  And here was this very cool photo of a small minnow mayfly, Acentrella nadineae, that I found last year at the Rapidan on 9/16.   It decided to ride out its time in my petri dish on the back of the case of this humpless case-maker, Brachycentrus appalachia.

When I returned to the Rapidan in October (10/15/13), this move was repeated by a very young spiny crawler mayfly nymph, Ephemerella subvaria.

When I have more than one insect in my petri dish when I take photos, it isn't uncommon for a small insect -- especially a mayfly nymph -- to cling  to something else, especially to the case of a case-maker caddis.  It's good to remember that this happens in real life in the streams as well.  Why waste energy swimming or crawling about on your own when there's a free ride to be had?

So I decided to look through my files to see if there were other times in the past when this has resulted in my getting some pretty nice photos.  Here we go.  I'll use a chronological order.

1. A "baby" Giant stonefly clings to the tail of a more mature nymph.  Both nymphs are Pteronarcys proteus.  6/1/11.

2. A very small stonefly, Isogenoides hansoni,  "peeks" inside the case of a humpless case-maker.
9/12/11.

3. A small minnow mayfly, Acentrella turbida, rides the back of a Giant stonefly.  10/9/11.

4.  Mayfly nymphs and caddisfly larvae also enjoy riding atop the cases of other caddis case-makers.
Here's an Ameletid mayfly nymph (Ameletus lineatus) and a Lepidostomatid caddis riding on the three-sided leaf case of a Pycnopsyche gentilis (Northern case-maker).  2/29/12.

5.  A small minnow mayfly, Baetis intercalaris, holding on to the tail end of a humpless case-maker case.  9/22/12.

6.  Not really a case of "hitching a ride," but a nice photo of two caddisfly case-makers lying side-by-side in my dish: humpless case-maker, Brachycentrus appalachia, and a "little mountain case-maker," Apatania incerta.  Rapidan River, 1/4/13.

7.  And I totally missed my first sighting of an Isoperala orata Perlodid stonefly nymph.  It was clinging to the case of a Northern case-maker, Pycnopsyche scabripennis.  Rapidan River, 5/13/13.

8.  Another mayfly holding on to the pebble case of a Pycnopsyche gentilis (Northern case-make): this time a small spiny crawler.  4/20/14.

9. And quite recently -- 8/15/14 -- at the Rapidan River, I found this midge larva, inside of a case, attached to another Pycnopsyche scabripennis Northern case-maker larva.  8/15/14.

10.  One more.  I don't have the date, but it was sometime early this year.  A Uenoid caddisfly larva in its little case made of pebbles and sand, crawling around on a Pycnopsyche gentilis Northern case-maker in a case that's part leaves and part pebbles.

More "oddities" to come in future entries -- but first, let's see what we can find in that clean mountain stream.

The autumn stoneflies in the Rivanna

I went to the Rivanna at Crofton this morning, where I found pretty much what I expected to see: the common stonefly, Agnetina annulipes; the giant stonefly, Pteronarcys dorsata; and the same common stonefly that I found at Darden Towe Park, Acroneuria arenosa (or possibly A. evoluta).

In the photo above, a fairly immature A. annulipes (measured about 9 mm), and here's one even smaller (only 5 mm).

Agnetina Perlids have a complete setal row at the back of the head (occiput), and anal gills are present.  While neither feature is crystal clear in my photos, if you look closely, they can be seen in this one.

Other key features: "head pattern roughly M-shaped with arms directed posterolaterally," "dorsum of abdomen banded, with dark bands on anterior half of segment," and "tergum 10 mostly dark including apex."  (Beaty, "The Plecoptera of North Carolina," p. 15)  The banding shows up the best on the small nymph in the photo above: we can see the arms on the M, and dark color of tergum 10, in just about all of the photos.  Actually, in this photo we can see all of the features.

I saw a lot of these Perlids today, and I'll continue to see them well into the fall.  They're fairly common in the Rivanna at this time of year.
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Giant stonefly, Pteronarcys dorsata

And I saw a lot of these "giants" -- Pteronarcys dorsata.   This is a fairly tolerant species -- 2.4 -- and it's the only species of giant I've seen in the Rivanna.   Characteristics: "Lateral angles of pronotum produced, anterolateral ones almost hook-like;  no lateral projections on abdominal segments; each tergite with anterior and posterior abdominal spots sometimes confluent to give the abdomen a longitudinally striped appearance (3-5 stripes possible)."  (Beaty, "Plecoptera," p. 28)  Very striped it is.

