Saturday, February 9, 2013

Isoperla season at the Rapidan River

If you follow this blog in a regular way, you know that the streams in our part of Virginia -- especially those in or near the mountains -- are rich in Isoperla Perlodid stoneflies (sometimes called "stripe tails").  The most common species we see is Isoperla namata, and I saw them in large numbers today.  But the Perlodid in the photo above is a small Isoperla nr. holochlora.  I was surprised to see one this early, but the Rapidan is the only stream in which I've seen this particular species.  (For a list of the Isoperlas I've found, so far, in our streams, see the entry posted on 9/8/12: for photos, see the entry for 10/3/12.)

The "nr." in Isoperla nr. holochlora stands for "near," since this species looks a lot like Isoperla holochlora.  But we can tell them apart with no need for a microscope view.  There are two things to check: 1) does the front part of the large yellow spot on the head merge with the labrum, or just barely touch it?  2) are there obvious longitudinal "stripes" on the abdominal segments?  If it's Isoperla nr. holochlora, the answers are "no": if it's Isoperla holochlora, the answers are "yes."  Have a look at these mature nymphs from last year.

They're both beautiful stoneflies.  But the Perlodid that we're more likely to see -- and in the spring, we see a lot of them in a lot of our streams (TV is 2.5) - is Isoperla namata.  This one.

They were a dime a dozen today in the leaf packs, and note that the wing pads on the nymph in the second photo are already starting to spread out from the body.

The I. nr. holochlora was a surprise in my findings today: so too was this little small minnow mayfly, even though I was hoping to see one.

Baetis tricaudatus -- the first of the year.  This is a fairly intolerant small minnow mayfly with a TV of 1.5, so we only see it in clean mountain streams.  I've found them in the Rapidan, the South river in Greene county, and in some of the small streams in Sugar Hollow.  There are two ways to tell this one if you're using a loupe: 1) B. tricaudatus nymphs have a short, center tail with no banding on the tails, and 2) they have a thin, pale stripe that runs down the center of the abdominal segments.

And when they're mature they look like this (Rapidan River last winter).


For the most part, I saw insects today that I expected to see, but the variety in this stream is really amazing.

1. Flatheaded mayfly, Epeorus pleuralis.  This is the most common insect I saw on the bottoms of rocks.

The wing pads are fairly long on this one as it prepares for the early spring hatch of "Quill Gordons."

2. Pronggilled mayfly, genus Paraleptophlebia.  These were common in leaf packs.

3. Spiny crawler mayfly, Ephemerella subvaria.  Always a favorite.

4. Spiny crawler mayfly, Ephemerella invaria.  Note the pale yellow stripe between the eyes.

5. Common netspinner caddisfly larva, Ceratopsyche alhedra.  This is the only common netspinner species I've seen up here in the winter.

6. Freeliving caddisfly larva, Rhyacophila fuscula.  This was a BIG one, probably an inch long.

7. Humpless case-maker caddisfly larva, Brachycentrus appalachia.

8. Lepidostomatid case-maker caddisfly larva, genus Lepidostoma.  I've never seen "mixed media" cases at the Rapidan River.

9.  Uenoid caddisfly larva, Neophylax consimilis.

10. And Uenoid caddisfly larva, species to be determined.

The water remains high in most of our streams -- but at least there was sunshine today!

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