Two interesting shots.  The first, a ventral view of the thoracic gills of the giants:

the second, a side view of one of my nymphs as it climbed on top of another.

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Common stonefly, Acroneuria arenosa/evoluta

Since I've already covered the problem we have with the ID of this particular Perlid,  I'll just post some of my photos.  The anal gills (black/gray) are clearly visible on both of the nymphs that I photographed: on one, the M pattern is clearly there but faint -- which points to A. arenosa.   But, both nymphs are still immature.  What I need to see is what the head will look like on these nymphs when they're mature.

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Common netspinner, Hydropsyche venularis

Though I found very few -- not sure I saw any -- common netspinners last week at Darden Towe Park, at Crofton, there were still some crawling in the vegetation on rocks.  I got some fairly good photos of this one, and it turned out to be Hydropsyche venularis, one of two species that I commonly see in this river.

The yellow marks on the head are key to identification.  On H. venularis, there are two pair of yellow dots on the top of the head, but the two often merge into one.

Still, to distinguish venularis from rossi, we must look at the muscle scars on the side of the head.  On H. venularis the rows of muscle scars "curve dorsad posteriorly." (Beaty, "The Trichoptera of North Carolina," p. 76)  You can see that with the top row in this microscope photo.

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The weather is changing at last: cooler temps on the way.  So I should start to get out more often and start looking at smaller streams.

The "brown" Acroneuria Perlids we find in the Rivanna: Two additional comments

On Monday I argued that this "brown" Acroneuria Perlid (Common stonefly) that I found at Darden Towe Park appears to be Acroneuria arenosa.  I'm still persuaded that that is the case, but I have two additional comments that need to be made.

1) I sent my photos to Steven Beaty, and he confirmed -- that yes indeed -- those are anal gills on our nymph, and that it could be A. arenosa.

He cautioned, however, that we have to consider another ID: Acroneuria evoluta.  Here is how he describes both of those species ("The Plecoptera of North Carolina," p. 14).

A. arenosa...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.

A. evoluta...dorsum of head with interrupted M-shaped head pattern, appearing as a transverse row of 3 light spots in front of anterior ocellus; abdomen not banded; anal gills present.  Uncommon.  Occurs in Piedmont, Slate Belt, and southern Coastal Plain frivers.  Nymphs more common during summer thru late fall.

Critical to the distinction is the "M-pattern" on the dorsum of the head.  Before we decide which description seems most appropriate for our nymph, here is a photo of a similar nymph that I found in the Rivanna last year (11/14/13).

It has the same anal gills, and for that matter, the same type of thoracic gills on the sides: gray/black in color, and not very frilly.  And the abdomen is uniformly brown; unbanded.

What about the "M" on the head?  Well, it is "interrupted" (=A. evoluta), but does it appear "as a transverse row of 3 light spots"?  I could go with that on the nymph that I found on Monday, but not with the nymph from last year.  I could just as well say that on both nymphs there is an "M" pattern present (=A. arenosa),but not, in this case, "faint to absent."

Tough call.  One I can't really make.  But there is one other thing to consider: A. arenosa is attested in Virginia, specifically in our part of Virginia, A. evoluta is not.  On this point, see: http://explorer.natureserve.org/servlet/NatureServe?searchSciOrCommonName=Acroneuria+arenosa&x=0&y=0, and http://explorer.natureserve.org/servlet/NatureServe?searchSciOrCommonName=Acroneuria+evoluta&x=10&y=7.  Stewart and Stark make the same point (see Nymphs of North American Stonefly Genera, p. 316).  Of course it is possible that we have found an Acroneuria species that, to date, has been overlooked in Virginia.

At the moment, I'm leaning towards A. arenosa, but the safe call is A. arenosa/evoluta.
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2) Point two is that it is not the case that every "brown" Acroneuria nymph that I've found in the Rivanna has anal gills.  Look at this nymph, found on 10/4/13, also at Darden Towe Park.

No anal gills (also note that the thoracic lateral gills are different; they're frilly and white).

This would surely be the 3rd type of A. abnormis noted by Beaty: "dorsum of head without M-shaped head pattern and abdomen uniformly brown." (p. 14)  Here is another example.

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I would conclude that A. arenosa/evoluta and A. abnormis (brown type) are both present in the Rivanna.  This is the only stream in which I've seen these brown Acroneuria Perlids.

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.
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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.

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The tolerance value of Acroneuria arenosa is 2.4.

(For a second photo of A. arenosa, go to: http://www.discoverlife.org/mp/20p?see=I_DSC2&res=mx&flags=no_slide_show.)